The liver is unique, however, in that most of the acetyl coenzyme A it forms from fatty acids during the post-absorptive state does not enter the Krebs cycle (a series of enzyme catalyzed reactions that reduce the acetyl portion of acetyl coenzyme A in the mitochondrial matrix) but is processed into three compounds collectively called ketones, or ketone bodies. Ketones are released into the blood and provide an important energy source during prolonged fasting for the many tissues, including those of the nervous system. One of the ketones is acetone, some of which is exhaled and accounts in part for the distinctive breath odor of individuals undergoing prolonged fasting.
The net result of fatty acid and ketone utilization during fasting is the provision of energy for the body while at the same time sparing glucose for the brain and nervous system. Moreover, as just emphasized, the brain can use ketones for an energy source, and it does so increasingly as ketones build up in the blood during the first few days of a fast. The survival value of this phenomenon is significant: When the brain reduces its glucose requirement by utilizing ketones, much less protein breakdown is required to supply amino acids for gluconeogenesis. Consequently, the protein stores will last longer, and the ability to withstand a long fast without serious tissue damage is enhanced.
Widmaier, E.P., & Raff, H. (2008). Vander’s Human Physiology: The Mechanisms of Body Function. New York, NY: McGraw-Hill.
Weight loss in low-carbohydrate diet group and low-fat diet group resulted predominantly from reduced energy intake. However, the method of reducing energy intake differed greatly. The low-fat diet group received counseling to restrict intake of fat, cholesterol, and energy, whereas the low-carbohydrate diet group received counseling to restrict intake of carbohydrates but not energy. The voluntary reduction in energy intake among recipients of the low-carbohydrate diet merits (值得) future research. These participants may have restricted intake because of limited food choices, or the low-carbohydrate diet may have appetite suppressant properties. Other possible explanations for the discrepancy in weight loss between groups include loss of energy through ketonuria and the increased thermic (thermal) effect of a high-protein diet.
With regard to the composition of weight loss, both groups lost predominantly fat mass over 24 weeks, and the percentage of total weight loss that was fat was similar in both groups. The low-carbohydrate group lost a greater amount of water in the first 2 weeks than did the low-fat diet group; this finding confirms report of diuresis (尿多) with the low-carbohydrate diet. After the first 2 weeks, however, estimations of total body water were similar in the low-carbohydrate diet group and the low-fat diet group. Moreover, the changes in fat-free mass in both groups were largely explained by changes in total body water, not lean tissue mass.
Perhaps the biggest concern about the low-carbohydrate diet is that the increase in fat intake will have detrimental (有害的) effects on serum lipid levels. We found that the LDL cholesterol level did not change on average but did increase by more than 10% from baseline to week 24 in 30% of recipients of the low-carbohydrate diet who completed the study. Because the low-carbohydrate diet may adversely affect the LDL cholesterol level, it is prudent (審慎的) to monitor the serum lipid profiles of followers of this diet.
The changes in body weight, blood pressure, and serum lipid levels that we observed suggest that research may be warranted on the effects of the low-carbohydrate diet in patients with the metabolic syndrome, which is characterized by increased blood pressure, hypertriglyceridemia, low HDL cholesterol levels, abdominal adiposity, and insulin resistance. We did not measure insulin sensitivity, but previous studies of the low-carbohydrate diet have shown that serum glucose and insulin levels decrease.
Recipients of the low-carbohydrate diet reported symptomatic adverse effects more frequently than did recipients of the low-fat diet. Symptomatic adverse effects that typically occur at initiation of a low-carbohydrate diet (for example, weakness, orthostatic hypotension, headaches, constipation, and muscle cramps) are short-lived and may be reduced by copious (大量的) fluid intake, consumption of the allowed amounts of vegetables, bouillon (清湯), and a daily multivitamin and mineral supplement.
It is also possible that the nutritional supplements helped to prevent potential adverse effects of the low-carbohydrate diet. For example, a recent study suggests that this type of diet at a daily energy intake of 2000 kcal may increase the risk for kidney stones. Citric acid contained in the supplements may have helped to prevent the formation of kidney stones.
Olsen, M. K. (2004). A Low-Carbohydrate, Ketogenic Diet versus a Low-Fat Diet To Treat Obesity and Hyperlipidemia (A Randomized, Controlled Trial). Annals of Internal Medicine. https://doi.org/10.7326/0003-4819-140-10-200405180-00006
