After a few days of fasting, or of drastically reduced carbohydrate consumption (below 20 grams a day), glucose reserves become insufficient for the supply of glucose to the central nervous system (CNS). The CNS cannot use fat as an energy source; hence, it normally utilizes glucose. After three to four days without carbohydrate consumption the CNS is ‘forced’ to find alternative energy sources, and this alternative energy source is derived from the overproduction of acetyl coenzyme A (CoA). This condition, seen in prolonged fasting, type 1 diabetes and high-fat/low-carbohydrate diets, leads to the production of higher-than-normal levels of so-called ketone bodies (KBs), that is, acetoacetate, b-hydroxybutyric acid and acetone. This is a process called ketogenesis, and it occurs principally in the mitochondrial matrix in the liver.
Weight loss: There is conflicting evidence on the exact mechanisms of weight loss from ketosis. Some researchers suggests an improved metabolic advantage of going low carb and its effects on metabolism. The majority of evidence, however, suggests there is no advantage of macronutrient composition and instead suggests the induced weight loss is governed more so by the law of thermodynamics. Nonetheless, most people who undergo ketosis have a suppressed appetite that can reduce calories and hence lead to weight loss.
Therapeutic: There has been strong evidence to suggest that ketogenic diets have a beneficial effect on cardiovascular risks, epilepsy and type 2 diabetes. Ketogenic diets may help alleviate symptoms of such diseases, but ultimately, lifestyle and exercise will also have a role to play.
Athletes who under-go high-intensity exercise may wish to avoid going keto. Carbohydrate is the body’s preferred fuel source at exercise capacity above 60% (Romijn et al, 1993). If an individual decides to follow a high-fat diet and undergo high-intensity exercise, the duration at which they compete at may be compromised due to insufficient supplies of glucose. Indeed, it’s been highlighted that although these iterations of high(er) fat, low(er) CHO diets result in increased rates of fat oxidation during exercise of varying intensities, evidence that this substrate shift translates to a clear enhancement of sports performance in athletic populations is lacking (Burke & Kiens, 2006; Burke, 2015).
Indeed, it’s also shown to aid in weight loss; however, a large majority of people fail to adhere to this diet in the long term. Remember, there is no perfect diet for weight loss, I’m not promoting or downplaying any weight-loss diet. The best diet for weight loss is one where there is a caloric deficit and you can adhere to it.
Steve O'Mahony BSc MSc
Romijn JA, Coyle EF, Sidossis LS, Smalley KJ, Polansky M, Kendrick ZV, et al. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol 1993;265:E380e91.
Burke LM & Kiens B (2006). “Fat adaptation” for athletic performance: the nail in the coffin? J Appl Physiol 100, 7–8
Burke LM (2015). Re-examining high-fat diets for sports performance: Did we call the ‘nail in the coffin’ too soon? Sports Med 15 Suppl 1, S33–49