A vegan diet claims to hearken back to primitive days but proponents leave out that it wasn't by choice. Whenever possible, early man ate everything on and inside a bone they could, including brains.

The biological basis for that is evident when we consider that the brain is only 2 percent of our body weight but uses 20 percent of our caloric intake. Meat provides a calorie dense way to acquire the diverse nutrition we need.

If you raise your toddler on a vegan diet even now, you are likely going to end up in jail. Yet even in adults it is not healthy unless you offset the nutrition loss with supplements, a study shows. You can't get creatine, carnosine, taurine, omega-3, haem iron and vitamins B12 and D3 from a vegan diet.

That's far different than the imagery they sell.

Good luck functioning at full brain power without B12. And yet half of recent participants in a vegan analysis lacked B12 even though supplements are a $35 billion market just in the US. Half of the people who actually need them because of their diet fad don't take them. 

Read more: "How a vegan diet could affect your intelligence" By Zaria Gorvett, BBC