Once upon a time humans walked around collecting food as they found it. Most of it was either animal origin that nutritionally means lots of fat and some protein and hardly any carbs. Or it was edible plants which had fibre, small amounts of protein, and some complex carbs. Seasonally berries and fruit would be available which tended to be more simple sugars, but most of the time these would be in short supply.
Humans would walk for long periods hunting for animals to kill. And there'd be the odd bit of running. maybe a wrestle with a bear too
Natural selection would mean that humans would develop systems for living off a low carb, high fat diet, with the facility to process occasional boosts of sugar.
Until a few thousand years ago that remained the case when sowing crops started to develop. Adaption takes a while though and as the human diet has changed the most in the last 50 years when consumption of sugar has skyrocketed, this is why we have such a problem with diabetes. A few centuries ago diabetes was a disease of the wealthy, those who could afford 'sweetmeats' and sugar rich food.
A co-morbid change has been in the timing of human meals. We now seem to have bigger meals later in the day.
More studies show that people tend to convert sugar taken later in the day direct to fat stored predominately around the belly. And in general, sugars taken during the day but not used for supporting exercise also get laid down as abdominal fat.
It's believed that abdominal fat impairs the function of the pancreas, liver and kidneys, as well as possibly harming the heart.
The pancreas is programmed to release insulin to enable blood born sugar to be used. However sugar in excess harms the mitochondrial that converts energy in food form to muscle energy. As this deterioration happens, the uptake of blood sugar by the muscles reduces which causes the pancreas to issue increasing amounts of insulin in an effort to get the energy across.
After prolonged periods if left unchecked, this ramping up of insulin production to counteract the reduction of insulin sensitivity of the mito whatnot, cause the pancreas to burn out. This is diabetes.
More and more studies are showing that high fat low carb diets stop diabetes from developing, and can (with care) reduce existing diabetes (type 2) possibly as much as stopping medication all together.
Fat must be good quality, not processed fats such as trans. Despite earlier concerns, dairy fats aren't that bad for you either. Certainly better than as many calories of high energy drink.
Clinical research also shows that the body is fooled into thinking that artificial sweetners are sugar, and so signals the pancreas to produce a dollop of insulin. This then finding no sugar to use, it messes around and you end up with even more fat shuttled off to storage around the abdomen.
Of course all this might be proved to be bogus science promoted by purveyors of 'healthy' and natural food. But the statistics really do seem to support the later theory