Ketosis (not the same as ketoacidosis common in diabetic patients) occurs on a so called ketogenic or high-fat, low-carbohydrate diet.
"Normally, human bodies are sugar-driven machines: ingested carbohydrates are broken down into glucose, which is mainly transported and used as energy or stored as glycogen in liver and muscle tissue. When deprived of dietary carbohydrates (usually below 50g/day), the liver becomes the sole provider of glucose to feed your hungry organs – especially the brain, a particularly greedy entity accounting for ~20% of total energy expenditure. The brain cannot DIRECTLY use fat for energy. Once liver glycogen is depleted, without a backup energy source, humanity would’ve long disappeared in the eons of evolution.
The backup is ketone bodies that the liver derives primarily from fatty acids in your diet or body fat. These ketones – β-hydroxybutyrate (BHB), acetoacetate and acetone – are released into the bloodstream, taken up by the brain and other organs, shuttled into the “energy factory” mitochondria and used up as fuel. Excess BHB and acetoacetate are excreted from urine, while acetone, due to its volatile nature, is breathed out (hence the characteristically sweet “keto breath”). Meanwhile, blood glucose remains physiologically normal due to glucose derived from certain amino acids and the breakdown of fatty acids – voila, low blood sugar avoided!"In a few words: when we eat meat and fat, our brain is able to survive on a fuel of ketone bodies derived from dietary fat or the fat deposited in our fat tissues. The lack of the hormone insulin does not prevent our fat tissues from breaking down, leading to weight loss.
If carbohydrates are consumed, the primary fuel for the brain and muscles is glucose from these carbohydrates. The fatty acids will remain in the fat tissues as a high insulin level is preventing their release. A constantly high carbohydrate diet (leading to chronically elevated insulin levels) will lead to a few pounds of excess fat in our fat tissues over the years. Burning glucose - when it's available - before using fatty acids, makes a lot of sense, since high glucose levels are toxic, so our bodies adapted to get rid of this excess glucose by using it first.
The question now is what would be the ideal fuel for our body: glucose or fatty acids. It turns out that the carbohydrates we are consuming today led to the development of a number of chronic diseases, almost entirely absent in native populations on their traditional diet.
If we turn out to be healthier and have a higher chance of avoiding chronic diseases with less carbohydrates, then a lower carbohydrate diet is able to provide the necessary fuel to our bodies. This study (Study of the ketogenic agent AC-1202 in mild to moderate Alzheimer's disease: a randomized, double-blind, placebo-controlled, multicenter trial) seems to confirm this theory.