What Can Fruit Flies Tell Us About a Human Diet?

A whole lot, apparently.  At least, if the experiments of Michael Rose are anything to go by.  I recently found out about Michael Rose through Michael Eades’ newsletter The Arrow (highly recommended) – and I found it truly eye-opening.  Be warned, this blog post is a bit of an excuse to nerd out on my part, so feel free to skip it if that sort of thing doesn’t interest you – but there is a practical takeaway; the older you get, the less well-adapted you are to an agricultural diet, and the more you stand to benefit from switching to a primal, hunter-gatherer diet.  Why, you ask?  Stay tuned to find out!

The Dilemma: How Well-Adapted are We to Agriculture?

The gist of a primal approach to health (and the reasoning behind many current diets, including primal, paleo, keto, carnivore, and Paul Saladino-style “animal-based” diets), is that we should eat the “original” human diet, the diet that humans were eating way back in prehistoric times before we invented modern food processing techniques or even agriculture.

This premise has a lot of theoretical plausibility.  After all, we feed animals in captivity basically what they eat in the wild.  This is because biological adaptation takes time; your dog may enjoy eating human junk food, but it still has essentially the same digestive system as a wild wolf or jackal.  Likewise, you may enjoy toaster pastries and chicken nuggets, but biologically you are still basically a hunter-gatherer.  

Or are you?  Critics of a paleo diet point out that, even by conservative estimates, agriculture has been around for thousands of years.  This may be the blink of an eye in geological or biological terms, but it is certainly long enough for biological adaptation to set in.  While people may think of animals “evolving” very slowly and gradually, we now know that organisms can adapt to new environments in a matter of just a few generations.   

Furthermore, we know of plenty of societies both historically and currently who eat a traditional agricultural diet and are not seemingly plagued by the chronic diseases of modern civilization.  When the dentist Weston A. Price made his famous observations of traditional societies in the early twentieth-century, and demonstrated that they were largely free of chronic disease, many of these societies (but not all of them) were eating an agricultural diet.  So, have we adapted to agriculture?  

Hamilton’s Forces of Natural Selection

Michael Rose is a longevity researcher, and whatever one thinks of his ideas about longevity and life-extension in humans, his research on fruit flies has some truly fascinating implications for human health and diet.

Rose’s research is based on a principle called “Hamilton’s forces of natural selection”.  The principle can be mathematically formulated, but the gist of it is that natural selection acts more effectively on organisms when they are at or below reproductive age, and gets increasingly less effective the older an organism gets.  Thus, adaptation shows rapidly in younger animals, but more slowly in older animals.

Hamilton’s laws are well-established, but it also just makes common sense when you think about it.  To take an extreme example, a mutation or trait which is so detrimental that it inevitably kills an organism before it is even old enough to reproduce will be eliminated in a single generation.  A mutation or trait which allows an organism to reach reproductive age but then die very shortly afterward might get passed down, but will probably get eliminated from a population pretty quickly, as the individuals with this trait will be at a huge reproductive disadvantage compared to individuals who live at reproductive age for some time.  But a mutation or trait which will kill you after you’re already too old to reproduce anyway?  You’ve already got ample opportunity to pass on your genes, so the survival benefits of fixing such a trait are pretty minimal.  

If I can anthropomorphize natural selection a bit, natural selection has a very strong incentive to fix a problem that shows up when you are too young to reproduce, but the more time goes on and the more time you have had to reproduce, the less important it is to fix problems.  Thus, because the effect of natural selection gets weaker the older you get, it takes longer for adaptations to new conditions to manifest in old age than in youth.  (Note that natural selection still has some effect even after one is not old enough to reproduce.  There are several reasons for this, including the so-called “grandmother effect”; because humans in particular depend so much on acquired knowledge for survival, surviving well past reproductive age allows you to impart your wisdom to your children and grandchildren, thus increasing the odds that they will live long enough to have children of their own.  My oversimplified description of Hamilton’s forces might give the impression that natural selection stops working entirely after reproductive age, but it doesn’t.)  

How does this relate to Rose’s fruit fly experiments?  Because fruit flies have such a short lifespan, (they live for mere days), they adapt very quickly compared to long-lived animals such as humans.  Over the course of several decades, Rose has conducted experiments in fruit flies which would correspond to observing 30,000 years of human evolution – longer than we have had to adapt to agriculture.  Thus, one elegant way to try to figure out if humans have had enough time to adapt to agriculture is to make an analogous dietary change in fruit flies, then watch an equivalent number of fruit fly generations and see what happens. 

Michael Rose’s Fruit Fly Experiments

Michael Rose (and his student Michael Rutledge) took a population of fruit flies who had evolved on an ancestral diet of apples.  Then, he put them on a diet of bananas for multiple years – the equivalent of 30,000 years of human evolution.  

After the flies had been on the banana diet and presumably had time to adapt, he then split the flies into three groups: one he kept on the banana diet they had all been eating for years, another he put on an apple diet meant to replicate their original ancestral diet, and then he put a third group on a completely novel diet of oranges.  Then, he looked at the mortality rates in these three groups of flies.

In all age groups, the flies eating the oranges had the highest rate of mortality. 

In young and middle-aged flies, the apple and banana groups did about the same.  They apparently had had enough time on the bananas to adapt to them, but even the young and middle-aged apple group did about as well as the banana group, indicating that they had not significantly lost their ability to thrive on apples.

In old age, however, the apple group was by far the healthiest.  The mortality rate of the banana group was pretty high in old age, closer to the orange group.  Thus, despite having hundreds of generations to adapt to the new diet, the flies were still not fully adapted to the bananas – they had fully adapted up to reproductive age, but old age hadn’t caught up yet.

The experiment elegantly demonstrates Hamilton’s forces of natural selection at work.  It also shows, at least in fruit flies, that dietary adaptation follows the pattern predicted by Hamilton’s forces; adaptation happens rapidly in youth, but takes much longer in old age.

Human Application

Of course, you can’t just assume that the results of a fly experiment carry over in the case of humans.  Having said that, it seems very reasonable to assume that these results are at least similar to what we would expect in a human population, since Hamilton’s forces are extremely general and apply to pretty much all organisms that are subject to natural selection (which is pretty much all organisms period, if you think about it).  

The three fruit fly diets very closely correspond to what we might call an ancestral human diet, a traditional agricultural human diet, and a modern diet.  An ancestral human diet would be a meat-heavy paleo or primal diet also featuring some fruit, vegetables, nuts, seeds, and tubers: this is the diet that modern humans evolved on and that hominins have been eating some variation of for at least two million years.  This closely corresponds to the apple diet for the fruit flies.

A traditional agricultural diet has many different forms, but it encompasses what humans ate starting around the agricultural revolution maybe ten thousand years ago up until the mid to late nineteenth century when the ability to genetically modify crops more efficiently and to mass-produce food in factories led to the modern diet.  Many of the healthy societies observed by Weston A. Price in the 1930s were following such a diet.  While such traditional societies included grains, legumes, and dairy, unlike ancestral diets, they differed from modern diets in that they contained no modern hyper-processed foods and that the grains, legumes, and dairy products were prepared using traditional methods, including soaking, sprouting, and fermentation, which increased their nutrient availability and made the food easier to digest.  This roughly corresponds to the banana diet for the fruit flies.

Finally, the orange diet corresponds to a modern human diet.  The human diet has been changing with increasing speed over the last 150 years.  Notable changes include the introduction of GMO and other genetically modified plant foods, significantly increased use of pesticides, antibiotics, preservatives, and artificial colors and flavors, the introduction of significant quantities of processed white flour and processed sugar and corn syrup, and the introduction of industrial seed oil.  Like the flies on the orange diet, humans have had basically no time to adapt to this diet.  

If the fruit fly experiment is anything to go on, then we would expect the ancestral diet to be healthy at all stages of life, the traditional agricultural diet to be healthy in youth and up to middle age, and the modern diet to be unhealthy for everyone.  

Why It Makes Sense

Michael Rose’s experiments have the implication that the paleo folks and the pro-agriculture folks might both be partially right.  The pro-agriculture folks are right that 10,000 years is enough time for evolution to accomplish quite a bit, and the paleo folks are right that humans have not yet fully-adapted to an agricultural diet.  

The reason that Michael Rose’s idea struck me as so plausible is because it made sense of my own anecdotal experience.  It is pretty common knowledge that we can “get away” with more dietarily when we are younger.  We all know people who could pound carbs all day and stay thin as a rail – until they couldn’t.  Some might just write that off as more evidence that getting older stinks.  I find that a rather unsatisfying explanation.  If adaptation to an agricultural diet gets weaker as we get older, and an agricultural diet is almost by default a more plant-based and carbohydrate-based diet than an ancestral diet (which it usually is), then this is exactly what we would expect. 

 A quick perusal of any social media platform will bear this out.  You will see many lean, muscular individuals going on about how they eat 700 grams of carbs a day and have only 8% body fat, so paleo and keto are bogus, blah blah blah. Leave aside for a moment the fact that what a gym bro can get away with may not be what the average person can get away with, and leave aside for a moment the fact that many of these guys are in fact metabolically unwell despite looking lean and strong.  Rather, notice that most of these guys are young.  The older the population, the less people you will find pounding carbs and getting away with it.  This is my observation, not a scientifically rigorous study, but check it out.  I think it is highly telling.  

Practical Takeaways

This has all been rather theoretical.  What practical lessons can we learn from this, you ask?

First, the banana flies did way better (for a while) than the orange flies.  Applying this to humans, an agricultural diet is not the same as a modern diet.  An agricultural diet can be a pretty healthy diet, as Weston A. Price showed (although you might need to tighten it up as you get older, according to the fruit fly data and according to my personal experience).  This doesn’t mean that most of the crap you can find on the shelves in a standard supermarket is healthy.  The introduction of refined flour and sugar, seed oils, and other chemicals and processing techniques led to a food revolution over the past 150 years or so perhaps as radical as the agricultural revolution 10,000 years ago.  Like the fruit flies eating the oranges, we have had no time to adapt to that.

Furthermore, a traditional agricultural diet is defined not only by what plant foods are on the menu, but also how they are prepared.  Eating large quantities of raw plant matter is a pretty new phenomenon.  As you will discover by checking out the work of Weston A. Price, or other specialists in traditional diets such as Bill Schindler, traditional societies knew that plant foods contained less bioavailable nutrition and more toxins than most animal foods, and so they processed them accordingly.  Vegetables were thoroughly cooked or fermented, and grains, nuts, and legumes were soaked, sprouted, or fermented, and then thoroughly cooked.  If you want to eat these foods, do some research on how to prepare them properly. 

Unfair as it may seem, age really does matter.  You can get away with more when you’re younger.  Having said that, this doesn’t give you carte blanche to eat crap when you are young.  You can trash your metabolism, and even if the effects of years of metabolic abuse, such as obesity, can largely be reversed, you may never go back to baseline.  For example, if you are healthy from a young age and never become insulin resistant, you may never develop carbohydrate intolerance and may be able to enjoy moderate amounts of high-carb foods, such as fruit, sweet potato, maybe even some quinoa and wild rice, for the rest of your life.  If you make yourself insulin resistant, sick, and overweight, you may need to permanently avoid these foods, even if you can radically improve your metabolic health.  

Individual tolerance also matters quite a bit.  Some groups of humans have been eating agricultural foods for thousands of years;  others literally made the switch within living memory.  Generally speaking, the longer ago your ancestors began farming, the more tolerant you are likely to be of carbohydrates, grains, and dairy, and the later in life the switch to paleo will become meaningful; this puts those of European and Asian ancestry in the most agriculture-tolerant class, while, at the other end, Australian Aborigines have literally no agricultural adaption whatsoever.  Most other people fall somewhere in the middle.  Having said that, since human adaption to agriculture is still a work in progress, this will vary quite a bit from person to person, so the best way to figure out what diet is best for you is to experiment.

Finally, Paleo is still a “safe” “baseline” diet for most people.   We have partially become able to digest and use grains; we still don’t need them.  There are no nutrients in grains, legumes, and dairy which we cannot get from other sources, and animal products remain the most nutrient-dense and easily digested food sources for humans.  If you want to pivot towards a more ancestral diet early in life while you still seem to be able to “get away” with eating more agricultural foods, there is no danger in that.

Does this paradigm make sense to you?  Have any of you seen that what you feel good eating has changed as you’ve gotten older?  Let me know in the comments!