Fasting ketosis occurs “after a few days of fasting…when liver glycogen stores are depleted” and “the body shifts from a glucose-based metabolism to a fat-based one,” Lucas Tafur explained in 2012 on his now defunct website. It is a physiologic state that the ketone expert, Dr. Richard Veech at The National Institutes of Health, calls “the normal state of man.” But Tafur avers, “It can either be triggered by fasting or by diet.” Therein lies the message.
Dietary ketosis is the adaptive response, or “acquired evolutionary mechanism, [that] shifts [the body] from a glucose-based metabolism to a fat-based one,” Tafur explains. This shift occurs when the carbohydrates are unavailable for fuel. The enzymes that break down fat for energy are controlled by insulin which is very responsive to the presence of carbohydrate. Dietary ketosis is achieved by the sustained and sharply curtailed intake of carbohydrates. How sharply? It varies. Your mileage may vary (YMMV), but for me it’s 20 to 30g of carbs a day.
What is the mechanism and how does it work? (Sorry, but I think some of my readers will find the science both useful). A part of everything we eat becomes fuel. Carbohydrates break down to mostly glucose. That’s good because glucose quickly converts to energy, and the body (the brain, especially) needs a little glucose, NOT carbs – about 30-35 grams a day, either directly as glucose or in a form (e.g. ketone bodies) that substitutes for it perfectly.
Fat (both body fat and ingested) are triglycerides, made up of 3 fatty acid molecules and 1 glycerol molecule. Note the stem ‘gly.’ When each triglyceride molecule is broken down into free fatty acids for fuel, it leaves a glycerol molecule to join with another to make glucose. Thus, about 10% of ingested or stored body fat becomes glucose.
When protein is digested into its component amino acids and is taken up by the body, what is left over goes to the liver. There, when the body needs glucose, the liver makes glucose from those stored amino acids by a process called gluconeogenesis (“glucose-new-creation”). About 54% of the protein we eat is glucogenic, i.e. can become glucose, especially if we eat too much at any one meal. It is stored and then reconstructed and utilized as glucose!
Carbohydrates – almost all of them, from simple sugars to complex starches – digest to glucose. Some starches digest slowly, but simple sugars and highly processed carbs (in products sold in boxes and bags) break down fast to the single-cell sugars glucose, fructose or galactose. In the case of fructose, they are shunted directly to the liver to protect your body from them. Your liver will convert fructose to glucose to glycogen to store in the liver, but if the liver is already full of glycogen, it will convert the fructose to fat by a process called de novo lipogenesis.
The way to achieve the condition called dietary ketosis is this: eat a very low carbohydrate, moderate protein and high fat diet. Expressed as a formula – sorry, again – where K = ketogenic molecules and G = glucose molecules.
K/G ratio = (0.9*FAT+0.46*PRO)/ (0.1*FAT+0.54*PRO+1*CHO.)
The fat, protein and carbohydrates (CHO) are all entered in grams (weight). A ketogenic ratio of numerator (K) to denominator (G) is >1.5. Thus, the ratio should be at least 1.5 to 1, stated >1.5:1, in each meal, every day.
Because this dietary regimen is very high in fat and very low in carbs, you will not be hungry between meals, so long as you don’t eat carbohydrates. You will be satisfied because fat is satiating and protein digests slowly. But all carbs break down to glucose, and circulating glucose will raise you blood insulin levels and take you out of ketosis.
Once you are in dietary (nutritional) ketosis, fatty acids and ketone bodies are used as the major sources of fuel. But, the “balanced diet” establishment says, this could cause a problem for the brain because fatty acids do not cross the blood-brain barrier. Fortunately, the liver uses the fatty acids from the breakdown of triglycerides (both body fat and ingested fat) to make ketone bodies which enter the brain and substitute for glucose. Ketone bodies are actually a more efficient fuel for the brain than glucose. Ketones as brain fuel are also a desired alternate to glucose if the brain has started to develop Insulin Resistance (aka “Type 3” Diabetes). Increasingly, the ketogenic diet is being used as a therapeutic diet for Mild Cognitive Impairment (MCI), aka early-stage Alzheimer’s Disease.