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Educate your patients on the importance of 3-A-Day of Dairy: Here's
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every day for stronger bones.
Developed in conjunction with The American Academy of Family
Physicians, The American Academy of Pediatrics, The American Dietetic
Association, and The National Medical Association.
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Dairy Council Digest Archives
Dietary Protein & Bone Health: New Perspectives
Protein's Effect on Bone Health
Protein is an important structural component of bone. It is incorporated into the organic matrix of bone for collagen formation, upon which mineralization occurs. By weight, bone tissue is 70% mineral (primarily calcium and phosphorus), 22% protein, and 8% water (9). Although dietary protein has been alleged to adversely affect bone health, this effect remains unclear. Furthermore, many, but not all, epidemiological studies point to a beneficial role of dietary protein in bone health.
Is Excess Protein Harmful to Bones? Accumulating findings from epidemiological studies indicate that high protein intakes – intakes within the range commonly consumed – favorably affect bone health (i.e., decrease bone loss or fracture risk) (21-25). In a three-year prospective study of more than 30,000 postmenopausal women who participated in the Iowa Women's Health Study, higher protein intake, particularly animal protein, was associated with a 70% reduction in hip fractures, even after controlling for major confounding variables (21). Similarly, when dietary protein (mostly animal protein) and bone mineral density were examined in a four-year follow-up study of older men and women from the Framingham Osteoporosis study, higher protein intake was associated with increased bone mineral density of the femoral neck and spine (22). Also, dietary protein and total hip bone mineral density were positively associated in a study of postmenopausal women in the NHANES III survey (23).
Accumulating evidence supports a beneficial role for protein in bone health. This is supported by findings in older adults that low protein diets are harmful to bones.
High intakes of total and animal protein were significantly associated with increased bone mineral density of the hip and total body in older women, but not in older men, in a four-year prospective study of 960 participants of the Rancho Bernardo study in California (24). In contrast to the beneficial effect of animal protein, higher intake of vegetable protein was significantly associated with decreased bone mineral density at the hip, femoral neck, and spine in both men and women (24). When the relationship between intake of various nutrients and bone mineral density was examined in a cross-sectional study of136 healthy postmenopausal women, intake of protein was positively associated with bone mineral density/content of the total body, hip, and hand (25).
Not all epidemiological studies support a beneficial effect of protein on bone health. For example, in a cohort of the Nurses' Health study, postmenopausal women who consumed more than 95g protein/day were at a higher risk of forearm fractures than women whose intake was less than 68g protein/day (26). A similar association was found for animal protein, but there was no association between total protein intake and risk of hip fractures (26). Tufts University researchers recently found that the association between protein intake and bone mineral density in older adults depended on calcium intake (27).
Protein intervention studies also support a beneficial effect of dietary protein on bone health (28,29). In a six-month randomized, double-blind, placebo-controlled trial, older adults with a hip fracture and low usual protein intake all received calcium and vitamin D, and then either a protein supplement (20g/day) or a placebo (28). Compared to the placebo group, patients given the protein supplement had less bone loss, improved muscle strength, and reduced length of stay in a rehabilitation hospital (28). In another six-month randomized, placebo-controlled dietary intervention study, no adverse effect on bone mineral density was found in overweight or obese adults who consumed a diet providing108g protein/day (based on low fat dairy products and lean meat) compared to those who consumed a diet containing 70g protein/day during weight loss (29). The high protein diet may have helped to preserve bone mineral content during weight loss (29).
Although the mechanism(s) is unknown, protein's favorable effect on bone may be explained by its ability to provide the necessary substrates for bone formation and remodeling (11). Another possibility is that dietary protein may increase the production of insulin-like growth factor-1 (IGF-1), a hormone that stimulates bone formation (28,30a,31-33). In some studies, dietary protein has been demonstrated to increase serum IGF-1 in subjects fed food sources or supplements of protein (28,30a,31,32), however, another study found no association between dietary protein and IGF-1 (27). Furthermore, reduced IGF-1 has been associated with reduced bone breaking strength in rats and increased fracture risk in humans (11,34).
Inconsistencies regarding protein's role in bone health may be explained by the presence of other dietary components. For example, adequate dietary calcium may allow protein to have a positive effect on the skeleton.
Is A Low Protein Diet Harmful to Bones? Epidemiological studies have associated low protein intakes in older adults with low bone mineral density, high rates of bone loss, and increased risk of hip fractures (21-24,35,36). A low protein diet was significantly associated with low hip bone mineral density in non-Hispanic white women participating in the NHANES III survey (23). Similarly, low levels of protein were significantly associated with higher rates of bone loss at the hip and spine in elderly women in the Framingham Osteoporosis study (22). Increased risk of hip fractures was found in older women in the Iowa Women's Health study who consumed the lowest amounts of protein (21). In a recent study of 225 postmenopausal women in Minnesota who were followed for an average of 16 years, a decrease in both total and animal protein was associated with a 44% increased risk of all osteoporotic fractures (36). In this same study, a decrease in calcium intake was associated with a 29% increase in all osteoporotic fractures (36).
The mechanism(s) by which low dietary protein adversely affects bone health is unknown. University of Connecticut researchers, based on a series of short-term experiments, suggest that low protein diets may induce a reduction in intestinal calcium absorption that is accompanied by secondary hyperparathyroidism, which increases bone remodeling, particularly bone resorption (12,37-39). However, protein's effect on calcium absorption may be transient. Other studies have found no effect of dietary protein on intestinal calcium absorption (20,27,35,40).
What About the Type of Protein? Inconsistencies regarding protein's effects on bone health have raised the question of whether or not the type of protein influences the findings. However, there is no overall pattern regarding the effect of animal versus vegetable protein on bone health (21,22,24,27,40,41). These inconsistencies may be explained by other constituents in foods and in the diet or, in particular, the synergy among nutrients which may have a greater impact on protein's effect on bone than the type of protein (10).
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