I'm not a scientist but as I understand it we eat to get hold of various substances that are essential for the functioning of our bodies. Foods containing protein give us amino acids. Foods containing fat give us essential fatty acids. Eating a good range of foods of all types gives us an excellent chance of getting all the vitamins and minerals we need.
The other key requirement we have is an energy source. We need glucose, a simple sugar, to be carried round in the bloodstream to every cell in the body. The glucose is our fuel, like the petrol in a car. We get it from digesting our food. All carbohydrates except cellulose can be broken down in the body into glucose. So that means sucrose, fructose and lactose (all sugars), but also starch. In an extremity the body can break down both fat and protein to convert them into glucose. If there is more glucose available from our food than we need at the time it gets converted into fat and stored for another time.
The problem with refined carbohydrate is that it's incredibly easy to eat a lot in a short time and scarcely notice we're doing it. Eating high-fibre foods takes a long time because of the chewing and the fibrous part bulks out the stomach and makes us feel full. Eating fat and protein for some reason makes us feel full quite quickly so we stop eating. Eating bread, sugary foods and so on unfortunately doesn't.
I assume that's why low carb diets help people lose weight. Like most weight loss diet programmes, if someone follows the advice carefully they will end up eating fewer calories than they did before.
I'm sure for many people there is also the advantage of eliminating things their body finds difficult to tolerate, like newer additives or gluten.
PS Reverting to the fascinating topic of how our species adapted to the introduction fo agriculture, isn't it the case that:
- Once we grew our own food rather than relying on what we could get from hunting and gathering, we usually had a reliable food supply and could put food by for lean years, so population increased.
- Once population increased it became possible to settle down in villages, towns, cities and develop specialist trades, and once somebody does a job full-time they get better and better at it and technology/culture develops in leaps and bounds.
- However, living in close proximity to our domesticated animals meant we got a lot more infectious diseases from them (cf bird flu now) and living in much more crowded conditions meant those diseases could spread very quickly through populations. Probably didn't help that the diet would have been less varied and obviously sanitation would have been an issue too.
So we ended up with higher mortality from diseases and permanent health effects on those who survived, e.g. not being as tall as a hunter gatherer, but there were far more of us than ever before.
The spread of our species right across the globe and the fact that we survived in almost every climate is proof of the fact that our bodies are incredibly adaptable and can keep us alive on almost any diet.