“Eat fat”, that is what we have evolved to do over millions of years. Some interesting excerpts taken from
Fat Detection: Taste, Texture, and Post Ingestive Effects
Frontiers in Neuroscience
Editors: Jean-Pierre Montmayeur, PhD and Johannes le Coutre, PhD.
Increasingly, biomedical researchers are coming to recognize the importance of an evolutionary perspective for understanding the origin and nature of modern human health problems. This is particularly true when examining “nutritional/metabolic” disorders such as obesity and cardiovascular disease. Research in human evolutionary biology over the last 20 years has shown that many of the key features that distinguish humans from other primates (e.g., our bipedal form of locomotion and large brain sizes) have important implications for our distinctive nutritional needs (Aiello and Wheeler, 1995; Leonard and Robertson, 1997; Leonard, 2002). The most important of these features is our high levels of encephalization (large brain:body mass). The energy demands (kcal/g/min) of brain and other neural tissues are extremely high—approximately 16 times that of skeletal muscle (Kety, 1957; Holliday, 1986). Consequently, the evolution of large brain size in the human lineage came at a very high metabolic cost.
In addition to improvements in dietary quality and greater fat intakes, the increased metabolic cost of larger brain size in human evolution also appears to have been supported by developmental changes in body composition. During the human life course, the metabolic demands of our large brains are most dramatic in infancy and early childhood, when brain:body weight ratios are largest and when brain growth is most rapid. Whereas brain metabolism accounts for 20%–25% of resting needs in adults, in an infant of under 10 kg, it uses upwards of 60% (Holliday, 1986)! .
To accommodate the extraordinary energy demands of the developing infant brain, human infants are born with an ample supply of body fat (Kuzawa, 1998; Leonard et al., 2003). At ~15%–16% body fat, human infants have the highest body fat levels of any mammalian species (cf., Dewey et al., 1993; Kuzawa, 1998). Further, human infants continue to gain body fat during their early postnatal life. During the first year, healthy infants typically increase in fatness from about 16% to about 25% (see Table 1.4). Thus, the very high levels of adiposity seen in early human growth and development coincide with the periods of greatest metabolic demand of the brain.
Compared to other primates and mammals of our size, humans allocate a much larger share of their daily energy budget to “feed their brains.” The disproportionately large allocation of our energy budget to brain metabolism has important implications for our dietary needs. To accommodate the high energy demands of our large brains, humans consume diets that are of much higher quality (i.e., more dense in energy and fat) than those of our primate kin (Leonard and Robertson, 1992, 1994). On average, we consume higher levels of dietary fat than other primates (Popovich et al., 1997), and much higher levels of key long-chain polyunsaturated fatty acids (LC-PUFAs) that are critical to brain development (Crawford et al., 1999; Cordain et al., 2001). Moreover, humans also appear to be distinctive in their developmental changes in body composition. We have higher levels of body fatness than other primate species, and these differences are particularly evident early in life.
The need for an energy-rich diet also appears to have shaped our ability to detect and metabolize high-fat foods. Humans show strong preferences for lipid-rich foods. Recent work in neuroscience has shown that these preferences are based on the smell, texture, and taste of fatty foods (Sclafani, 2001; Gaillard et al., 2008; Le Coutre and Schmitt, 2008), and that our brains have the ability to assess the energy content of foods with remarkable speed and accuracy (Toepel et al., 2009). Additionally, compared to large-bodied apes, humans have an enhanced capacity to digest and metabolize higher fat diets. Our gastrointestinal (GI) tract, with its expanded small intestine and reduced colon, is quite different from those of chimpanzees and gorillas and is consistent with the consumption of a high-quality diet with large amounts of animal food (Milton, 1987, ). Finch and Stanford (2004) have recently shown that the evolution of key “meat-adaptive” genes in hominid evolution were critical to promoting enhanced lipid metabolism necessary for subsisting on diets with greater levels of animal material.
Comparative analyses of primate dietary patterns indicate that the high costs of large human brains are supported, in part, by diets that are rich in energy and fat. Relative to other large-bodied apes, modern humans derive a much larger share of their dietary energy from fat. Among living primates, the relative proportion of metabolic energy allocated to the brain is positively correlated with dietary quality. Humans fall at the positive end of this relationship, having both a very high-quality diet and a large brain.
In many respects, the human gut is more similar to that of a carnivore and reflects an adaptation to an easily digestible diet that is higher in energy and fat.
In addition, recent work in human evolutionary genetics suggests that the selection for key “meat-adaptive” genes were critical for allowing our hominid ancestors to more effectively exploit diets with higher levels of animal fat. Finch and Stanford (2004) argued that the evolution of the unique E3 allele in Homo at the apolipoprotein E (apoE) locus was important for allowing our ancestors to exploit diets with greater animal material. ApoE plays a critical role in regulating the uptake of cholesterol and lipids throughout the body (Davignon et al., 1988). The E3 allele is evident in humans, but not in chimpanzees and gorilla, and is associated with reduced metabolic and cardiovascular risks with the consumption of higher fat diets (Finch and Stanford, 2004).
When we look at the human fossil record, we find that the first major burst of evolutionary change in hominid brain size occurred at about 2.0–1.7 million years ago (mya), associated with the emergence and evolution of early members of the genus Homo (see Table 1.2). Prior to this, our earlier hominid ancestors, the australopithecines, showed only modest brain size evolution from an average of 400–510cm3 over a span of 2 million years from 4 to 2 mya. With the evolution of the genus Homo there is rapid change, with brain sizes of, on average, ~600 cm3 in Homo habilis (at 2.4–1.6 mya) and 800–900 cm3 in early members of Homo erectus (at 1.8–1.5 mya). Furthermore, while the relative brain size of H. erectus has not yet reached the size of modern humans, it is outside of the range seen among other living primate species.
The evolution of H. erectus in Africa is widely viewed as a “major adaptive shift” in human evolution (Wolpoff, 1999; Antón et al., 2002; Antón, 2003). Indeed, what is remarkable about the emergence of H. erectus in East Africa at 1.8 mya is that we find (a) marked increases in both brain and body size, (b) the evolution of human-like body proportions, and (c) major reductions of posterior tooth size and craniofacial robusticity (McHenry, 1992, 1994a,b; Ruff et al., 1997; McHenry and Coffing, 2000). These trends clearly suggest major energetic and dietary shifts: (a) the large body sizes necessitating greater daily energy needs; (b) bigger brains suggesting the need for a higher quality diet; and (c) the craniofacial changes suggesting that they were consuming a different mix of foods than their australopithecine ancestors.
The ultimate driving factors responsible for the rapid evolution of brain size, body size, and craniodental anatomy at this stage of human evolution appear to have been major environmental changes that promoted shifts in diet and foraging behavior. The environment in East Africa at the Plio-Pleistocene boundary (2.0–1.8 mya) was becoming much drier, resulting in declines in forested areas and an expansion of open woodlands and grasslands (Vrba, 1995; Reed, 1997; Bobe and Behrensmeyer, 2002; deMenocal, 2004; Wynn, 2004). Such changes in the African landscape likely made animal foods an increasingly attractive resource for our hominid ancestors (Harris and Capaldo, 1993; Behrensmeyer et al., 1997; Plummer, 2004).
Associated with the evolution of our high-quality diet, humans developed distinct molecular pathways for detecting and metabolizing high-fat foods. We show preferences for foods that are rich in fat and energy. Key genetic mutations during later hominid evolution were critical to promoting the enhanced lipid metabolism necessary for subsisting on diets with greater levels of animal material. Moreover, accumulating evidence highlights the remarkable capacity of the human brain and sensory system for accurately assessing the energy content of potential food items. In sum, the ability to effectively detect, metabolize, and store fats likely provided tremendous selective advantages to our hominid ancestors, allowing them to expand into diverse ecosystems around the world. Further research is needed to better understand the nature of the dietary changes that took place with the emergence of early human ancestors and how they are associated with distinctive aspects of our own nutritional biology.