In the graph of the metabolism of glucose, protein, and fat on the left, you will find all three nutrients are connected to each other in some forms and biochemical reactions. After absorbed by the small intestine, fats (triglycerids, diglycerides, momoglycerides, fatty acids, and cholesterol) are sent to fatty tissue and others where they are wanted. Except cholesterol, the final form of fats in the fatty tissue is triglycerides. When the body needs more energy in cases of either restriction of carbohydrates or starvation, the body breaks down fats for fuel. This breakdown is gluconeogenesis (gluco-neo-genesis) to generate or produce “new glucose.” Therefore, the “new glucose” supports a stable blood glucose level even in cases of starvation and carbohydrate restriction. However, not all the triglycerides will become glucose. First, breaking triglycerides down to glycerol and fatty acids. Glycerol will not become glucose. Fatty acids are composed of four or more carbons, in addition to hydrogen and oxygen atoms. Only the fatty acids with even number of carbons will be converted into glucose. The rest of fats can be used to produce energy through Tricarboxylic Acid Cycle (TCA Cycle) if there is a minimum amount of oxaloacetate (a derivative from glucose.) If not, they will become beta-hydroxybutyrate, which are ketones. The brain, muscles, and heart can use beta-hydroxybutyrates for energy production. Some reports said that beta-hydroxybutyrates are 25% more efficient in comparison to glucose in those tissues.On the other hand, after absorbed by the small intestine, the smallest unit of proteins – amino acid is sent to the tissues to become proteins (anabolism.) When the body needs more fuel in case of either restriction of carbohydrates or starvation, the body breaks down proteins (catabolism) into amino acids, which subsequently being use to produce glucose for producing energy. When converting protein or amino acids into glucose for producing energy, there is a energy loss of one kilocalorie per gram. Thus, replacing carbohydrates with proteins, gram for gram, promotes weight loss.Therefore, the statement that all fats and proteins “result in glucose” is not true.
Robert Su, Pharm.B., M.D.
Note: This is a reply to a question about metabolism of nutrients, originally published in the middle of January 2009.
