Low-protein diet

A low-protein diet is a diet in which people reduce their intake of protein. A low-protein diet is prescribed for those with inherited metabolic disorders, such as Phenylketonuria and Homocystinuria and reduced protein levels have been used by people with kidney or liver disease. Low protein consumption appears to alter the risk of bone breakage, presumably through changes in calcium homeostasis. Consequently, there is no uniform definition of what constitutes low-protein, because the amount and composition of protein for an individual suffering from phenylketonuria would differ substantially from one suffering homocystinuria. The amount used by those with liver disease would still result in individuals being in nitrogen balance.

Amino acids that are excess to requirement cannot be stored, but must be modified by deamination (removal of the amine group). As this occurs in the liver and kidneys, some individuals with damaged livers or kidneys may be advised to eat less protein. Due to the sulphur content of the amino acids methionine and cysteine, excess of these amino acids leads to the production of acid through sulphate ions. These sulphate ions may be neutralized by calcium ions from bone, which may lead to net urinary loss of calcium. This might lead to reduction in bone mineral density over time. Individuals suffering from phenylketonuria lack the enzyme to convert phenylalanine to tyrosine so low levels of this amino acid need to be provided in the diet. Homocystinuria is an inherited disorder involving the metabolism of the amino acid methionine leading to the accumulation of homocysteine. Treatment includes providing low levels of methionine and high levels of vitamin B6 in the diet.

Low-protein diets are in vogue among some members of the general public because of the impact of protein intake on Insulin/Insulin-like growth factor 1 Signalling (IIS) and the direct sensing of amino acid availability by mammalian target of rapamycin (mTOR), two systems that are implicated in longevity and cancer proliferation. Apart from low protein intake, such as in the 80:10:10 diet, other attempts to modulate IIS are through intermittent fasting and the 5:2 diet.

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