The biggest issue we have with diet and nutrition today is a lack of skepticism. We read a study and take it as gospel because it was written by a scientist who seems credible. The reality is, though, that many of the health policies and personal dietary choices we made are based on shaky evidence — and that's pretty dangerous.
To have a better understanding of the gravity of the situation, you should read the essay "Are some diets 'mass murder'?" by the BMJ's former editor Dr. Richard Smith. He perfectly sums it up in just one sentence: "In short, bold policies have been based on fragile science, and the long term results may be terrible."
Smith borrows insights from Nina Teicholz's book The Big Fat Surprise to explain why studies on nutrition and diet are so unreliable — focusing in on low-fat and Mediterranean diet crazes. He says that the studies in which we place so much trust, particularly related to saturated fat, were based on biased evidence. The research that came out that challenged these studies were methodically ignored and shaped by self-interested scientists linked to big food companies.
Major food manufacturers get their way. So when big food companies want people to believe that saturated fats cause cardiovascular disease, they will make people believe it. Unsurprisingly, it became, and still is, the global orthodoxy that people should eat low-fat diets in order to prevent heart disease.
The biggest test of the saturated fat hypothesis was with the Women's Health Initiative. For this study, researchers followed 49,000 premenopausal women for 10 years, and those with low-fat diets reduced their total fat consumption and saturated fat significantly. What about risk of heart disease and stroke, you ask? Well, it made no difference.
Since then, multiple reviews, including one by the Food and Agriculture Organization, have shown that there is "no probable or convincing evidence" that a diet high in fat causes heart disease. So, naturally, people started to notice the deterioration of the hypothesis, and Big Food got concerned. That's when scientists with conflicts of interest (for example, affiliations with olive oil manufacturers) came out of the woodwork with shoddy evidence supporting the Mediterranean diet.
Smith concludes, likely shaking his head: