Age and physical activity have profound effects on the accrual and retention of lean body tissues, but a range of nutritional factors can influence this process. Inadequate protein and energy intake can lead to malnutrition and wasting of muscle reserves.

In healthy populations, other nutritional influences may impart significant differences in the relative amounts of lean tissues. One factor that appears to be a significant contributor is the acidotic and alkaline effects of foods.

It has been well documented that renal populations with acidosis suffer increased catabolism of muscle stores, and this acidosis can be influenced by a diet that contains a higher potential renal acid load (PRAL). The PRAL is a calculation that takes into account the intake of protein, phosphorus, potassium, calcium, and magnesium and estimates the mEq acid load that the body incurs.

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Protein intake increases the PRAL primarily through the acidogenic effect induced by the metabolism of sulfur-containing amino acids. Phosphorus also contributes to an increased PRAL. The potassium, calcium, and magnesium cations are often bound to alkaline-inducing anions such as citrate and bicarbonate, which decrease the PRAL. Fruits and vegetables are the highest sources of these cations, whereas animal products and cereals contribute to the acidogenic factors.

A recent study by Welch AA et al reviewed data from 2,689 female twins from the UK Adults Twins Registry (Osteoporos Int 2012; published online ahead of print), including information obtained via food frequency questionnaires, physical activity levels, smoking status, and dual-energy X-ray absorptiometry scans analyzing body composition. PRAL was estimated based on the results of the food frequency questionnaire.

The population ranged in age from 18-79 years and 50% of the subjects were older than 50. The groups were stratified into PRAL quartiles. As expected, higher protein intakes correlated with increased PRAL and increased potassium and magnesium intakes correlated with a lower PRAL. Meat, fish, and cereals were significantly associated with an increased PRAL, while fruits and vegetables were associated with a reduced PRAL. When analyzing potassium specifically, the primary food contributions included fruits and vegetables, dairy products, and potatoes.

In addition, fat free mass (FFM) was significantly greater in those consuming a lower PRAL after adjusting for age, physical activity, smoking, fat mass, and energy intake to estimated energy expenditure ratio. This association continued after adjusting for protein intake as a percentage of total calories consumed. Similar results were found for FFM% and fat free mass index (FFMI). This translated into a difference of 1.3% and 1.9% difference in FFM% and FFMI, respectively, when comparing the lowest PRAL quartile to the highest. Percentage protein intake was negatively associated with FFM%. Notably, the average protein intake of the cohort was 1.26 g/kg/day compared with the recommended dietary allowance (RDA) recommendations of 0.8 g/kg/day.

This intake may suggest that an elevated intake of protein without metabolic need may induce reduction in FFM through increased PRAL. Stratifying the data into quartiles based on the ratio of fruits and vegetables to proteins and cereals, a significant and positive association was found between an increased ratio, such that the difference between the lowest and highest ratio were 1.05 kg FFM, 0.67% FFM%, and 0.32 kg/m2 FFMI.

It would appear from these data that, although stronger factors such as physical activity, smoking, and age exist, increased fruits and vegetables can not only promote a more favorable body composition, but also an overall greater content of lean body mass. The mean protein intake was noticeably higher than the RDA, but this intake is typical in Western nations.

Protein is generally regarded as crucial in helping to increase lean body mass, but this factor may only be true until nitrogen requirements have been met. The study may suggest that once general nitrogen needs have been met, endogenous acid production begins to outweigh the benefit unless it can be neutralized with increased fruits and vegetables.

A team of investigators recently used data from 145 participants in the DONALD Study to analyze associations between PRAL and non-alcoholic fatty liver disease (J Nutr 2012;142:313-319). The DONALD Study is a prospective cohort study that has been gathering dietary and other data from patients beginning in infancy starting in 1985. Among the female subjects, PRAL was significantly associated with alanine-aminotransferase (ALT), hepatic steatosis index (HSI), and fatty liver index (FLI) in multivariate regression models.

These results remained consistent after adjusting for age, sex, intake of fiber/kcal, saturated fat/kcal, protein/kcal, carbohydrates/kcal, body mass index (BMI), and waist circumference. Subjects with an adolescent PRAL in the highest tertile had 3.5, 4.4, and 4.5 higher values of ALT, HSI, and FLI, respectively, as adults compared with those with the lowest PRAL. These same associations were not found in men.

The results of these two studies indicate that excessive endogenous acid production and subsequent metabolic effects may be related to negative effects on liver health and fat free mass. In each study, the correlations were still found to be significant even when adjusting for factors such as BMI, waist circumference, energy intake, etc. that would suggest general excessive energy intake was to blame. These results further promote the concept that plant-based diets offer a multitude of health benefits beyond caloric density alone.