Although vegetarian diets are considered generally protective against chronic disease, nutrient deficiencies, including protein, are possible due to low bioavailability from plant-based sources. The consequences of inadequate dietary protein include reduced lean body mass (LBM) and muscle weakness. This study examined relationships between protein intake, strength, and LBM in 37 underactive vegetarians and recorded the impact of protein supplementation (18 g/day mung bean protein) on these indices utilizing an eight-week, randomized, controlled, feeding trial. Both handgrip and knee flexor and extensor strength were measured at baseline and week eight. At baseline, LBM was significantly related to grams of protein consumed daily. LBM was also correlated to grip strength (r = 0.569, p < 0.001) and lower body strength (r = 0.763 to 0.784; p < 0.001). Twenty-five vegetarians completed the feeding trial, including 11 in the protein supplementation group (PRO) and 14 in the control group (CON). At the end of the trial, LBM and strength did not differ significantly between groups. However, the average percent change for grip, flexor, and extensor strength did differ between PRO and CON participants (+2.9 ± 7.2% and −2.6 ± 7.3% respectively, p = 0.05). Thus, there were strong associations between dietary protein, LBM, and strength in vegetarians and an indication that supplementary vegetarian protein increased strength in the absence of exercise and independent of LBM.

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Vegan diets have gained popularity in recent years for reasons including health benefits and concerns for animal welfare. Although these diets are considered to be nutritionally adequate, questions remain over whether the current protein recommendation (0.8 g/kg/d) is sufficient. Protein status is determined through a nitrogen balance analysis when the protein content of the diet is known. A negative balance indicates a catabolic state, and a positive nitrogen balance indicates an anabolic state. In healthy adults, nitrogen equilibrium is the expectation reflecting the net synthesis and breakdown of proteins. Currently, there are no known studies measuring nitrogen balance in strict vegan men fed the protein requirement. Eighteen minimally active vegan men received a 5-day eucaloric diet (protein content: 0.8 g/kg/d). On day five, 24 h urine was collected for nitrogen analysis. Both the mean absolute nitrogen balance (−1.38 ± 1.22 g/d) and the mean relative nitrogen balance (−18.60 ± 16.96 mg/kg/d) were significantly lower than zero (equilibrium) (p < 0.001). There were no correlations seen between nitrogen balance and age, years as vegan, or fat-free mass. Consuming 0.8 g/kg/d of protein is not adequate to produce nitrogen balance in men adhering to typical strict vegan diets for at least one year.

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