As you know, humans do not live by glucose alone, we need other nutrients we need to run our body normally. Accordingly the cells of the human body seethe with biochemical reactions, synthesizing one amino acid from another, making fats from carbohydrates, and channeling surplus organic molecules of all types into energy storage or release. There are two kinds of metabolic transformation in discuss the term of energy relate to the diet. They are the production of ATP from fats and proteins, and the synthesis of fats from high calorie foods.
First about the production of ATP from fats and proteins, we should reconsider first that even the leanest person has some fat in his body. During fasting or starvation, the body mobilizes these fats reserves for ATP synthesis, because even the bare maintenance of life requires a continous supply of ATP and seeking out new food sources demands even more energy. Fat metabolism flows directly into the pathways of glucose metabolism.
In fat metabolism, the bonds between the fatty acids and glycerol are hydrolyzed (broken into subunits by the addition of water in the reaction). The glycerol which is part of a fat feeds directly into the middle of glycolisis pathway after its activation by ATP. Then the fatty acids are transported into the mitochondria where enzymes in the inner membrane and matrix chop them up into acetyl groups. These groups attach to coenzyme A to form acetyl CoA which then enters the Krebs cycle.
Second, our body could also synthesis fats from sugar. The body in addition to having developed ways of coping with fasting or starvation has also evolved strategies for coping with situations in which food intake exceeds current energy needs. The sugar and starch in corn flakes, candy bars, or the nectar of flowers can be converted into fats for energy storage. High calorie meals or Complex sugar such as starch and sucrose are first hydrolyzed into their monosaccharide subunits. Then they are broken down to pyruvate and converted to acetyl CoA. If the cell needs ATP, the acetyl CoA will enter the Krebs cycle. If the cell have plenty of ATP, acetyl CoA will be used to make fatty acid by a series reactions that are essentially the reverse of fatty acids breakdown. In human, the liver synthesizes fatty acids, but fat storage is relegated to fat cells under human skin (subcutan layer), with their all-to-familiar distribution in the body particularly around the waist and the hips.
Energy use, fat storage, and nutrient intake are usually precisely balanced. Where the balance point lies, however varies from person to person. Some people seem able to eat nearly continuously without ever storing much fat, other people crave high calorie foods even when they have a lot of stored fat.