I hope someday this concept of a healthy diet goes by the wayside because even just a cursory investigation makes it obvious how flawed it is. It's like people think that a food is "healthy" as long as it's not pizza or ice cream, and even most GPs don't totally know better (they're not stupid, it's just that they're not as educated in that subject).

A reasonable diet isn't even that complicated. Control the sugar and choose foods lower on the insulin index and it will be surprisingly difficult to not control your weight. You won't necessarily get ripped on that alone, but it would be really unlikely to get anywhere near as large as this guy or for him to stay that way.

For instance, even though oatmeal is pretty overrated to start with, it's by no means the worst thing one can be eating, so he would just need to not eat it with sugar. Oatmeal is a somewhat higher glycemic food if I remember correctly, but it's a good source of fiber. Bananas are pretty high on the glycemic and insulin indexes so they should only be eaten occasionally and as an alternative to something worse like ice cream. Salad is of course mostly fine if it has minimal dressing and no added sugar (and hopefully a good fat like olive oil). Peanuts are, surprisingly enough, very low on the insulin index and are fairly low on the glycemic index, and they're high in satiation. A peanut butter sandwich with natural peanut butter (no added sugar) and whole grain bread would be far better to a PB&J, though eating some peanut butter without bread would be optimal. Pasta probably isn't the best thing to be eating if you're 600+ lbs, but at lower weights it's not necessarily that bad relative to other things. Better to replace it with something closer to a low carb Mediterranean dish if you want a substantial dinner.

If you (the reader in general) read those two paragraphs then I just saved you countless hours in reading research papers and fad diet books/sites.

Metabolism is complicated, but the insulin response is the most important thing to understand for body fat control. If you don't have an insulin response, you can't gain adipose fat. The most successful diet plans usually select for foods that have a lower glycemic effect and a lower insulin response.

The leptin/ghrelin balance is also important, but even that is less important in the long term than insulin. Both diet and fasting help with those, the latter being of greater help in the short term.

> especially fructose

I would generally agree, although I think the fructose thing is a little overstated and misleads some people to lumping food into the same category is Coca Cola. Different fruits have different insulin index values, different amounts of sugar, and usually come with fiber which helps with blood sugar spikes among other things. With some exceptions (bananas and obviously fruit juices), if you're consuming whole fruit but you're not eating refined sugars then you'll probably be fine. Even mangos, which are super tasty IMO, aren't particularly high in calories relative to many other foods people normally consume, and if your source of sugar is a mango a day then most of that sugar will be processed by your liver and it's little enough that you can burn it off with some low intensity exercise.