A bit of physiology is in order too. Your blood cannot supply your muscles with enough energy for athletic activities, since it would have to have the chemical composition of cooking oil. (At peak, I burn off roughly 1200 - 1500 calories per hour.) Instead, the muscles store sugar and release it as needed. You have enough for roughly two plus hours of high-level activities or about 2,000 calories. Highly conditioned athletes have more. Long distance runners are well aware of this limit and refer to it as "hitting the wall" or "bonking" meaning that quite suddenly they are in an impaired state and have a hard time continuing their run. The cycle then is that as soon as you eat, muscles soak up the sugars. This has a very high priority in the body as a simple matter of survival: If muscle glycogen gets depleted, you cannot outrun that sabre-toothed kitty at all. This so important that if you fail to eat after a heavy activity, you will tend to get very sleepy and probably take a nap, like it or not. This is actually almost, but not quite, passing out from sport-induced hypoglycemia (low blood sugar) where the muscles have such a priority recharging that you shut down until it happens.
Training tip. Since sugar is stored in the muscles, sports performance is mostly independent of initial blood sugar levels. Most athletes know that feeling a bit pokey, as long as they have adequately watched over their nutrition, will almost miraculously abate within a few minutes once they start training. Putting yourself on a training schedule so that you know you are able to perform well is a crucial step in consistency. If you must rely on motivation to get you to the gym you will tend to miss more training.
Now we come to the problem: insulin resistance. This is a dysfunction in which healthy cells (like fat and muscle) have reduced ability to use glucose in the presence of insulin. As a result, more and more insulin is produced by the pancreas. Eventually, this causes the pancreas to simply fail and then type 2 diabetes ensues. Among other issues are sharp weight gain at the same time there is an inability to utilize glucose, so the body starves at the cellular level while accumulating fat. You are constantly, gnawingly hungry getting fatter and fatter all the time and attempting to eat less gives you a host of unwanted symptoms associated with starvation. It is very difficult indeed to reverse this once it happens.
How people become insulin resistant is very well understood: Too many carbohydrates too often, especially ones with a high glycemic index (which just means they are readily converted to glucose). Kids who binge constantly on sugar can develop it and there are now many recorded cases of teenagers getting adult onset diabetes, which was unknown even 50 years ago in that age group. A low fat diet, as mentioned above, will reduce fatty deposits from arteries, but also will begin making people much more insulin resistant. Low fat diets were used as short-term therapy for quite some time before becoming a fad. Now people are on them non-stop and metabolic syndrome as well as a sharp increase in the national waistline seem to be the result. Note too that the country with the most abnormally high levels of heart disease and diabetes is India, where strict vegetarian cooking is the norm. (To make this more real for you, diabetes and heart disease occur 10 - 15 years earlier in India, and nearly 1/3 of all a fatalities in India are due to this. In the US, the rate is slightly less than 1/4. It is estimated by the WHO that by 2020 60% of all heart attack fatalities worldwide will be in India. It is referred to now as the "cardiac crisis in India".) In theory vegetarian diets can be very similar to non-vegetarian diets, but in practice, they tend to be extremely carbohydrate heavy. I was married to a vegetarian for many years and did much of the cooking, so I know.
The major problem is the definition of what is healthy. "Healthy" now refers to a highly sedentary, middle-aged office worker. They are the ones who develop insulin resistance and metabolic syndrome and are the one who require the highest amount of health care. Rather than teach people to listen to their bodies and treat food as fuel -- which is hard and requires they take ownership of their health -- the baseline for healthy has been redefined. There are also the usual list of agenda driven groups pushing for this or that program. If your activity levels allow it, train hard, eat right (and lots) and enjoy good health. If they do not, you are probably stuck with whatever the doctor prescribes to offset years of inactivity and poor eating. Any advice that does not directly relate your activity levels and specific health concerns to nutritional requirements is probably just ideology and should be treated with enormous skepticism.