Sometimes greed comes before honesty.
There is a lot of money to be made in selling nutrition advice and products that are based on particular kinds of nutrition advice, and sometimes there’s even more money to be made when the advice is grounded in partial or complete falsehoods. The ultimate example is Robert Atkins, who almost single-handedly spawned a low-carb diet industry that peaked at $15 billion a year based on a largely bogus dietary philosophy.
It’s important to understand that profit motives also color what is presented to us as nutritional fact in more insidious ways. University nutrition departments now depend heavily on large food and drug companies to fund research. (I’ve interviewed veteran researchers who remember when it was unthinkable to accept funding from such sources.) Even when these relationships don’t affect the results of research, they affect the types of studies that are done, and are not done. Studies designed to prove that a certain product solves a health or nutrition problem are much more likely to be funded than studies that merely serve to increase our knowledge of human nutrition.