Fat is the preferred fuel of the body
Our body will burn whatever we offer it when it comes to burning fuel. It has no choice... unless we give it that choice. When we were hunters and gatherers we flourished on healthy fat (in the form of free roaming antibiotic free meat, seeds, and nuts) along with any fruits and vegetables we could find. When food was sparse we broke down our fat stores and used this quite efficiently and successfully as fuel until the next meal could be obtained. Obesity, heart disease, and Diabetes did not exist.
Fast forward a few hundred years and what we now burn as fuel is predominantly sugar and processed carbohydrates. Our food choices are overwhelmingly laden with added sugar…and we are addicted to this sweet but not so innocuous molecule. One of the interesting results of the overconsumption of sugar is the resulting staggering increase in our insulin levels. If one measures the fasting insulin levels in the Maasai and Inuit cultures, where there is no access to added sugar or processes carbs, their levels range between 2.0 and 5.0. The average fasting insulin level in the U.S. Is 9.8. The majority of the levels I obtain in my overweight and obese patients are between 20 and 50! We are walking around with insulin levels that are orders of magnitude higher than our predecessors. And we spend a trillion dollars/yr combating the metabolic consequences of our diet in the form of those diseases our ancestors avoided.
While it is true our brain primarily relies on glucose to function, it can do quite well using the stores of glycogen in our liver, from gluconeogenesis (the production of glucose from protein), and from ketones (the break down product of fatty acids). We don't need carbohydrates to survive…they are not essential. I believe our body wants to burn fat as a fuel. It can achieve this cleanly and efficiently and without the health risks associated with our current sugar/carb rich diet.
One last caveat. If you want to lose weight you must lower your insulin levels. Insulin is your storage hormone. It locks excess energy in the form of fat in your fat cells…and it throws away the key! You cannot access that fat as energy as long as your insulin levels are elevated. As insulin falls (by avoiding the overconsumption of sugar) you can utilize that “freed” fat as energy. The more keto-adaptive you become the more weight you will lose; you are on your way to a longer, healthier, and more productive life!