Low protein intake: the impact on calcium and bone homeostasis in humans.
Increasing dietary protein results in an increase in urinary calcium. Despite over 80 y of research, the source of the additional urinary calcium remains unclear. Because most calcium balance studies found little effect of dietary protein on intestinal calcium absorption, it was assumed that the skeleton was the source of the calcium. The hypothesis was that the high endogenous acid load generated by a protein-rich diet would increase bone resorption and skeletal fracture. However, there are no definitive nutrition intervention studies that show a detrimental effect of a high protein diet on the skeleton and the hypothesis remains unproven. Recent studies from our laboratory demonstrate that dietary protein affects intestinal calcium absorption. We conducted a series of short-term nutrition intervention trials in healthy adults where dietary protein was adjusted to either low, medium or high. The highest protein diet resulted in hypercalciuria with no change in serum parathyroid hormone. Surprisingly, within 4 d, the low protein diet induced secondary hyperparathyroidism that persisted for 2 wk. The secondary hyperparathyroidism induced by the low protein diet was attributed to a reduction in intestinal calcium absorption (as assessed by dual stable calcium isotopes). The long-term consequences of these low protein-induced changes in calcium metabolism are not known, but they could be detrimental to skeletal health. Several recent epidemiological studies demonstrate reduced bone density and increased rates of bone loss in individuals habitually consuming low protein diets. Therefore, studies are needed to determine whether low protein intakes directly affect rates of bone resorption, bone formation or both.
The subject is complicated with a lot of conflicting data. The full article is great.
When BMD is the primary outcome, most (39–48), but not all (49–53), epidemiological studies show a positive relationship between protein intake and BMD. Stated another way, most of the epidemiological evidence shows that when other known dietary factors are controlled, those individuals who consume low protein diets have lower BMD. Using the National Health and Nutrition Examination Survey (NHANES) III database, we found that in 1882 non-Hispanic white women 50 y old and older, after adjusting for age and body weight, a low protein intake was associated with a significantly lower hip bone mineral density (Fig. 5) (47). Consistent with these data, Hannan and colleagues (46) studied 615 participants in the Framingham Osteoporosis Study over a 4-y period and found that lower levels of protein intake were associated with significantly higher rates of bone loss at the hip and spine. These findings confirm the earlier work of Freudenheim et al. (39), who reported that a low protein intake was associated with greater loss in bone density from the wrist in 35- to 65-y-old women. Most recently, Promislow et al. (48) found a positive association between total dietary protein intake and BMD in elderly men and women participating in the Rancho Bernardino study. Therefore, there is substantial agreement in those studies in which BMD is the primary outcome. Munger et al. (59), reporting data from the Iowa Women’s Health Study, found an increased risk of hip fracture in 55- to 69-y-old women consuming the lowest amounts of protein.
Adequate dietary protein may also help in fracture healing and in preventing bone loss after fracture. Bonjour and colleagues (73) studied the effects of 6 mo of protein supplementation, after osteoporotic hip fracture, in a group of elderly subjects. These patients had self-selected protein intakes that were very low (∼40 g). The administration of additional protein (+20 g) was associated with significant attenuation of proximal femur bone loss in the fractured hip such that, at 1 y, bone loss rates were 50% lower in the protein-supplemented individuals. The correction of poor protein nutrition also improved serum prealbumin and insulin-like growth factor 1 (IGF-1) concentrations and decreased the length of rehabilitation (73).
There is agreement that diets moderate in protein (in the approximate range of 1.0 to 1.5 g protein/kg) are associated with normal calcium metabolism and presumably do not alter skeletal homeostasis. Approximately 30–50% of adults in the United States consume dietary protein that could be considered moderate. At low protein intakes, intestinal calcium absorption is reduced, resulting in increases in serum PTH and calcitriol that persist for at least 2–4 wk. The long-term implications of these findings are unknown; however, recent epidemiological data suggest increased rates of bone loss in individuals consuming such diets. Individuals consuming high protein intakes, particularly from omnivorous sources, develop hypercalciuria that is attributable for the most part to an increase in intestinal calcium absorption. Whether high protein diets result in an increase in bone resorption and higher fracture rates remains uncertain.