What is Ketogenic Diet?
When carbohydrate intake is less than 50 g per day, ketosis provides the brain and skeletal muscles with an alternate energy source in the form of ketones derived from lipolysis (the breakdown of fat). Ketones are believed to improve satiety (suppress appetite), at least initially.
Low-carbohydrate and ketogenic diets provide rapid initial weight loss from diuresis secondary to the carbohydrate restriction; early weight loss may be ≥60% water. This diuretic effect is a result of depleted liver and muscle glycogen which holds three to four times its weight in water.
In a classic energy-nitrogen balance method study comparing an 800-calorie ketogenic diet with an 800-calorie mixed diet, subjects did lose weight more rapidly at the beginning of the ketogenic diet period, however the extra weight loss was due solely to excess water losses. Both diets led to the same amount of body fat and protein (LBM) losses . A recent much shorter version of this type of study by the NIH also found no advantage to the ketogenic diet.
The impact of a ketogenic diet on the microbiome and overall nutrient intake are two areas of concern. A recent study on the effect of a ketogenic diet on the gut microbiota found a bacterial group supposed to be involved in the exacerbation of the inflammatory condition of the gut mucosa associated with the ketogenic diet pattern. The recommended upper limit for dietary fat is 35% of calorie intake per the dietary reference intakes, which are intended to ensure adequate micronutrients and shield against preventable diseases.
High-protein (rather than high-fat) variants of low-carbohydrate and ketogenic diets include the Zone and South Beach Diets, which restrict carbohydrates to no more than 40% of total calories, with fat and protein each providing 30% of total calories. These diets are considered moderate choices within the low-carbohydrate category and include generous amounts of fiber and fresh fruits and vegetables, and they stress the kind of fat, with emphasis on monounsaturated and polyunsaturated fat and limitation of saturated fat. For more information about a ketogenic diet and the conditions for which it has been studied.
Unanswered questions about the ketogenic diet include:
• What are the long-term (1 year or longer) effects, is it safe?
• Do the diet’s health benefits extend to higher risk individuals with multiple health conditions
and the elderly? For which disease conditions do the benefits of the diet outweigh the risks?
• As fat is the primary energy source, what is the effect of such a high-fat diet that includes so much saturated fat?
• Is the high fat, moderate protein intake on a ketogenic diet safe for disease conditions that
interfere with normal protein and fat metabolism, such as heart, kidney, and liver diseases?
• Is a ketogenic diet too restrictive for periods of rapid growth?
• Is the ketogenic diet safe and is it effective for athletes?
The carbohydrate-insulin theory of obesity is the foundation of low-carbohydrate and ketogenic
diets. The basic theory is that carbohydrates stimulate insulin secretion causing increased fat storage, which increases appetite and suppresses metabolism resulting in weight gain. Low carbohydrate intake does decrease insulin secretion. The insulin theory however, only describes postprandial energy metabolism while ignoring the rest of the 24-hour energy metabolism picture. Insulin levels don’t remain elevated, and overnight — in the fasting state — fat oxidation increases, reducing fat stores. A net gain in fat stores only occurs with positive energy balance.
Recent carefully controlled metabolic laboratory studies appear to have invalidated the insulin theory of obesity. A recent systematic review of high quality randomized controlled trials (RCTs) comparing low-carbohydrate with isoenergetic (having the same total calories) balanced diets found essentially no difference in weight loss, measures of glycemic control, blood pressure, or blood lipid between the two diets .
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