There are multiple lines of evidence that an animal-based diet best supports human brain development in infants and young children.
In brief / contents
- Human fetuses and infants rely on ketones for brain building.
- Weaning onto meat increases brain growth.
- The brain is an energy-intensive organ that required an animal-based diet to evolve.
- Because humans wean early, and human brain growth is extended past weaning, the post-weaning diet must support fetal-like brain growth.
- A high-fat animal-based diet best supports brain growth.
Human fetuses and infants rely on ketones for brain building.
In a previous post, we wrote about the known (but little-spoken-of) fact that human infants are in mild ketosis all the time, especially when breastfed. In other words, ketosis is a natural, healthy state for infants. Infancy is a critical time for brain growth, so we expect that ketosis is advantageous for a growing brain. Otherwise, there would have been a selective advantage to reduced ketotis in infancy. This species-critical, rapid brain growth continues well past weaning. For that reason, we suggest in our article that weaning onto a ketogenic diet would probably be preferable to weaning away from ketosis.
In response to that post, a reader sent us a paper called Survival of the fattest: fat babies were the key to evolution of the large human brain.  The authors discuss the apparently unique human trait of having extremely fat babies, and explain it in terms of the unique need for growth of extremely large brains.
A key point they make is that a baby's ample fat provides more than simply a large energy supply, (much more than could be stored as glycogen or protein; by their calculations, more than 20 times more), but that ketone bodies are themselves important for human brain evolution.
They repeat the usual unwarranted assumption that adult brains use mainly glucose for brain fuel by default, and that ketone bodies are merely an alternative brain fuel. Nonetheless, when talking about fetuses, they are willing to say that the use of ketones is not merely an "alternative":
In human fetuses at mid-gestation, ketones are not just an alternative fuel but appear to be an essential fuel because they supply as much as 30% of the energy requirement of the brain at that age (Adam et al., 1975).
Second, ketones are a key source of carbon for the brain to synthesize the cholesterol and fatty acids that it needs in the membranes of the billions of developing nerve connections.
Ketones are the preferred carbon source for brain lipid synthesis and they come from fatty acids recently consumed or stored in body fat. This means that, in infants, brain cholesterol and fatty acid synthesis are indirectly tied to mobilization and catabolism of fatty acids stored in body fat.
In other words, the claim is that ketones are the best source of certain brain-building materials, and specifically, that fetuses use them for that purpose.
Moreover, the thesis is that the extra body fat on human babies is there specifically for the purpose of supporting extra brain growth after birth, through continued use of ketones.
Weaning onto meat increases brain growth.
[ Please note that by convention weaning refers to the gradual process of transitioning from exclusive breastfeeding (starting with the first foods introduced, while breastfeeding is still ongoing), to the end of breastfeeding, not just the end itself. ]
We aren't the only ones who have thought weaning onto meat would be a good idea. A couple of studies have compared weaning onto meat rather than cereal.
One showed a larger increase in head circumference , which is a good index of brain growth in infants  and young children . Moreover, higher increases in head circumference in infants are correlated with higher intelligence, independently of head circumference at birth . In other words, the amount of brain growth after birth is a better predictor of intelligence than the amount of brain growth in gestation.
That study also found the meat-fed infants to have better zinc status, and good iron status despite not supplementing iron as was done in the cereal arm . Zinc and iron are abundant in the brain, and zinc deficiency is implicated in learning disorders and other brain development problems . Iron deficiency is a common risk in infants in our culture, because of our dietary practices, which is why infant cereal is fortified with it .
Another study showed better growth in general in babies weaned onto primarily meat .
Weaning onto meat is easy. Here's how I did it.
It is believed likely that early humans fed their babies pre-chewed meat . I did that, too, although that wasn't my first weaning step. Influenced by baby-led weaning, I waited until he was expressing clear interest in my food, and then simply shared it with him. At the time this meant:
- Broth on a spoon, increasingly with small fragments of meat in it.
- Bones from steaks and chops, increasingly with meat and fat left on them.
- Homemade plain, unseasoned jerky, which he teethed on, or sucked until it disintegrated.
- Beef and chicken liver, which has a soft, silky texture, and is extremely nutrient-dense.
The brain is an energy-intensive organ that required an animal-based diet to evolve.
In 1995, anthropologists Leslie C. Aiello and Peter Wheeler posed the following problem :
- Brains require an enormous amount of energy.
- Humans have much larger brains than other primates.
- However, human basal metabolic rates are not more than would be predicted by their body mass.
Where do we get the extra energy required to fuel our brains, and how could this have evolved?
Aiello and Wheeler explain this by noting that at the same time as our brains were expanding, our intestines (estimated as comparably energy-intensive) were shrinking, by almost exactly the same amount. thereby freeing up the extra metabolic energy needed for the brain. Both adaptations, a large brain and small guts, independently required them to adopt a "high-quality" diet, for different reasons.
Let's mince no words; "high-quality" means meat . Meat is more nutrient dense than plants, both in terms of protein and vitamins. Plants are simply too fibrous, too low in protein and calories, and too seasonal to have been relied on for such an evolutionary change , . It is widely accepted that meat became an important part of our diets during this change. This is the mainstream view in anthropology .
Although the need for protein and brain-building nutrients is often cited as a reason for needing meat in the evolutionary diet, energy requirements are also important to consider. It would have been difficult to get caloric needs met from plants (especially before cooking) , because they were so fibrous. Herbivores with special guts (such as ruminants like cows with their "four stomachs") and primates with much larger intestines than we have, actually use bacteria in their guts to turn significant amounts of fiber into fat, see eg. . This strategy is not available to a such a small gut , , which is why we had to find food that was energy dense as is.
Fortunately, insofar as we were already using animal sources to get protein and nutrients, we also had access to an abundance of fat. The animals we hunted were unlikely to have been as lean as modern game. Evidence supports the hypothesis that human hunting was the most likely cause of the extinction of many megafauna (large animals that were much fatter than the leaner game we have left today) . Humans, like carnivores, prefer to hunt larger animals whenever they are available . It has been proposed that the disappearance of the fatter megafauna exerted a strong evolutionary pressure on humans, who were already fat-dependent, to become more skilled hunters of the small game we have today, to rely more on the fat from eating brains and marrow, and to learn to find the fattest animals among the herds .
Animal fat and animal protein provided the energy, protein, and nutrients necessary for large brains, especially given the constraint of small guts.
Because humans wean early, and human brain growth is extended past weaning, the post-weaning diet must support fetal-like brain growth.
Humans wean much earlier than other primates, and yet their brains require prolonged growth. Our intelligence has been our primary selective advantage. Therefore it is critical from an evolutionary standpoint that the diet infants were weaned onto was supportive of this brain growth.
In a (fascinating and well-written) paper on weaning and evolution, Kennedy puts it this way:
"[A]lthough this prolonged period of development i.e., ‘‘childhood’’ renders the child vulnerable to a variety of risks, it is vital to the optimization of human intelligence; by improving the child’s nutritional status (and, obviously, its survival), the capability of the adult brain is equally improved. Therefore, a child’s ability to optimize its intellectual potential would be enhanced by the consumption of foods with a higher protein and calorie content than its mother’s milk; what better foods to nourish that weanling child than meat, organ tissues (particularly brain and liver), and bone marrow, an explanation first proposed by Bogin (1997)."
"Increase in the size of the human brain is based on the retention of fetal rates of brain growth (Martin, 1983), a unique and energetically expensive pattern of growth characteristic of altricial [ born under-developed ] mammals (Portmann, 1941; Martin, 1984). This research now adds a second altricial trait—early weaning—to human development. The metabolically expensive brain produced by such growth rates cannot be sustained long on maternal lactation alone, necessitating an early shift to adult foods that are higher in protein and calories than human milk."
The only food higher in protein and calories than breast milk is meat.
A high-fat animal-based diet best supports brain growth.
Taking these facts together:
- Even modern fetuses and breastfed infants are in ketosis, which uniquely supports brain growth.
- Infants who are weaned onto meat get essential nutrients to grow brains with: nutrients that are currently deficient in our plant-centric diets today. Moreover, experiments have found that their brains actually grow more than babies fed cereal.
- Human brains continue to grow at a fast rate even past weaning.
- It is likely that in order to evolve such large, capable brains, human babies were weaned onto primarily meat.
A meat-based, inherently ketogenic diet is not only likely to be our evolutionary heritage, it is probably the best way to support the critical brain growth of the human child.
We would like to thank Matthew Dalby, a researcher at the University of Aberdeen, for helpful discussions about short-chain fatty acid production in the large intestines.
Cunnane SC, Crawford MA.
Comp Biochem Physiol A Mol Integr Physiol. 2003 Sep;136(1):17-26.
Evidence type: experiment
Meat as a first complementary food for breastfed infants: feasibility and impact on zinc intake and status.
Krebs NF, Westcott JE, Butler N, Robinson C, Bell M, Hambidge KM.
J Pediatr Gastroenterol Nutr. 2006 Feb;42(2):207-14.
"This study was undertaken to assess the feasibility and effects of consuming either meat or iron-fortified infant cereal as the first complementary food.
"Eighty-eight exclusively breastfed infants were enrolled at 4 months of age and randomized to receive either pureed beef or iron-fortified infant cereal as the first complementary food, starting after 5 months and continuing until 7 months. Dietary, anthropometric, and developmental data were obtained longitudinally until 12 months, and biomarkers of zinc and iron status were measured at 9 months.
"Mean (+/-SE) daily zinc intake from complementary foods at 7 months for infants in the meat group was 1.9 +/- 0.2 mg, whereas that of the cereal group was 0.6 +/- 0.1 mg, which is approximately 25% of the estimated average requirement. Tolerance and acceptance were comparable for the two intervention foods. Increase in head circumference from 7 to 12 months was greater for the meat group, and zinc and protein intakes were predictors of head growth. Biochemical status did not differ by feeding group, but approximately 20% of the infants had low (<60 microg/dL) plasma zinc concentrations, and 30% to 40% had low plasma ferritin concentrations (<12 microg/L). Motor and mental subscales did not differ between groups, but there was a trend for a higher behavior index at 12 months in the meat group.
"Introduction of meat as an early complementary food for exclusively breastfed infants is feasible and was associated with improved zinc intake and potential benefits. The high percentage of infants with biochemical evidence of marginal zinc and iron status suggests that additional investigations of optimal complementary feeding practices for breastfed infants in the United States are warranted."
Evidence type: authority
[Head circumference and brain development. Growth retardation during intrauterine malnutrition and catch-up growth mechanisms (author's transl)]. [Article in German]
Klin Wochenschr. 1981 Sep 1;59(17):995-1007.
"Today the close correlation between head circumference growth and brain development in the last weeks of gestation and in the first two years of life is no longer disputed. A recently developed formula even allows for calculations of brain weight based upon head circumference data. Between the ages of 32 postmenstrual weeks and six months after expected date of delivery there is a period of very rapid brain growth in which the weight of the brain quadruples. During this growth spurt there exists an increased vulnerability by unfavorable environmental conditions, such as malnutrition and psychosocial deprivation. The erroneous belief still being prevalent that the brain of the fetus and young infant is spared by malnutrition, can be looked upon as disproved by new research results. Severe malnutrition during the brain growth spurt is thought to be a very important non-genetic factor influencing the development of the central nervous system (CNS) and therewith intellectual performance. In the past a permanent growth retardation of head circumference and a reduced intellectual capacity usually was observed in small-for-gestational age infants (SGA). Nowadays, however, there can be found also proofs of successful catch-up growth of head circumference and normal intellectual development after early and high-energy postnatal feeding of SGA infants. The development of SGA infants of even very low birth weight can be supported in such a way that it takes a normal course by providing good environmental conditions, such as appropriate nutrition - especially during the early growth period - and a stimulating environment with abundant attention by the mother."
Evidence type: experiment
Relationship between head circumference and brain volume in healthy normal toddlers, children, and adults.
Bartholomeusz HH, Courchesne E, Karns CM.
Neuropediatrics. 2002 Oct;33(5):239-41.
"To quantify the relationship between brain volume and head circumference from early childhood to adulthood, and quantify how this relationship changes with age.
"Whole-brain volume and head circumference measures were obtained from MR images of 76 healthy normal males aged 1.7 to 42 years.
"Across early childhood, brain volume and head circumference both increase, but from adolescence onward brain volume decreases while head circumference does not. Because of such changing relationships between brain volume and head circumference with age, a given head circumference was associated with a wide range of brain volumes. However, when grouped appropriately by age, head circumference was shown to accurately predict brain volume. Head circumference was an excellent prediction of brain volume in 1.7 to 6 years old children (r = 0.93), but only an adequate predictor in 7 to 42 year olds.
"To use head circumference as an accurate indication of abnormal brain volume in the clinic or research setting, the patient's age must be taken into account. With knowledge of age-dependent head circumference-to-brain volume relationship, head circumference (particularly in young children) can be an accurate, rapid, and inexpensive indication of normalcy of brain size and growth in a clinical setting.
Evidence type: experiment
Gale CR1, O'Callaghan FJ, Godfrey KM, Law CM, Martyn CN.
Brain. 2004 Feb;127(Pt 2):321-9. Epub 2003 Nov 25.
"Head circumference is known to correlate closely with brain volume (Cooke et al., 1977; Wickett et al., 2000) and can therefore be used to measure brain growth, but a single measurement cannot provide a complete insight into neurological development. Different patterns of early brain growth may result in a similar head size. A child whose brain growth both pre‐ and postnatally followed the 50th centile might attain the same head size as a child whose brain growth was retarded in gestation but who later experienced a period of rapid growth. Different growth trajectories may reflect different experiences during sensitive periods of brain development and have different implications for later cognitive function.
"We have investigated whether brain growth during different periods of pre‐ and postnatal development influences later cognitive function in a group of children for whom serial measurements of head growth through foetal life, infancy and childhood were available."
"We found no statistically significant associations between head circumference at 18 weeks’ gestation or head circumference at birth SDS and IQ at the age of 9 years."
"In contrast, there were strong statistically significant associations between measures of postnatal head growth and IQ. After adjustment for sex, full‐scale IQ rose by 2.59 points (95% CI 0.87 to 4.32) for each SD increase in head circumference at 9 months of age, and by 3.85 points (95% CI 1.96 to 5.73) points for each SD increase in head circumference at 9 years; verbal IQ rose by 2.66 points (95% CI 0.49 to 4.83) for each SD increase in head circumference at 9 months of age, and by 3.76 points (95% CI 1.81 to 5.72) for each SD increase in head circumference at 9 years; performance IQ rose by 2.88 points (95% CI 0.659 to 5.11) for each SD increase in head circumference at 9 months of age, and by 3.16 points (95% CI 1.16 to 5.16) for each SD increase in head circumference at 9 years."
"[W]e interpret these findings as evidence that postnatal brain growth is more important than prenatal brain growth in determining higher mental function. This interpretation is supported by the finding that head growth in the first 9 months of life and head growth between 9 months and 9 years of age are also related to cognitive function, regardless of head size at the beginning of these periods."
Evidence type: review
Pfeiffer CC, Braverman ER.
Biol Psychiatry. 1982 Apr;17(4):513-32.
"The total content of zinc in the adult human body averages almost 2 g. This is approximately half the total iron content and 10 to 15 times the total body copper. In the brain, zinc is with iron, the most concentrated metal. The highest levels of zinc are found in the hippocampus in synaptic vesicles, boutons, and mossy fibers. Zinc is also found in large concentrations in the choroid layer of the retina which is an extension of the brain. Zinc plays an important role in axonal and synaptic transmission and is necessary for nucleic acid metabolism and brain tubulin growth and phosphorylation. Lack of zinc has been implicated in impaired DNA, RNA, and protein synthesis during brain development. For these reasons, deficiency of zinc during pregnancy and lactation has been shown to be related to many congenital abnormalities of the nervous system in offspring. Furthermore, in children insufficient levels of zinc have been associated with lowered learning ability, apathy, lethargy, and mental retardation. Hyperactive children may be deficient in zinc and vitamin B-6 and have an excess of lead and copper. Alcoholism, schizophrenia, Wilson's disease, and Pick's disease are brain disorders dynamically related to zinc levels. Zinc has been employed with success to treat Wilson's disease, achrodermatitis enteropathica, and specific types of schizophrenia."
Evidence type: authority
"Who is most at risk?
Young children and pregnant women are at higher risk of iron deficiency because of rapid growth and higher iron needs.
Adolescent girls and women of childbearing age are at risk due to menstruation.
Among children, iron deficiency is seen most often between six months and three years of age due to rapid growth and inadequate intake of dietary iron. Infants and children at highest risk are the following groups:
Evidence type: experiment
High protein intake from meat as complementary food increases growth but not adiposity in breastfed infants: a randomized trial
Minghua Tang and Nancy F Krebs
Am J Clin Nutr October 2014 ajcn.088807
"Background: High intake of cow-milk protein in formula-fed infants is associated with higher weight gain and increased adiposity, which have led to recommendations to limit protein intake in later infancy. The impact of protein from meats for breastfed infants during complementary feeding may be different.
"Objective: We examined the effect of protein from meat as complementary foods on growths and metabolic profiles of breastfed infants.
"Design: This was a secondary analysis from a trial in which exclusively breastfed infants (5–6 mo old from the Denver, CO, metro area) were randomly assigned to receive commercially available pureed meats (MEAT group; n = 14) or infant cereal (CEREAL group; n = 28) as their primary complementary feedings for ∼5 mo. Anthropometric measures and diet records were collected monthly from 5 to 9 mo of age; intakes from complementary feeding and breast milk were assessed at 9 mo of age.
"Results: The MEAT group had significantly higher protein intake, whereas energy, carbohydrate, and fat intakes from complementary feeding did not differ by group over time. At 9 mo of age mean (± SEM), intakes of total (complementary feeding plus breast-milk) protein were 2.9 ± 0.6 and 1.4 ± 0.4 g ⋅ kg−1 ⋅ d−1, ∼17% and ∼9% of daily energy intake, for MEAT and CEREAL groups, respectively (P < 0.001). From 5 to 9 mo of age, the weight-for-age z score (WAZ) and length-for-age z score (LAZ) increased in the MEAT group (ΔWAZ: 0.24 ± 0.19; ΔLAZ: 0.14 ± 0.12) and decreased in the CEREAL group (ΔWAZ: −0.07 ± 0.17; ΔLAZ: −0.27 ± 0.24) (P-group by time < 0.05). The change in weight-for-length z score did not differ between groups. Total protein intake at 9 mo of age and baseline WAZ were important predictors of changes in the WAZ (R2 = 0.23, P = 0.01).
"Conclusion: In breastfed infants, higher protein intake from meats was associated with greater linear growth and weight gain but without excessive gain in adiposity, suggesting potential risks of high protein intake may differ between breastfed and formula-fed infants and by the source of protein."
"Premastication is complementary to breastfeeding in the health practices of infants and young children, providing large amounts of carbohydrate and protein nutrients not always available through breast milk, and micronutrients such as iron, zinc, and vitamin B12 which are essential nutrients present mainly in meat. Compounds in the saliva, such as haptocorrin also helps increase B12 availability by protecting the vitamin against stomach acid.
"Infant intake of heme iron
"Meats such as beef were likely premasticated during human evolution as hunter-gatherers. This animal-derived bioinorganic iron source is shown to confer benefits to young children (two years onwards) by improving growth, motor, and cognitive functions. In earlier times, premastication was an important practice that prevented infant iron deficiency.
"Meats provide Heme iron that are more easily absorbed by human physiology and higher in bioavailability than non-heme irons sources, and is a recommended source of iron for infants."
Leslie C. Aiello and Peter Wheeler
Current Anthropology, Vol. 36, No. 2 (Apr., 1995), pp. 199-221
Evidence type: review
J Nutr. 2003 Nov;133(11 Suppl 2):3886S-3892S.
(The whole paper is worth reading, but these highlights serve our point.)
"Without routine access to ASF [animal source foods], it is highly unlikely that evolving humans could have achieved their unusually large and complex brain while simultaneously continuing their evolutionary trajectory as large, active and highly social primates. As human evolution progressed, young children in particular, with their rapidly expanding large brain and high metabolic and nutritional demands relative to adults would have benefited from volumetrically concentrated, high quality foods such as meat."
"If the dietary trajectory described above was characteristic of human ancestors, the routine, that is, daily, inclusion of ASF in the diets of children seems mandatory as most wild plant foods would not be capable of supplying the protein and micronutrients children require for optimal development and growth, nor could the gut of the child likely provide enough space, in combination with the slow food turnover rate characteristic of the human species, to secure adequate nutrition from wild plant foods alone. Wild plant foods, though somewhat higher in protein and some vitamins and minerals than their cultivated counterparts (52), are also high in fiber and other indigestible components and most would have to be consumed in very large quantity to meet the nutritional and energetic demands of a growing and active child."
"Given the postulated body and brain size of the earliest humans and the anatomy and kinetic pattern characteristics of the hominoid gut, turning increasingly to the intentional consumption of ASF on a routine rather than fortuitous basis seems the most expedient, indeed the only, dietary avenue open to the emerging human lineage (2,3,10,53)."
"Given the probable diet, gut form and pattern of digestive kinetics characteristic of prehuman ancestors, it is hypothesized that the routine inclusion of animal source foods in the diet was mandatory for emergence of the human lineage. As human evolution progressed, ASF likely achieved particular importance for small children due to the energetic demands of their rapidly expanding large brain and generally high metabolic and nutritional demands relative to adults."
Evidence type: review
J Hum Evol. 2005 Feb;48(2):123-45. Epub 2005 Jan 18.
"Although some researchers have claimed that plant foods (e.g., roots and tubers) may have played an important role in human evolution (e.g., O’Connell et al., 1999; Wrangham et al., 1999; Conklin-Brittain et al., 2002), the low protein content of ‘‘starchy’’ plants, generally calculated as 2% of dry weight (see Kaplan et al., 2000: table 2), low calorie and fat content, yet high content of (largely) indigestible fiber (Schoeninger et al., 2001: 182) would render them far less than ideal weaning foods. Some plant species, moreover, would require cooking to improve their digestibility and, despite claims to the contrary (Wrangham et al., 1999), evidence of controlled fire has not yet been found at Plio-Pleistocene sites. Other plant foods, such as the nut of the baobab (Adansonia digitata), are high in protein, calories, and lipids and may have been exploited by hominoids in more open habitats (Schoeninger et al., 2001). However, such foods would be too seasonal or too rare on any particular landscape to have contributed significantly and consistently to the diet of early hominins. Moreover, while young baobab seeds are relatively soft and may be chewed, the hard, mature seeds require more processing. The Hadza pound these into flour (Schoeninger et al., 2001), which requires the use of both grinding stones and receptacles, equipment that may not have been known to early hominins. Meat, on the other hand, is relatively abundant and requires processing that was demonstrably within the technological capabilities of Plio-Pleistocene hominins. Meat, particularly organ tissues, as Bogin (1988, 1997) pointed out, would provide the ideal weaning food."
Plants can become more nutrient dense through cooking. That is the basis of Wrangham's hypothesis:
"Wrangham's latest work focuses on the role cooking has played in human evolution. He has argued that cooking food is obligatory for humans as a result of biological adaptations and that cooking, in particular the consumption of cooked tubers, might explain the increase in hominid brain sizes, smaller teeth and jaws, and decrease in sexual dimorphism that occurred roughly 1.8 million years ago. Most anthropologists disagree with Wrangham's ideas, pointing out that there is no solid evidence to support Wrangham's claims. The mainstream explanation is that human ancestors, prior to the advent of cooking, turned to eating meats, which then caused the evolutionary shift to smaller guts and larger brains."
Evidence type: review
Popovich DG1, Jenkins DJ, Kendall CW, Dierenfeld ES, Carroll RW, Tariq N, Vidgen E.
J Nutr. 1997 Oct;127(10):2000-5.
"We studied the western lowland gorilla diet as a possible model for human nutrient requirements with implications for colonic function. Gorillas in the Central African Republic were identified as consuming over 200 species and varieties of plants and 100 species and varieties of fruit. Thirty-one of the most commonly consumed foods were collected and dried locally before shipping for macronutrient and fiber analysis. The mean macronutrient concentrations were (mean ± SD, g/100 g dry basis) fat 0.5 ± 0.4, protein 11.8 ± 8.2, available carbohydrate 7.7 ± 6.3 and dietary fiber 74.0 ± 12.9. Assuming that the macronutrient profile of these foods was reflective of the whole gorilla diet and that dietary fiber contributed 6.28 kJ/g (1.5 kcal/g), then the gorilla diet would provide 810 kJ (194 kcal) metabolizable energy per 100 g dry weight. The macronutrient profile of this diet would be as follows: 2.5% energy as fat, 24.3% protein, 15.8% available carbohydrate, with potentially 57.3% of metabolizable energy from short-chain fatty acids (SCFA) derived from colonic fermentation of fiber. Gorillas would therefore obtain considerable energy through fiber fermentation. We suggest that humans also evolved consuming similar high foliage, high fiber diets, which were low in fat and dietary cholesterol. The macronutrient and fiber profile of the gorilla diet is one in which the colon is likely to play a major role in overall nutrition. Both the nutrient and fiber components of such a diet and the functional capacity of the hominoid colon may have important dietary implications for contemporary human health."
We disagree, of course, with the authors' suggested interpretation that humans, too, could make good use of the same dietary strategy, as we haven't the colons for it.
The maximum amount of fat humans could get from fermenting fibre in the gut is unknown. The widely cited value of 10% of calories comes from:
E. N. Bergman
Physiological Reviews Published 1 April 1990 Vol. 70 no. 2, 567-590
"The value of 6-10% for humans (Table 3) was calculated on the basis of a typical British diet where 50-60 g of carbohydrate (15 g fiber and 35-50 g sugar and starch) are fermented per day (209). It is pointed out, however, that dietary fiber intakes in Africa or the Third World are up to seven times higher than in the United Kingdom (55). It is likely, therefore, that much of this increased fiber intake is fermented to VFA and even greater amounts of energy are made available by large intestinal fermentation."
However, it should not be concluded that SCFA production could rise to 70% of energy requirements!
For one thing, as a back-of-the-envelope calculation, you can get up to about 2 kcal worth of SCFA per gram of fermentable carbohydrate. That would come from soluble plant fiber, resistant starch and regular starch that escapes digestion. To get 70% of calories this way on a 2000 kcal/day diet, you'd need to ingest 700g of fibre.
Even if you achieved this, it is unlikely you could absorb it all, and in the process of trying, you would experience gastrointestinal distress, including cramping, diarrhea or constipation, gas, and perhaps worse. Indeed, this would probably happen even at 100g/d, which would provide about 10% of energy in a 2000 kcal/d diet. Moreover, it would interfere with mineral absorption, rendering it an unviable evolutionary strategy. Even the ADA, which extols the virtues of fiber, cautions against exceeding their recommendations of 20-35g. See Position of the American Dietetic Association: health implications of dietary fiber.
Evidence type: review
May 22, 2009
"As the mathematical models now seem quite plausible and the patterns of survivors versus extinct species seem inexplicable by climate change and easily explicable by hunting (7,11), it is worth considering comparisons to other systems. Barnosky et al. note that on islands, humans cause extinctions through multiple synergistic effects, including predation and sitzkrieg, and “only rarely have island megafauna been demonstrated to go extinct because of environmental change without human involvement,” while acknowledging that the extrapolation from islands to continents is often disputed (7). The case for human contribution to extinction is now much better supported by chronology (both radiometric and based on trace fossils like fungal spores), mathematical simulations, paleoclimatology, paleontology, archaeology, and the traits of extinct species when compared with survivors than when Meltzer and Beck rejected it in the 1990s, although the blitzkrieg model which assumes Clovis-first can be thoroughly rejected by confirmation of pre-Clovis sites. Grayson and Meltzer (12) argue that the overkill hypothesis has become irrefutable, but the patterns by which organisms went extinct (7,11), the timing of megafauna population reductions and human arrival when compared with climate change (5), and the assumptions necessary to make paleoecologically informed mathematical models for the extinctions to make accurate predictions all provide opportunities to refute the overkill hypothesis, or at least make it appear unlikely. However, all of these indicate human involvement in megafauna extinctions as not only plausible, but likely."
Evidence type: review
William J. Ripple and Blaire Van Valkenburgh
BioScience (July/August 2010) 60 (7): 516-526.
"Humans are well-documented optimal foragers, and in general, large prey (ungulates) are highly ranked because of the greater return for a given foraging effort. A survey of the association between mammal body size and the current threat of human hunting showed that large-bodied mammals are hunted significantly more than small-bodied species (Lyons et al. 2004). Studies of Amazonian Indians (Alvard 1993) and Holocene Native American populations in California (Broughton 2002, Grayson 2001) show a clear preference for large prey that is not mitigated by declines in their abundance. After studying California archaeological sites spanning the last 3.5 thousand years, Grayson (2001) reported a change in relative abundance of large mammals consistent with optimal foraging theory: The human hunters switched from large mammal prey (highly ranked prey) to small mammal prey (lower-ranked prey) over this time period (figure 7). Grayson (2001) stated that there were no changes in climate that correlate with the nearly unilinear decline in the abundance of large mammals. Looking further back in time, Stiner and colleagues (1999) described a shift from slow-moving, easily caught prey (e.g., tortoises) to more agile, difficult-to-catch prey (e.g., birds) in Mediterranean Pleistocene archaeological sites, presumably as a result of declines in the availability of preferred prey."
Evidence type: review
Man the fat hunter: the demise of Homo erectus and the emergence of a new hominin lineage in the Middle Pleistocene (ca. 400 kyr) Levant.
Ben-Dor M1, Gopher A, Hershkovitz I, Barkai R.
PLoS One. 2011;6(12):e28689. doi: 10.1371/journal.pone.0028689. Epub 2011 Dec 9.
"The disappearance of elephants from the diet of H. erectus in the Levant by the end of the Acheulian had two effects that interacted with each other, further aggravating the potential of H. erectus to contend with the new dietary requirements:
"The absence of elephants, weighing five times the weight of Hippopotami and more than eighty times the weight of Fallow deer (Kob in Table 3), from the diet would have meant that hunters had to hunt a much higher number of smaller animals to obtain the same amount of calories previously gained by having elephants on the menu.
"Additionally, hunters would have had to hunt what large (high fat content) animals that were still available, in order to maintain the obligatory fat percentage (44% in our model) since they would have lost the beneficial fat contribution of the relatively fat (49% fat) elephant. This ‘large animal’ constraint would have further increased the energetic cost of foraging."
"Comparing the average calories per animal at GBY and Qesem Cave might lead to the conclusion that Qesem Cave dwellers had to hunt only twice as many animals than GBY dwellers. This, however, is misleading as obligatory fat consumption complicates the calculation of animals required. With the obligatory faunal fat requirement amounting to 49% of the calories expected to be supplied by the animal, Fallow deer with their caloric fat percentage of 31% (Kob in Table 3) would not have supplied enough fat to be consumed exclusively. Under dietary constraints and to lift their average fat consumption, the Qesem Cave dwellers would have been forced to hunt aurochs and horses whose caloric fat ratio amounts to 49% (the equivalent of buffalo in Table 3). The habitual use of fire at Qesem Cave, aimed at roasting meat , , may have reduced the amount of energy required for the digestion of protein, contributing to further reduction in DEE. The fact that the faunal assemblage at Qesem Cave shows significantly high proportions of burnt and fractured bones, typical of marrow extraction, is highly pertinent to the point. In addition, the over-representation of fallow deer skulls found at the site ,  might imply a tendency to consume the brain of these prey animals at the cave. Combined, these data indicate a continuous fat-oriented use of prey at the site throughout the Acheulo-Yabrudian (400-200 kyr).
"However, the average caloric fat percentage attributed to the animals at Qesem Cave – 40% – is still lower than the predicted obligatory fat requirements of faunal calories for H. sapiens in our model, amounting to 49% (Table 2). This discrepancy may have disappeared should we have considered in our calculations in Table 3 the previously mentioned preference for prime-age animals that is apparent at Qesem Cave , . The analysis of Cordain's Caribou fat data (: Figure 5) shows that as a strategy the selective hunting of prime-age bulls or females, depending on the season, could, theoretically, result in the increase of fat content as the percentage of liveweight by 76% from 6.4% to 11.3%, thus raising the caloric percentage of fat from animal sources at Qesem Cave. Citing ethnographic sources, Brink (:42) writes about the American Indians hunters: “Not only did the hunters know the natural patterns the bison followed; they also learned how to spot fat animals in a herd. An experienced hunter would pick out the pronounced curves of the body and eye the sheen of the coat that indicated a fat animal”. While the choice of hunting a particular elephant would not necessarily be significant regarding the amount of fat obtained, this was clearly not the case with the smaller game. It is apparent that the selection of fat adults would have been a paying strategy that required high cognitive capabilities and learning skills."