Not a disease, but one thing I suspect is very important is:
Back in either the late 50's or early 60's, there was a study done to determine the effects of linoleic acid (one of the omega-6 fatty acids, heavily present in corn, safflower, canola, sunflower, and other such oils) on the body.
Historically it is likely that these fats were little consumed by man. How much oil is there in an ear of corn? Hardly any. I doubt man was consuming much of these other oils either. Maybe some sunflower seeds, but certainly high-linoleic-acid fats were not a major part of the diet.
They used older people as the subjects. While these sorts of oils were certainly coming onto the market at this time, though nowhere near to the extent of today, older people probably were "sot in their ways" and tended to eat as they always did, so were not high consumers of these oils.
Very importantly, the study actually biopsied the body fat of the subjects and determined the fatty acid content.
Approximately speaking, the subjects averaged about 10% linoleic acid content in their bodyfat at the start of the study.
They were put into three groups: low, moderate, and high linoleic acid consumption and they were tracked for many years.
The moderate, and especially the high, linoleic acid groups experienced increasing levels of LA in their own bodyfat.
If I recall correctly, higher rates (compared to the low LA intake group) of obesity, diabetes or insulin resistance, and other adverse markers started appearing as bodyfat LA content reached 15%. Most certainly that was the case by the time it reached 20%.
In all this, the low LA intake group continued to maintain about 10% LA content in their bodyfat and did not have increased rates of such problems.
Now look at the situation today. People actually think it is healthy to consume all these "healthy vegetable oils" instead of nasty saturated fat.
I would not be surprised if obese people today average much higher LA intake than even the high intake group in that study.
Their bodies are, that study gives reason to believe, loaded with LA to a degree that simply is not natural to man and which this study gives reason to believe leads to problems. Including obesity developing in individuals which would not be obese but for this factor. (If that last were not true, then the study wouldn't have shown markedly higher rates.)
I'm not saying they couldn't lose the fat anyway, but it doesn't help them and quite possibly hurts them greatly that their diet includes so much of this, and ironically, as they try to eat "healthy," they load up on LA. Or even if they are not trying to eat healthy: this stuff is just ubiquitous these days. You really have to make an effort to avoid it.