Protein deficiency is particularly common in developing countries because animal protein is expensive as so is often substituted in the diet for cheaper vegetable based protein sources. However, because vegetable sources of protein are of lower quality, the protein intake often falls as a consequence. The protein intake required to maintain positive nitrogen balance will depend on the individual, with factors such as age and activity level playing a part. Insufficient protein intake in the elderly is often a concern because normal calorie and protein intakes decline with age and this can be exacerbated by poverty. Negative nitrogen balance can cause a decrease in lean body mass and depress the immune system and this can increase the risk of mortality. Nitrogen balance studies have therefore been used by researchers to determine the amount of protein required in order to maintain health.
For example, researchers1 have investigated the effects of varying protein levels in a 9 week study of 12 elderly subjects. The subjects were fed either a weight maintenance diet containing 1.47g protein per kg body weight, or the same diet with 2.94g protein per kg body weight. The low protein diet caused the nitrogen balance to remain negative throughout the study and this caused a significant loss of lean body tissue, muscle strength and muscle function. Mean body cell mass decreased by 8% in the low protein group, but remained unchanged in the control group. Both groups had a 6% decrease in lean body mass, but the low protein group then lost a further 8% that was not seen in the high protein group. In addition, immune function as measured by a skin antigen test declined by 50% in the low protein group, but increased by 47% in the high protein group.
These results suggest that a low protein diet causes detrimental changes in muscle mass, muscle function and immunity in elderly women. This study was relatively short-term but the results support other studies that have created a negative nitrogen balance in individuals. Over time, adaptation does occur to low protein diets but a longer-term study would be needed to show this process. Studies have shown that initial feeding of low protein diets to individuals results in negative nitrogen balance, but after 30 days nitrogen balance is restored if the protein restriction is not too severe. Determining the amount of protein required to prevent negative nitrogen balance is difficult because biochemical individuality and external factors can change the daily requirement. Current recommendations are to ingest 0.8g protein per kg body weight, an intake below the one in this study.
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