The basic reason why evolution has endowed us, and indeed every animal, with the ability to accumulate fat is quite obvious – the accumulation of an energetic buffer to be used at times of shortage.
What is less appreciated however is how important for hunter gatherers’ survival is not to weigh more than is absolutely necessary. Chasing and fleeing situations are obvious reasons to not be overweight but there is also a more subtle aspect – an excessive level of Daily Energetic Expenditure (DEE). DEE is composed of Basic Metabolic Rate (BMR), which is a function of weight and a multiplier, based on the activity level that for contemporary human foragers is estimated at 1.77. There is a formula, called the Klieber formula, that estimates BMR in calories as Weight in kgs ^ 0.75 X 70 so a BMR for a typical 65 kgs (143 lbs) individual would be 1602 calories. 1602 calories multiplied by 1.77 gives us a DEE of 2836 calories. Now let’s assume that the individual has added 10 kgs (22 lbs) to his weight. His DEE comes out at 3158 calories. So presumably every day this individual forager has to find an additional 322 calories.
But we are not done yet. With diminishing marginal returns, a basic law of nature (the easy calories have all been taken already,) that individual has to spend significantly more energy to get those 322 calories than the original calories so we can assume, just for argument’s sake, that the multiplier is now 1.9 instead of 1.77 which means she/he has to obtain almost 3390 calories. So all in all a 10 kgs additional weight may result in the need to obtain an additional 554 calories every day.
It is clear that the chances of survival are reduced with the need to obtain additional calories and with the diminishing ability to chase or flee. There are other considerations like additional time spent hunting/gathering and the probable need to obtain a large part of these additional calories from fat, but we shall leave that for another post.
So the simple “Thrifty Gene” hypothesis is obviously faulty. It is just another example of the misconception of the hunter gatherer’s life as a constant state of hunger resulting in a constant need to store fat. Rather it makes more sense that a fitness advantage must have been conferred to those who had the ability to strike a clever compromise between the security that a large quantity of body fat provides and its fitness diminishing effect.
Does that mean that we have some magic fat percentage number that our genes have evolved to maintain?
I don’t think so. I think evolution is smarter than that. What if the body could predict what is the likelihood that it will have to use its fat reserves in the near future and thus maintain a dynamic optimum amount of fat? Surely such a capability would confer a relative fitness advantage to whoever poses it.
Here is my hypothesis: In order to maintain a dynamic optimum the body uses cues from the environment to gauge the probability that famine or abundance is imminent. Since meat is the preferred source of calories for humans, ingestion of plenty of meat is translated into a message of abundance and ingestion of larger than normal quantities of non-meat food is translated into a message of higher probability of famine.
In other words the body interprets ingestion of sub optimal foods as an indication of a shortage of optimal foods and begins to accumulate fat. Sensing plenty of optimal foods, the body reduces fat accumulation to a minimum in order to allow for optimum DEE and efficient chase or flight.
It also makes sense that the body would benefit from predicting future levels of locomotive needs and utilize cues from the present level of activity (or inactivity) in order to regulate fat accumulation so that, for example, given predicted times of plenty, a higher predicted future activity would translate into less fat accumulation. Conversely high activity levels coupled with a message of famine, namely high starch consumption, would not be as effective in reducing fat reserves. Then the highest fat accumulation happens when we signal to the body that supply of proper foods is becoming scarce while we won’t be needing to move much in the near future.
I am not going to repeat the arguments here but I’m convinced (and have written in the past) that the consumption of large quantities of starch was not the norm during human evolution. It is quite expected that, unless you live on an island like Kitava, where quick genetic adaptation can take place, large quantities of starch would be interpreted by the body as a sign of an impending food shortage and large quantities of meat as times of forthcoming plenty. Thus plenty of starch, little meat and fat and low levels of activity all point the body in one direction – accumulating fat. Plenty of meat and fat, little starch and a good measure of physical activity send the body a clear message – it is time to reduce fat reserves.
The same model can be applied to consumption of other non-evolutionary foods like dwarf wheat, omega 6 oils etc. Also, some of us have adapted to consume a higher quantity of starch and may not react negatively to increased quantities of “Safe Starch”. It is difficult to find out how well adapted you are to higher starch diets so it may pay off to err on the side of caution.
I believe most readers of this blog have experienced these exact results themselves so are not as adapted to a high starch diet. Those who haven’t tried communicating the message of plentitude to their bodies please look up Volek/ Feinman / Lindberg / Westman / Phinney at Pubmed or read the countless number of anecdotes in the Paleo/Low carb blogs.
In summary, fat accumulation must be cleverly regulated by the body. Fat accumulation does not happen as a result of an “over abundance” of food, as the “Thrifty Gene” hypothesis suggests, but as a result of an over abundance of the wrong type of foods, seemingly the very same foods that the USDA recommends.