Here is the short explanation. When you eat any time of food, your body starts breaking it down to more simple compounds and eventually to glucose. Every cell in the body needs glucose to function. The glucose is in the blood and the your body releases insulin to get it to the cells. If there isn’t enough insulin, the blood sugar will stay high, making you tired, thirsty and generally feeling like crap. Now, in the type 2 diabetes, which is the one fats have, there is an insulin resistant, i.e. the cells resist to the effect of the insulin. The weight, and particularly the abdominal fat contribute to this effect, and then the insulin reitsnce results in more weight gain.  I have seen that correlation in real life with my former roommate who kept increasing his weight for a couple of years. The doctor was making him get a blood test every 3 months. Once the guy crossed into the overweight category, his blood sugar started rising and when he was approaching the obese BMI, he became was pre diabetic. He went on a diet, lost the weight, and never had high blood sugar again :)  They still check him every year because he was in a risk group.