The Omnivore (Non) Diet

With this post I'm wading into one of two shitstorms that I usually avoid: diet as it relates to endurance and evolution.  (The other no-fly zone is running foot strike.  I can hear the barefoot runners and the Hoka disciples sharpening their blades already.)  I avoid this topic because everyone has an opinion about it- and you know what they say about opinions.... but with evolutionarily-informed diets being popular lately, people sometimes ask me what I think.  So, here goes.

How can evolution inform diet? What do we know?

As far as I'm concerned, there's only one major point we should all agree on based on biological and evolutionary evidence: we are consummate omnivores.  The huge variation in human diets, particularly traditional diets, is evidence for this.  Arctic peoples thrive on a diet based almost entirely on animal fat.  Traditional Masai live primarily off of the meat, milk and blood from their cows.  In many parts of the world, meat is scarce, or culturally/religiously forbidden, and this vegetarian diet suits people just fine so long as they get all essential nutrients from plants.  (And yes, vegetarian diets seem to offer health benefits, though I'm not convinced that effects of physical activity and other dietary aspects have been properly controlled in these studies.)  Hipsters survive despite veganism, which I should add is a conscious lifestyle choice that would only occur to someone spoiled for dietary choice.  (I kid a bit.  I do respect the ethical basis of veganism, but the health arguments for it are simplistic and unconvincing.) Mike the Fruitarian is an ultra runner who gets 99% of his calories from fruit.  While he is egregiously misguided in his interpretation of its benefits (he was psyched to report that he no longer produces earwax) he has had success eating like a monkey. More on athlete fad diets in a bit. 

To me, the range of diets that we can survive on tells us one simple and interesting fact about ourselves: just as our minds are adapted to enable our survival in variable physical and social environments, our guts and metabolism are adapted to allow dietary flexibility. Remember, our ancestors were spread across several disparate ecological biomes by perhaps 1.8 million years ago. So we needed to survive in a range of habitats, each with different available foodstuffs.  What good is a highly flexible, problem-solving human mind without a metabolism to match? In short: You are an omnivore, biologically and culturally equipped to eat what's available (though of course now everything is available).  

This is not to say that our biology didn't change to accommodate our brand of dietary flexibility. Around the time of late Australopithecus or early Homo, our ancestors' guts were shrinking.  Comparative biology of living primates suggests that larger guts are useful for processing low-quality, high-bulk foods, such as leaves and other tough plant foodstuffs.  Herbivores have to eat and process lots of plants to get enough calories and nutrients, and a long digestive tract helps.  Carnivores tend to have shorter digestive systems because having lots of metabolically-expensive digestive tissue provides little benefit if your diet is high quality and digestible.  We shouldn't assume, however, that our ancestors were ever strictly carnivores, even though early stone tool discoveries and a dose of machismo led paleoanthropologists to argue for a meat-centered hominin diet for the 2nd half of the 20th century.  In fact, the pendulum swung far the other way with some researchers arguing that plant foraging led to our human-ness.  Others argue that reduced gut size means we must have been cooking our food-- both plant and animal-- nearly 2 million years ago, and that this made us human.  I'm only paraphrasing that last part-- this claim has been made.  The point is that paleoanthropologists are just as enticed by simplistic answers as are today's faddists.  

Lactase persistence. From nature.com.

Amylase copy number. From nature.com.

We did evolve some specific adaptations to local diets.  Recent research has identified 3 separate origins for lactose tolerance-- the ability to drink milk into adulthood-- in Africa and Europe.  Presumably, between 3,000 and 10,000 years ago, several populations who were dependent on dairying experienced drought, lending a big advantage to any individuals who happened to have a mutated gene allowing for lactose digestion.  These individuals outcompeted their neighbors and today many of us can thank them every time we eat a pint of Ben and Jerry's without crapping our pants.  Another example: Populations long dependent on grains have more copies of the amylase gene, amylase being a gene that breaks down starch. 

Is there an optimal diet for endurance athletes? What about the fad diets?

We can eat just about anything and survive.  Our guts are adapted for diverse but generally high-quality, nutrient dense food.  But while most diets sustain you, surely one is optimal, right?  And a diet that sustains a couch potato will not suffice for someone living at the outer bounds of human physicality, right?  Enter the endurance diet faddists. The observation that muscle cells love to metabolize glucose led Tim Noakes and others to promote a high carbohydrate diet for athletes, and indeed this approach has been king for decades. It has long been recognized that our bodies store only limited amounts of carbohydrate (as glycogen, in the muscles and liver), and from this we are told that we should therefore ingest carbohydrates during endurance activity to stave off the inevitable glycogen crash.  Now, Paleo diet folks and other low-carb, high-fat enthusiasts (like the Atkins folks of the early 2000's) argue that we should instead interpret our poor glycogen storage capability as evidence that it's a lousy fuel.  Instead, they say, we should train our muscles and other cells to utilize fat as fuel, which they say can be accomplished by shunning carbohydrates. 

I have a nagging soft spot for the Paleo Diet folks. "It's evolution, man!" is a glossy and seductive argument.  I like the basic premise of the Paleo diet: identify what we evolved to eat and then eat that.  There are problems with this.  1)  We don't know what those foods are.  We really don't.  2)  It's overly reductive thinking.  I love reductionism...science doesn't work without it, and I'll argue that to the death.  But diet can't be considered in a vacuum.  The health impacts of an individual food are moderated by lifestyle, other foods you eat, your genetics, etc.  And my biggest gripe of all is a philosophical one.  Matt Fitzgerald, a running coach and nutritionist and advocate for a dietary middle road, pointed out that the same personality traits that attract one to extreme diets-- those that exclude entire food groups-- also predispose one to obsessive, even disordered eating. The Paleo diet, then, is a shortcut to health, or in the case of athletes, better strength and endurance.  Regardless of whether aspects of it work, it's seen as a silver bullet.  If you're a Paleophile and you're wondering if you approach your Paleo diet in this way, I have devised a simple two-question test.  Do you consider yourself an endurance athlete?  Is the bulk of your training done in a gym, lifting up heavy things and putting them down, instead of doing endurance training?  If you answered "yes" to both of these, then chances are you've bought into an entire pre-packaged lifestyle (or lifestyle ). 

At this point you may say, "But this guy I know ran a few Ultras on a low-carb diet and he just did lots of Crossfit, running only 1 day a week".  Yes, you can finish an Ultra this way*.  But the best in the world, by and large, have not adopted this method. And anyway, ultra events are the most conducive to low-carb diets, because fat really can be (maybe should be) the primary fuel for ultra distances.  Fat can be stored almost anywhere under the skin, plus some in your muscles, so you can store a LOT of it.  Its primary energetic role in humans is probably as a buffer against starvation during lean times (which our ancestors certainly encountered).  But fat can't break down without oxygen, so being bereft of glycogen puts any anaerobic effort off-limits.  (Ok, except for <30 second maximal muscle contractions, like weight lifting, which are powered by creatine phosphate.)  

*What's the point in doing only strength training for endurance events? That doesn't make you an endurance athlete. You're a strength athlete who sometimes survives endurance events. You're not living an endurance lifestyle...and isn't that the point?

Most endurance athletes will need to go at least a little anaerobic.  Fom Gaston 2001.

So.....

An honest, no-bullshit, no-speculation, no-fad diet would be the Omnivore Diet, which is to say not really a "diet" in any way that can sell books or products or make somebody with a goatee famous.  If hard pressed to come up with a list of rules that I think aren't super speculative or faddist, I'd probably go with:

1) Eat real food.  See Michael Pollen's short book Food Rules.  Less processed foods, etc.

2) Sugar is probably bad.  Minimize it.  (I'm bad at this.)

3) Complex carbs and fats are both fantastic and necessary fuel sources. 

Happy eating!