Re Diet, BMI, and metabolic disease.
Yes, it sure is a complicated topic. I've been doing a review of scientific literature on chronic illnesses, and there are sufficient connections to this topic that I can appreciate the various narratives here.
Much of this area is under constant revision , so I'll share a few recent insights that the medical community is digesting (he-he).
The GI tract can taste, just like the tongue. It makes sense, as the method of digestion depends on what is there and the amount of sugar that will shortly be entering the bloodstream requires corrective action (if present). In fact, the sweet taste of non-caloric sweeteners, in contrast to its nutrient value, is something that has to be relearned by the GI system.
The microbiota matter in many ways, and taking a multi strain 30-50million/day dose for some amount of time after antibiotics is wise. There are many types of gut bacteria and the mix can make a tremendous difference in calorie adsorption and how it is managed. The bacteria have their rhythm, and regular meal times are a good idea. You're feeding billion of guests, be kind. Fiber is checked for post-adsorption and has its own receptors that are involved with crosstalk in mitochondrial settings.
Circadian clock matters. Besides the bacteria's rhythm, circadian clock dysfunction is a hallmark of many illnesses and theories abound as to why. This is the research I am doing, but regardless of why, one method of "tuning" the circadian clock is meals. The current studies with a daily 12 hour fast, from last food to first (with the last food 3 hours before sleep), are interesting.
If you have a circadian polymorphism called Per3, where the amplitudes of the signal is about 1/3 as strong, and parts of the circadian system are bypassed (these people tend to be night owls as they don't get the sleepiness signal), meals are the most effective way to maintain the clock.
Type of calories matter, and its a controversial topic. One thing that all agree with is straight sugar is to be avoided- some as far as eliminating sweet fruits.
I saw upthread a fibromyalgia reference, and this is true for most of the chronic diseases, metabolic dysfunction is at least a hallmark, and some would argue "causative" in FM, Alzheimer's (the most effective recent trial was intranasal insulin), mood disorders, diabetes, Crohns, etc. But, it seems to be induced as part of a stress response cascade which causes radical changes in system regulation.
Daily exercise sends signals which, at the very least, are countering the stress messaging. And, some of those various messages originate from the adipose tissue ( as P0rtia alluded to).
Gastric bypass has larger effects than just the calorie reduction and is likely affecting the messaging system as well. And, don't underestimate how large an impact that has, since our microbiota are responsible for the majority of our serotonin. Our GI system has a tremendous number of neurons (some call it the gut-brain), and the vagus nerve enters at the point that controls addiction, pain management through afferent modulation, etc.. It's numorouss neural receptor effects are still not understood. And, yes, part of that is the endocannabinoid system.
Aside from that jumble, aging sucks.
Living to be a 100 year old rooster means keeping the damages down.Some supplements get unpredictable results in clinical trials, but I've found Quercetin, Pterostilbene, CoQ10,, and melatonin studies to be consistently impressive. If reading medical research papers is intimidating, there are a number of Chrome extensions that help make it more accessible.
The Mercury Project