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Do spider monkeys hold the key to why we get fat?

IN EARLY 2004, a team of researchers spent nearly five months in a lowland subtropical forest in Bolivia following spider monkeys – an arboreal primate that primarily dines on ripe fruit and leaves. From dawn to dusk, the researchers recorded every detail of the dietary habits of these wild spider monkeys. They monitored the length of each feeding event, categories of ripe vs. unripe fruit and young vs. old leaves, and so on. They took detailed notes on which parts of the fruits were preferred and collecting samples of everything. Back in the lab, the protein, lipid (fat), starch, fiber and other micronutrient content of the 69 different plants eaten by our little primate friends were calculated. What the researchers discovered rocked the primate nutrition world and should be creating nutrition guidance ripples felt at the doorstep of every government or organization that issues dietary guidelines for its citizens.

In nutrition ecology – in which the rules apply to humans – researchers have put forth a number of models that govern why a primate would pursue a certain diet. It could range from 1) energy maximization (eat all you can when you can); 2) nitrogen (protein) maximization; 3) avoidance of certain toxic plants; 4) to limit fiber intake (too much bulk equals not enough macro- and micronutrients) and 5) nutrient balancing. Of these, researchers who study humans and nonhuman primates tend to focus on the models of maximizing daily energy intake while trying to maintain a balance at the same time. This two-pronged strategy is the basis of optimal foraging theory, which states that we must replace what calories we expend when acquiring new ones. Unless energy intake is counterbalanced with energy expenditure, then ones booty gets bigger.

Until 2004, primate researchers assumed fruit eaters were energy maximizers – that is, they would aim to maximize their daily energy intake. However, the detailed dietary studies in the Bolivian rainforest found that across all age groups and sexes, spider monkeys aim for a target amount of protein, regardless of how few or many calories from carbohydrates and fats they consumed in the process. In other words, the daily protein intake remained remarkably stable throughout the study period, but the overall calories from carbohydrates and fats fluctuated significantly. This meant the nutritional strategy was a daily protein target not a balancing of macronutrients and a maximizing of caloric intake. The potential implications of this study for understanding the modern obesity epidemic among humans are profound.

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Amazingly, this protein leverage hypothesis – as it has been dubbed – is not confined to spider monkeys. It has been demonstrated in numerous other species including pigs, rodents, birds, fish and even insects. But what about humans? That answer came in a study published in 2011. Researchers from Australia and New Zealand measured the protein, overall energy intake and hunger levels in a 4-day study among 22 lean subjects. In short, the researchers found that by diluting the dietary protein across meals with carbohydrates and fats promoted overconsumption. In other words, the test subjects would keep eating until they reached a target protein intake.

This would all be very academic if it weren’t for the fact that over consumption of energy-dense foods ‘may be’ in large (or some) part to blame for sky rocketing obesity in just about every corner of the planet (in addition to rising systemic inflammation, quality and types of carbs, and a lot of other variables). It’s also interesting to note that according to the most recent The National Health and Nutrition Examination Survey conducted in the U.S., a drop in the percent of dietary protein across 1971 to 2006 was associated with an increase in total energy. Also note that stacks and stacks of peer-reviewed research has demonstrated that diets high(er) in protein are known to be more satiating an lead to longer, and more sustained weight loss.

The findings among the spider monkeys has the potential for “understanding the evolutionary and ecological origins of human susceptibility to obesity.” Since we share a common ancestor with spider monkeys, humans are likely predisposed to a protein target and in the case of our modern food supply of highly processed carbs and novel fats (e.g., concentrated vegetable oils), overconsumption to achieve protein levels is highly plausible evolutionary mechanism contributing to obesity.

Since protein is the primary source of dietary nitrogen, which is needed for growth, why its regulated and targeted levels are required is seemingly straightforward. During our more preindustrial life, over consuming low-GI carbs and well-balanced fats from high protein but lean meats during the regulatory search for protein was not a problem. In other words, the “costs of eating either excesses or deficits of carbohydrates and fats on a given day to ensure ingesting the target amount of protein were small in relation to the costs of failing to meet the protein intake target.” Furthermore, unlike unutilized fats and carbs, which can be stored and drawn upon when needed during periods of negative energy balance, no such buffer exists for protein. This suggests that the evolutionary drive to keep eating until a target level of protein is met somewhat out of perceived control. (Note anyone persons daily or weekly protein target will vary across gender, age, physical activity levels, muscle mass, pregnancy etc.)

Average protein consumption in the U.S. hovers around 15% of total energy consumed. The National Institute of Medicine – whose reports heavily influence the USDA’s dietary guidelines for Americans – suggests a range of 10-35% for protein as a percentage of energy intake. At 15%, the average American is on the low end. Analysis of modern-day and historical hunter gatherer groups around the world suggest a protein intake on the upper limits of that suggested by the lab coats over at the National Institute of Medicine.

Endless studies of supermarket prices and the correlation with the energy density of foods consistently demonstrate that the best bang for your buck comes from highly processed carbohydrates and sugary foods and fats but not in protein (on average). Therefore, food prices – especially for those living on a tight budget – can create a bias towards less and less protein (that’s unless you are down with eating beans every night).

It’s tantalizing to consider how the relative costs of macronutrients (protein, fat, carbs) and our embedded protein regulation interact with the human desire for salts, sugars, and our responses to display packages, busy lifestyles, built environments and so forth. A new field of study? I call first shot on dubbing it Future Primitive. A degree in Future Primitive has got to be more interesting and more useful than pumping out yet another student with a default degree in psychology!

It is clear that environmental and economic change in our recent past has opened the door to obesity. However, we cannot divorce our strategies for dealing with obesity and associated disease from our biology. By combining biological and economic factors in our understanding of consumption patterns we may improve health outcomes. However, current dietary guidelines and the backgrounds of the scientists and health officials who put them together seldom involve expertise in the biological and evolutionary realities of the population that seek to inform. And this says nothing of the influence of the special interest groups that seek to nudge those guidelines in favorable directions. It’s also sobering to consider that the very agency in the U.S. tasked with cobbling together dietary guidelines for Americans – the United States Department of Agriculture – is actually and agency whose purpose is to, well, promote agriculture. Seems like a bit of a misalignment.

In our more distant past, access to protein and non protein sources of energy was dictated by environmental constraints (deserts versus near sea for example) and access to technologies (think nets, bows, spears) to acquire and process those foods. In other words, the playing field was leveled and humans lived in symbiosis with the landscape and its resources. Everyone had more or less equal access. Fast forward to today, and the costs of protein and non protein energy is heavily influenced by ecological differences in the cost of producing those foods, issues of shelf life, costs of refrigeration and transport and so on. Your economic status then dictates your access to this new nutritional landscape. Unless we figure out ways to produce protein from animal sources in a more economical way – that also reduces the devastating environment affects of animal production in the process – and promote the consumption of plant-based protein sources, we are unlikely to curb the obesity epidemic in a meaningful way. This all assumes, of course, that the protein leverage theory plays any role in all of this. Perhaps it doesn’t. But interesting to think about none the less.

It may be time governments to shift farm policy and handouts to those farmers delivering protein sources. If the costs of protein where subsidized – and the environment impact could be dealt with – then we might have a strategy. In the meantime, give a wink to the spider monkeys next time you are at the zoo.


6 Comments

  1. “the devastating environment affects of animal production ” due to grain-based feeds.

    Pastured animals may be raised on land unsuitable for crops, and responsible ranchers improve the land – replacing lost topsoil.

  2. Any idea on how much protein per kilogram we should be aiming for then?

    • not sure – the robb wolf’s and mark sisson’s of the world might have a better idea.

    • 1–2g per kg depending on activity level. 1 (or 0.5-0.8 if you listen to the WHO) is a basic maintenance intake.

  3. Very interesting and well written. Thanks.

  4. I think if one is in ketosis 1 to 1.5 (upper limit of 2) grams of protein per kilogram of bodyweight per day is recommended. Any more than that and then gluconeogenesis converts amino acids to glucose and knocks one out of ketosis. This suggests to me that protein needs are in this range (depending on age, gender, activity, muscle mass, etc). I don’t know if this metric holds true for someone running on a more “normal” mix of fats and carbs, but I suspect 1-1.5 gm/kg/day is a good ballpark to meet daily requirements without going overboard.

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