carbs=sugar. Sugar exists in many forms once food are digested and converted by the body, fructose, alcohol, lactose, and so on. most forms are simple sugars and are burned extremely fast by the body (glucose, white flour, honey are examples), others are slow to convert and are know as complex carbs (Farro, quinoa, tapioca, Brown rice, Oatmeal) .Whole grains are also an important source of fiber, minerals and other nutrients, so its not just a "carb"They allow the body to use them over a period of time, extending their energy release. Athletes use a blend of fast and slow burning cabs for endurance sports. Your body needs fat, it uses them for protection of organs and other key functions but once your body fat levels get too low, your body will resort to burning your muscle for fuel if it doesn't have other energy sources. It also leads to organ failure. This is part of starvation.
Nitrate-free meat doesn't make the meat better, nitrates are just not good for you. If you want to look at "healthier" meat, look at grass-fed and grass finished beef and pork. Corn is not a natural food of cows and buffalo (for instance) and it does effect the fat make-up, specifically Omega content. it is not digested well by them. (lots of info on this)
we can certainly debate human evolution and diet, most modern humans evolved eating over 150 different types of food sources and there is evidence that once humans figured out how to cook meat, their diets changed but it wasn't until about 15K (give or take few thousand years depending on geographic location) humans changed from nomadic hunter-gathers to living in small groups and later to cities and began to grow and develop new animal species and crops with grains (not to mention organized religions and written language). Thats when our diets truly changed adding in "caborhydrate-based foods" but that allowed humans to increase their lifespan and supplanted more protein-based diets. But, what really is the culprit, is processed foods and processed foods contain high amounts of salt, sugar and saturated fats (not to mention lots of chemicals). They are as addictive as cocaine and other drugs. they are "empty" calories as they fill a person up but do not provide any nutrition and result in the body converting the extra calories into fat and lack essential nutrients and fiber. And, lets not overlook the effects of alcohol on our bodies. want to lose weight? Cut out alcohol. it has very little benefit.
However, as humans diets changed so did their DNA and continues to evolv at the fastest pace in our total evolution. A result of this is that some people are much more prone to high cholesterol regardless of diet, Some are lactose intolerant (USA vs Euro) as well as Gluten intolerance. So to say Keto (for example) is good for everyone is a misnomer. I do think minimizing simple carbs is a good idea as they provide very little benefit however fruits are essential to humans diets unless you are taking supplements including additional sources of fiber.
Testing on Keto in fact does show many benefits however, few people are able to maintain this over a life-time and long-term studies have not been conduct. I think the majority of benefits can be obtained by simply cutting out processed foods, drastically limiting simple carbs, (including all sugar-based drinks and white four) cutting out alcohol and eating a heathy balanced diet including complex carbs. Grass-fed beef and pork And, drinking more water. Exercise is also critical and without it, you may lose weight but you aren't doing anything to help your overall health and exercise helps the body to burn fuel more efficiently. Building muscles burns more calories. And, at the end of the day, Calories in vs calories out = weight loss or gain. However, if your goal is to add muscle or increase endurance, you need to fuel your body adequately.