Does the special kind of diet really matter, when making a radical change?

Question

Ok, so this might not be the most medically sound question, since I am a mathematician by training, but please bear with me.

The question originates from an observation I made from being confronted with many advocates for different types of diets (vegan, vegetarian, carnivore, paleo, gluten-free, sugar-free etc) and what baffles me is that no matter what the special type of diet is, there seem to be individuals that have very good success with less depression, less overweight, more vitality, less pills to take and so forth. These individuals are often presented by the respective advocates as good rolemodels.

This really struck me, since it indicates that it might not be the actual diet itself, but maybe something else. The only thing I could make out to be present across all individuals is the fact that these individuals got their life together at least with respect to their food intake. So could the correlation between a consequent and dedicated change of diet and the overall improvement of the individual be caused by psychological phenomena that stem from the new approach / mindest these successful individuals all share?

Is there any research on that? How could one even figure out something like this?

Answer 1

There is no convincing evidence to say that any specific diet is more beneficial than others for all people. On the other hand, there is enough evidence to say that a certain diet change can be beneficial for a particular person.

Example 1: Sugar-free diet and weight loss

According to two study reviews (Nutrients, 2017, Wiley, 2012), no specific diet is better for weight loss than other diets, but every hypocaloric diet is good.

On the other hand, if you, for example, consume a lot of sugar, you can gain excessive weight, not because of sugar, but because of excessive calorie intake (BMJ, 2013). Calories also come from fats, proteins and alcohol, but if you are addicted to sugar, the main reason for your weight gain is probably sugar. In this case, it is quite likely you will lose weight much easier by a sugar-free diet than by other diets.

Example 2: Gluten-free diet and placebo effect

Gluten-free diet is obviously beneficial for people with celiac disease (Celiac Disease Foundation).

There is another condition called non-celiac gluten sensitivity; the affected people report improvement of symptoms after starting gluten-free diet (Celiac Disease Foundation). There is no test to prove non-celiac gluten sensitivity, so it is not clear if it even exists; it could be a psychosomatic disorder similar to irritable bowel syndrome, and a gluten-free diet could just act as a placebo.

In conclusion, a specific diet does not have an inherent ability to be beneficial for everyone, but it can be good for someone, because foods can have different effects on different people. Saying that, often, a perceived benefit of a special diet is just an illusion.