People who know about human biodiversity often argue that low-carb diets are too broad-brush of an approach because different races adopted agriculture at different times, so while the late adopters might get really dinged by the novelty of grains and sugars, surely those who adopted it first have had enough time to evolve a decent (if not total) genetic resistance to poor diets. They point out that recent work shows how fast natural selection can work on humans -- just look at how quickly Europeans got lighter in their skin, hair, and eye color, or how quickly the dairying populations evolved lactase persistence. That's all within the past 10,000 years!
Unfortunately that logic only applies to easy changes where only a single mutation gets the job done. For skin, hair, and eye color, there are under 10 genes that make most of the difference, and really just a handful of those do most of the work. Lactase persistence is due to a single mutation. Why are these changes so easy to make? Because the basic machinery was already there, the result of millions of years of painstaking, gradual tinkering by natural selection. All these new traits represent are the dialing up or down of some knob -- break a gene that colors your skin, and you're pale; break a gene that shuts off lactase production, and you have lifetime lactase activity.
What they do not do is invent things from scratch or fundamentally re-draw the blueprint of any of the body's systems. That takes too long for 10,000 years to yield anything useful. Henry Harpending and Greg Cochran emphasize this point in their excellent and very readable book on the topic of recent human evolution, The 10,000 Year Explosion (check the Amazon box above). A mutation or two might break the process by which neuronal growth slows down, and that might make the person smarter. But inventing neurons in an organism that had no neurons is not going to happen in a short time and not by just a handful of mutations. The body's systems are too complex for random mutation to hit on them in a single shot.
In particular, the human digestive system is not going to change much at all in 10,000 years. Agricultural populations have relied far more on plant foods than hunter-gatherers ever did, and yet no human makes cellulase, the enzyme that lets herbivores digest plants. Wouldn't it be useful for us to have it too, rather than pass those plant foods through as undigested fiber? Sure it would -- it just takes a long time to invent it if you don't have it at all. More relevant to agriculture, eating grains like a bird or a rodent won't give you their digestive system unless this goes on for millions of years.
Just to check that this is true, let's have a look at the prevalence of obesity worldwide and focus on the Middle East where agriculture began. In the US, where most of the population came from later adopters, we have roughly 1/3 overweight and another 1/3 obese. To compare, check whoever you want, but the picture is clear -- the earliest adopters of agriculture are fat as hell right now. Here is an NYT article on how obese Qataris have become, which has the usual Western baloney about obesity being caused by too much food and not enough exercise. (Another case of harmful foreign aid -- shaming people in poor countries into becoming vegan joggers.)
But we know from fat marathoners and bike riders that even enormous amounts of exercise won't melt fat, and we know from hunter-gatherers like the Ache -- who eat between 3000 and 4000 calories a day and are still lean and muscular -- that quantity of food has little to do with it. What are Qataris eating so much of that makes them fat? The article says fast food, as well as home meals of rice, clarified butter, and lamb. Well, butter and meat are just about all the Maasai live on (plus cow's blood) and they're lean and muscular, even though they don't live life on a stairmaster. It must be the heapings of rice.
And what exactly does "fast food" include? Judging by the picture of boys chowing down on McDonalds, we see that there's almost no animal products at all -- I think I can make out a 2-oz slice of beef and perhaps a slice of cheese (which I doubt came from an animal anyway -- probably another soy-based abomination). Instead, they're sucking down five pounds of straight up sugar (Pepsi, ketchup, and barbecue and honey sauces), starch (a mountain of French fries), grains (white bread buns), and fibrous carbs (tomatoes, onions, and other toppings). All of that glucose is going to send their insulin sky-high, and insulin is the hormone that signals fat to stay locked away in fat cells.
You might not see this pattern where people don't get much to eat at all -- there would be little fat to lock away in the first place. High-carb diets don't conjure fat out of nothing, but if you are getting enough to eat, a high-carb diet will keep the fat you're eating locked in fat cells rather than dumped into your bloodstream to be burned for fuel. Places like India where people get little to eat aren't teeming with obese people, but they sure are hit by diabetes. This is another member of the full range of symptoms of Metabolic Syndrome (along with high blood pressure, hypertension, etc.), and you don't need to be eating a lot to get diabetes. Here is an article showing that over 4% of Indians have diabetes and that this will only get worse over time. And those Middle Eastern countries with high obesity levels? They have some of the highest diabetes rates in the world, around 15-20%.
No matter where your ancestors came from, their digestive system and metabolism was not designed at all for relying on nuts, grains, green plants, fruits, tubers, etc. If adapting to agricultural diets were only a matter of dialing up the level of some enzyme that we already had, they could have adapted quickly. But because we have to invent an agricultural digestive system from one designed for consumption of animal products, it would take a million or so years. The other prime examples of rapid selection are resistance to infectious diseases, but again these solutions just jigger with something that's already been invented, like taking a red blood cell and bending it so that it's somewhat sickle-shaped. You're not going to evolve red blood cells where there were none before in only 10,000 years.
There is individual variation relating to diet, since obesity is heritable -- some of the differences between people in body fat composition are related to genetic differences between them, even if they all eat the high-carb diets typical of the modern world. So some people get harmed more than others by bad food. Still, the real difference between individuals or groups when we look at the many facets of Metabolic Syndrome is the split between hunter-gatherers -- who have none of those many diseases -- and grain-munching farmers who are plagued by diabetes, obesity, and heart disease (not to mention depression and gout). You may have to fine-tune a low-carb diet because of your unique set of genes, but to a first approximation the reason you're fat, diabetic, or not thriving is because you're eating an agricultural diet.
Let the nutritionist classes tremble at a Paleolithic revolution. The dieters have nothing to lose but their grains. They have a full stomach to win.
Meat-eating Men of All Countries, Unite!