To be clear, the transformation between sugar and fat is two-way, but the degree of transformation is different. Sugar can form fat in large quantities. For example, when yeast is cultured in sugary medium, fat can be produced in cells, and the fat synthesized by a single species of yeast can be as high as 40% of the dry weight of yeast. However, fat cannot be converted into sugar in large quantities. For example, when some animals hibernate, fat can be converted into sugar.

Relationship between carbohydrate metabolism and protein metabolism

Firstly, the concepts of essential amino acids and non-essential amino acids should be clarified: the so-called non-essential amino acids refer to amino acids that may be synthesized in human cells; The so-called essential amino acids refer to amino acids that can't be synthesized in human cells, or the synthesis speed can't meet the needs of human body and must be ingested from food. There are eight kinds of essential amino acids, namely lysine, tryptophan, phenylalanine, leucine, isoleucine, threonine, methionine and valine.

The transformation between sugar and protein can also be bidirectional: the intermediate products of sugar metabolism can be transformed into non-essential amino acids, but sugar cannot be transformed into essential amino acids, so the process of transforming sugar into protein is not comprehensive; However, almost all natural amino acids that make up protein can be converted into sugar after deamination. For example, if dogs with artificial diabetes are fed with protein, more than 50% of protein can be converted into glucose.

Relationship between protein metabolism and lipid metabolism

The transformation between protein and lipids varies from organism to organism. For example, it is not easy for humans and animals to synthesize amino acids from fat, while plants and microorganisms can produce amino acids from fatty acids and nitrogen sources. Some amino acids can also be converted into glycerol and fatty acids in different ways. For example, by feeding animals food containing only protein, animals can accumulate fat in their bodies.

The metabolism of sugar, protein and lipid are mutually restricted.

Sugar can be converted into fat in large quantities, but fat can't be converted into sugar in large quantities. Only when sugar metabolism is disturbed can energy be supplied by fat and protein. When the intake of sugar and fat is insufficient, the decomposition of protein will increase. For example, when the glucose metabolism of diabetic patients is impaired, the decomposition function is completed by fat and protein, so the patient is emaciated.

The three nutrients are abundant in nature, especially in human body, and their basic functions in life are the guarantee of maintaining normal life functions.