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United States Department of Agriculture

Agricultural Research Service

Proteins can play role in bone health
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Jay Cao

Osteoporosis is a disease where bones become spongy and are easily broken. It usually occurs at the onset of old age, when peoples’ lifestyles change significantly. Reduced physical activity, decreased intake of nutrients, increased use of medications such as glucocorticoids or a sudden decline in the female hormone, estrogen, all can contribute to the condition.

Osteoporosis affects at least 10 million Americans and each year, an estimated 1.5 million people suffer an osteoporotic-related fracture. The direct medical cost for osteoporosis is more than $14 billion annually. Postmenopausal women are particularly at risk of developing the disease and to have bone fracture. That’s because during menopause, the production of a hormone called estrogen—which helps prevent bone loss—actually decreases.

Bones need protein

Physical activity and adequate nutrition are two important lifestyle choices in the prevention of many chronic diseases including osteoporosis. Physical activity is the effective way to increase bone formation and decrease bone loss. Nutritionally, the role and requirements of two important nutrients — calcium and vitamin D — for bone health have been established. In contrast, our knowledge on how dietary protein affects calcium balance and bone metabolism is limited and somehow controversial.

Proteins are large, complex molecules that are required for the growth, maintenance and repair of body tissues. They are part of our muscle, internal organs, hair, skin, blood, enzymes and hormones. About half of bone volume or one-third of bone mass is made up of protein. Dietary protein is required for maintaining bone growth and quality. People with low-protein intake usually have low bone mineral density and low intestinal calcium absorption rate.

On the other hand, generally increasing protein intake would increase urinary calcium excretion. That is because protein can generate acid when it is digested and metabolized in the body. It is assumed that bone would release calcium to neutralize the dietary acid and high-protein intake would cause bone to lose calcium. Since protein from meat sources produces more acid than that from vegetable sources, a high-dietary meat protein intake, as commonly seen in Western diets in past several decades, often is considered bad for bones, and limiting meat consumption is recommended by some researchers.

Protein and calcium

With new research data accumulating, scientists start questioning whether high-protein intake affects overall calcium balance and cause bone to lose calcium.

To examine how high-protein diet influences calcium balance, we recently conducted a controlled feeding study at the Grand Forks Human Nutrition Research Center. We recruited 16 postmenopausal women living in our community to eat two diets with different protein levels. One diet was high in protein from beef and other meats with limited fruits and vegetables, and the other was low in meat protein with more fruits and vegetables. The purpose of the study was to investigate whether the acid production from meat and other components in the diets affects calcium balance. The women consumed two diets for seven weeks each. We labeled dietary calcium with radio-isotope and measured calcium absorption with a sensitive whole body counting method.

We found that high-meat protein diet increased calcium absorption although it also increased urinary calcium excretion. The increase in calcium absorption with high meat was nearly as great as the increase in calcium excretion. We also found that the high-meat protein diet increased a blood hormone known to stimulate bone formation (insulinlike growth factor I, or IGF-I). These findings indicate that eating beef and other meats is not detrimental but may be beneficial to bone health.

Many epidemiological studies have found long-term, high-dietary protein actually improves bone mineral density. Considering about 15 percent to 38 percent of adult men and 27 percent to 41 percent of adult women in the United States have dietary protein intake lower than the current Recommended Dietary Allowance, maintaining adequate protein intake is essential to bone health. Elder people would benefit the most because they usually don’t have enough protein intakes.

Key to bone health

In a cross-sectional study, researchers in the UK examined the effects of diets on bone health in more than 14,500 men and women. They found that the acid production of the diet (protein) had only minimal negative effect on bone. Other factors, such as being old, low body weight, smoking, physical inactive or decline in female hormone (estrogen), are more risky than protein intake in affecting bone health.

For many Americans, the benefit of protein for human health including bone health is far greater than it has on urinary calcium excretion. It is no reason to reduce the intake of protein or meat consumption simply because protein increases calcium excretion in the urine. Keep in mind that being physically active and eating a balanced diet, as recommended in the U.S. Department of Agriculture sponsored MyPyramid, are two important keys to a better bone health.

Last Modified: 8/17/2009
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