- the human body has adapted to eat only particular foods, and
- we need to eat like our ancestors who did not eat grains and legumes to avoid modern-day chronic diseases
The latter idea is also flawed because it's controversial whether our ancestors ate grains or legumes. Also, it's been believed that grains cause inflammation but this occurs only for a small amount of people (less than 10% due to celiac disease and gluten sensitivity).
There is an assumption that the cause of modern chronic diseases (i.e., obesity, diabetes, heart disease, etc.) is the result of consuming grains. It is believed that those who follow the Paleo diet don't get chronic diseases because grains are not eaten. But this belief is misguided because the Paleo diet, which consists of whole foods including high fiber, lean proteins, lots of omega-3 fatty acids, unsaturated fats, vitamins and minerals, and less saturated fat and sodium, contributes to good health regardless of grain consumption. The characteristics of the Paleo diet make it a much healthier diet than a diet of processed foods high in sugar, fats and sodium predominately eaten today. Thus, the issue is not the consumption of grains but rather processed foods. The increased consumption of fruits and vegetables alone will surely lessen the prevalence of chronic diseases in our modern society.
BOTTOM LINE: Excluding dairy, legumes and grains from your diet, as the Paleo diet recommends, is not prudent as the evidence to support this is shaky. The human body can continually adapt to evolutionary changes in the diet so a one-size-fits-all approach to eating is suspect. In fact, any diet which espouses restricting particular foods should be a red flag. Healthy eating need not be complicated and consuming more fruits, vegetables, nuts, and seeds is certainly recommended. There are no inherently "good" or "bad" foods. Just be sure to eat any food in moderation.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."