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First of all, I'd like to point out that very low carb (VLC) and ketogenic diets can be effective therapeutic tools for treating many neurological disorders. I touched on this briefly a while back in my podcast with Emily Deans, and initial studies on low-carb diets and mental health have shown promise. (1, 2, 3, 4)
Because Dr. Perlmutter is a neurologist, it makes sense that he would be a proponent of low-carb diets for his patients based on these therapeutic effects.
However, recommending a low-carb diet as an intervention for sick people is very different from promoting it as a preventative measure for the entire population, which is what Dr. Perlmutter does in Grain Brain. His approach would be somewhat akin to recommending that everyone go on the Autoimmune Protocol to prevent autoimmune disease, which would be unnecessarily restrictive and unhelpful.
Comment: While Chris Kresser is a well-respected researcher in the Paleo world, he is showing his bias here. There is a great deal of evidence that a ketogenic diet is beneficial for many conditions, not just neurological and that yes, it is indeed beneficial in a preventative manner. For a comprehensive review of the literature, check out "The Ketogenic Diet: An Overview".
It's important to realize that just because a low-carb diet can help treat neurological disorders, doesn't mean the carbs caused the disorder in the first place. While I don't argue with the idea that refined and processed carbs like flour and sugar contribute to modern disease, there's no evidence to suggest that unrefined, whole-food carbohydrates do. In fact, there are three compelling reasons why this is not the case.
#1 - We evolved eating whole-food carbohydrates
The first reason it doesn't make sense that carbohydrates cause neurological disorders is that we've been eating carbs for a very long time, and we're well adapted to digesting and metabolizing them. For instance, fruit has been part of the human diet for longer than we've been recognizably human, and while starch hasn't been part of the human diet for quite as long, it's clear that we've evolved mechanisms to digest and utilize it efficiently.
Compared with most primates, humans have many more copies of the gene AMY1, which is essential for breaking down starches. (5) This gene is unusual in that the number of copies varies greatly between populations, with more copies present in populations that consume more starch. This indicates that starch played a significant role in our evolution, and some scientists have even argued that consumption of starch was partially responsible for the increase in our brain size.
In addition to possessing the ability to break down complex carbohydrates, our bodies require glucose to function properly and maintain homeostasis. The fact that humans can produce glucose from protein is often used as an argument that we don't need to eat glucose, but rather than viewing this as evidence that that glucose isn't important, we might view it as evidence that glucose is so metabolically essential that we evolved a mechanism to produce it even when it's absent from the diet.
Comment: Yes, humans are able to survive on a truly staggering number of foods when compared to other species in the animal kingdom. But that doesn't mean those foods that we can subsist on are the ideal. It is a fact that our brains run more efficiently in fat burning mode, dependant upon low carbohydrate consumption.
#2 - There are many traditional cultures with high carb intake and low or nonexistent rates of neurological disease
If carbohydrates cause neurological disorders, one would expect to see high rates of dementia and similar diseases in populations where carbs constitute a significant portion of the diet. But as it turns out, many of the cultures that maintain the lowest rates of neurological and other inflammatory disease rely heavily on carbohydrate-dense dietary staples. For example, the Hadza of north-central Tanzania and the Kuna of Panama obtain a high percentage of their total calories from foods that are high in natural sugars, such as fruit, starchy tubers and honey, yet they are remarkably lean, fit and free of modern disease. (6, 7)
Other examples include the Kitava in the Pacific Islands, Tukisenta in the Papa New Guinea Highlands, and the Okinawans in Japan. The Kitavan diet is 69% carb, with a high reliance on starchy tubers such as yams, and sugary tropical fruits such as banana and papaya. (8) The Okinawan diet is even more carb-heavy at 85% carbohydrate, mostly from sweet potato. (9) Finally, the Tukisenta diet is astonishingly high in carbohydrate at over 90%. (10) All of these cultures are fit and lean with practically non-existent rates of neurological disorders and other modern chronic disease. (11)
Comment: For one thing, these remote populations are a far cry from what modernized populations are living through, so comparing their diets and state of health to ours is asking for trouble. It again comes back to the ideal. Yes, these populations are thriving on carbohydrates in a pristine environment with nary a processed vegetable oil, GMO or wi-fi signal in sight. We don't live there.
Kresser is probably right that eating whole food, non-grain or legume source carbohydrates in a pristine environment is not going to cause any disease. But eating a less-than-ideal diet in a profoundly toxic environment just adds to the already overwhelming toxic load.
#3 - Modern research does not support the notion that 'safe' carbs are harmful
The claim that carbohydrates from whole-food sources cause neurological disorders is not supported by anthropological evidence. In addition, modern studies on the health effects of carb-dense foods such as fruit also fail to support Perlmutter's hypothesis. In fact, studies overall suggest that eating whole, fresh fruit may actually decrease the risk of health issues such as obesity and diabetes, and that limiting fruit intake has no effect on blood sugar, weight loss or waist circumference. (12, 13)
As you may know if you've been following my website, there is plenty of modern research demonstrating that diets rich in refined and processed carbohydrates are harmful. However, this is not due to carb content alone, and there's no evidence that whole-food carbs have the same effect. When an author or expert recommends excluding or severely limiting one of three macronutrients that humans consume, the evidence demonstrating harm should be strong - not only because of the inconvenience of following such a restricted diet, but because extreme diets (ketogenic or VLC diets in this case) are not always harmless. In my practice I've seen many patients who've worsened on long-term VLC diets, including those with adrenal issues and poor thyroid function. Long-term VLC diets can also lead to imbalances in gut bacteria due to a lack of prebiotic fiber, which can result in digestive issues.
Comment: It's probably true that some people are unable to switch to a ketogenic diet. We are, after all, all unique with our own sets of issues, genetic and otherwise. But that shouldn't stop those who are perfectly capable from giving it a try. The best evidence for the fact that this diet works long term is the Inuit and other Native American populations prior to the invasion of the European dietary practices. For more info, check out Wonders of the Ketogenic diet: The Andersen and Stefansson 'Eskimo Experiment.
As I've always maintained, you need to find out what works for you and tailor your diet to your specific health goals, rather than follow a canned approach. This is exactly what I'll teach you to do in my book, Your Personal Paleo Code, which is coming out at the end of December.
Comment: It's a misnomer to label a ketogenic a "canned approach". There is a great deal of personalization that can be accomplished on a ketogenic diet, as will naturally happen with any diet. In fact, Kresser's Personal Paleo code could likely be adjusted to allow for a ketogenic approach.