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The two main sources of the body’s energy are glucose (carbohydrate) and fatty acids (fat). Because of the bad effects of excessive glucose (hyperglycaemia) on the body, insulin is mobilised to get it out of the blood and safely into muscle (to be metabolised) or into fat tissue (to be stored). Under low-carb conditions, the body signals the liver to make just enough glucose for the cells that need it (there is usually 1.0 U.S. teaspoon circulating in the blood at any given time) and uses the fat in the diet (and any excess in storage) to produce ketone bodies. Although we enter ketosis within a day or so after cutting our carb intake, f we have been eating too much carbohydrate for any length of time, the skeletal muscles will need to re-adapt to metabolising fatty acids and to heal the damage from metabolising too much glucose, which normally takes somewhere between six and eight weeks of a sufficiently low-carb diet. This latter process is called keto-adaptation, or fat-adaptation.
Failing to give the body an adequate supply of energy will result in fatigue, lethargy, a sensation of cold, and so forth. The body does not normally metabolise protein for energy, preferring to use the amino acids from dietary protein to repair and rebuild tissues, but it can rob proteins from tissues and make glucose out of them—although this process carries a heavy energy cost, so the net yield is much lower than the yield from fats or glucose. So it’s the body’s equivalent of burning the furniture when the wood pile runs out.
How little carbohydrate one needs to eat depends on one’s degree of insulin-resistance. A perfectly insulin-sensitive person secretes reasonable amount of insulin in response to dietary carbohydrate, and such a person may be able to eat, say, 100 g/day and still keep insulin low enough to be in ketosis. A highly insulin-resistant person may find that his or her response to even 20 g/day of carbohydrate is enough insulin to keep him or her out of ketosis. It all depends.
The insulin response to dietary fat is the bare minimum required for survival (some insulin is necessary, it’s an excessive amount that causes problems). Fat also does not cause metabolic damage, the way glucose does.
The insulin response to protein amplifies the response to dietary carbohydrate. A sufficiently low carb intake renders the insulin response to protein minimal (that is a simplification but is essentially true); when protein accompanies dietary carbohydrate above the ketogenic level (for that person), then there is a significant response to the protein as well as the carbohydrate.
We recommend a diet that avoids all sugar (not just because it is a carbohydrate, but for other health reasons, as well), all grains, starches, tubers, and legumes (since they are just glucose molecules arranged in various ways) and allows primarily leafy greens and cruciferous vegetables (because they also contain indigestible fibre that mitigates the digestible carb content). It also encourages a reasonable amount of protein, and advises eating fat to satiety (in other words, cook with butter, lard, tallow, or coconut oil, and don’t shy away from fatty cuts of meat and fish). The standard dietary advice from the government is almost exactly backwards, in terms of promoting metabolic health.
Not knowing your precise circumstances or current diet makes it hard to advise you. People on a well-formulated ketogenic diet have eaten 3000-5000 calories a day and still shed excess fat. People on a well-formulated ketogenic diet who have stinted the calories have found their bodies surprisingly reluctant to shed fat (this is a response to famine). So the level of caloric intake is not nearly as important as the body’s hormonal response to food choices.
I hope that provides enough information for you to begin to make some choices. If you have further questions, we will be glad to try to come up with answers. Meanwhile, there is a wealth of information in our Newbies forum. Have a look around and see if any of it helps.