The transition from using glucose as energy source, to using fat as energy source, creates a strong hunger response. For those that consume normal western diets, with typically a high glycemic response, the intensity of hunger during calorie restriction will reach levels that may be intolerable. Staying in keto will keep your body on fat burning mode, and your hunger pangs will be less even if you are consuming less calories than you are using.
Note: The use of your own fat for fuel can reduce growth hormone levels, as the body switches from feast to famine mode. Although there seem to be workarounds, because growth hormone secretion is also biorhythmic.
One research paper found a link between a high fat diet and adipocyte apoptosis (fat cell death).
Answer by Robert Cruder on Quora:
Let me be clear that I practiced keto dieting for many years but no longer do so. That is partly because BOTH ultra low carb and ultra low fat diets reduce longevity compared to a more balanced approach. It is also because my kidney function has already been degraded by about 50% and permanently so by excess animal protein.
People are different and one’s glucose needs depend on both genes and activity. No one’s experience can determine another’s minimum glucose requirement. Adhering to anyone’s numbers is not a diet but a religion. Find the level of glucose that your body needs.
Please do not use only protein to achieve that level. As an example, whey powder leaves the stomach almost instantly and digests almost instantly. There is no way tissues can utilize amino acids at that rate and the excess is also almost instantly converted into glucose. Do not pretend that you would be drinking protein and avoiding glucose. You would still be taking in glucose but leaving behind a residue of phosphate and sulfate which can damage your kidneys.
How do fat and glucose burning relate to hunger?
Some muscles, mostly in the legs, have a high proportion of type I fibers that burn fat. The proportion is genetically determined. Those fibers and heart muscle are the only ones with transporters to absorb fatty acids from the bloodstream.
Neither other muscles nor the brain nor blood cells can intake fatty acids even if they could burn them. Those must depend on either glucose or ketone bodies.
The other muscles, also genetically determined, especially in males and especially in the upper body are composed of type II fibers which burn ONLY glucose. No matter how hard you work them they NEVER have a fat-burning zone.
Those type II fibers can take in ketone bodies instead of glucose, convert them into glucose and store that as glycogen. They do that at a much lower rate than they could glucose and never achieve the same level of glycogen storage. One can tell that from the weight one loses early in a keto diet. That eight to ten pounds of initial loss is approximately 1 kg or 4000 Calories of glycogen stored in type II muscles, each gram of which is associated with three to four grams of water.
There will always be some activity level at which the lesser stored glycogen is exhausted and ketone body intake cannot keep up. It is common for the performance deficit to run as much as 80%. Some muscles will always feel tired and only recover as they refill glycogen.
Worked muscles expose the GLUT4 transporter even in the absence of insulin and can easily exhaust the low level of circulating glucose associated with keto diets. Regardless of the amount of fat that one already carries and regardless of the amount of fat that one eats, one would be hypoglycemic and hungry. That would continue until either type II muscle fibers achieve a minimum level of glycogen storage or the GLUT4 transporters recycle.
Fat, fiber and large proteins DO fill the stomach and make one feel full. They do not release glucose rapidly enough to keep up with exercise-induced GLUT4. One can still be hypoglycemic and still feel very hungry. One cannot dispel that hunger with still more volume in one’s stomach.
Meat digests slowly, often over 6 to 8 hours. As it digests, the released amino acids almost always exceed what can be utilized by growing tissues (2 - 3 grams per hour) and the liver converts any excess minute-by-minute and gram-for-gram into glucose. That may suffice when asleep or sedentary.
Any significant activity burns glucose more rapidly than that. The liver continues to fill the gap but must get amino acids from somewhere. It does so by retrieving protein, not fat, from tissues and burn existing lean tissue, muscle, bone density, skin and blood proteins.
Your hunger should be telling you that the “weight” you lose is not all fat. Females find it difficult to restore bone density after their early twenties. Both sexes find it difficult to regrow muscle past their thirties but women suffer an additional deficit post menopause.
I have tried a meat and non-starchy-vegetable only diet (high-protein), for 6 weeks, and I was always in a state of hunger no matter how much I ate, but I think that this only applies to my own body type. Or, it only applies to a body and mind with a life long history of a high sugar and starch diet. Your experience could be completely different. Regardless of choosing a high protein, or high fat diet, I think most people would benefit from reducing the amount of carbohydrates, especially, simple sugars. Edit: I found out the body can convert protein into glucose, and so maybe, I was always transitioning in and out of ketosis. Likely, the transition in and out of ketosis was causing periods of elevated hunger.
Diets to lose weight in the short term may not be the solution: Diets Are Not the Answer. Lifestyle changes in what you eat, will lead to sustained weight loss, and better health. It's up to you to experiment, and find out what works best for your own energy processing engine.
You can use ketone strips to estimate your level of ketosis. “Test strips only show the amount of ketones your body ISN'T using, so how it really works is:
1. when you fist start trying to get into ketosis the strips should be light because your body isn't really producing many ketones.
2. the strips will get darker as your body starts producing more Ketones but cant effectively use them, so you excrete most of them.
3. the strips will get lighter after your body learns to use ketones and now you excrete less of them.” (Tim Gunther, Amazon)