There are countless claims out there about how much better one will feel after changing one's diet. Many emphasize weight loss, but others promise all sorts of additional benefits. Many of them do not even require long-term changes but insist that one will experience dramatic results after only a week or two.
The simplest version of this comes in the form of someone promoting a particular food and claiming that incorporating it into his or her diet for 2-3 meals a week will result in noticeable improvements in one or more areas. More complex versions might require one to make more extensive changes (e.g., adopting a vegan diet) and tend to make even more dramatic claims (e.g., you'll lose weight, be full of energy). While I tend to be quite skeptical of these claims, I cannot deny that I could stand to eat healthier.
With that in mind, I do make an effort to minimize consumption of foods I know are unhealthy and eat more of those with alleged health benefits. This has been somewhat helpful for things like blood pressure, weight, and regularity; however, I can honestly say that I have never felt better in most of the ways those making the claims say I should. I have never, for example, felt more alert, focused, or energetic, from changing my diet. I have never found that replacing a few less healthy foods with a few of the trendy "superfoods" has produced any noticeable benefit whatsoever. And the fact that these foods almost always cost considerably more than their less "super" counterparts fuels my suspicion that most of these claims are little more than nutritional snake oil.
Setting aside the issue of money for a moment, I'll admit that it can be fun to experiment on oneself in this way. For me, it has meant trying a variety of new foods with which I was previously unfamiliar. In some cases, I discovered I didn't like the food and probably wouldn't continue eating it even if it did produce noticeable changes (e.g., quinoa, chard, sprouts). In other cases, I liked it enough to incorporate it into my diet even though it did not deliver on any of the claims (e.g., tofu, kale, almonds, yogurt). And so, this has been a case where I have maintained my skepticism without it detracting in any way from my enjoyment of what I eat.