Wednesday, January 18, 2012

Milking Your Bones

Bone Density: The Big Dairy Fallacy

While the National Osteoporosis Foundation tells us we need more calcium to build stronger bones, especially from cow's milk, the scientific evidence does not support this.

The Chinese University of Hong Kong performed successive studies in the 1990s analyzing milk and calcium intake as they relate to the growth of children. This was an ideal place and time for such an investigation, because cow's milk was just making its way into popular use in that country, and the traditional diet was not high in calcium.

The first study looked at children from birth to five years of age. With 90 percent of the study children drinking milk, their average calcium intake was 550 mg. At age five, the current level of calcium intake for each child did not correlate with their bone mineral levels. The calcium intake during the second year of life proved to be the strongest predictor of bone "hardness" at age five (breast milk would provide the same or more advantage as cow's milk or other sources).1

In the second study, seven-year-old children were given calcium supplements to bring their daily calcium intakes up to 800 mg. Over 18 months no increases were seen in height or arm or leg bone density beyond those not supplemented, although some improvement in spinal bone density was seen.2 At ages 12 to 13, calcium intake did not correlate to bone mineral content, except that the girls who consumed the higher levels of calcium had lower bone density in their arms.3

In all these studies, higher weight and greater physical activity were strongly associated with higher bone mineral content. By adolescence, neither calcium intake nor physical activity had any more influence on bone mineral improvement.4 A 14-year British study found that in young adults, their early teenage body weight and physical activity levels determined their bone mineral densities, with a slight effect from calcium intake.5

In a widely quoted study by a British hospital, researchers provided an extra glass of cow's milk to adolescent girls' diets, comparing their growth to those who drank an average of just over one half cup per day. Total daily calcium intake at the beginning of the study was 750 mg, and the extra milk group increased to 1,100 mg. The researchers reported about 10 percent greater bone growth rate for those with the extra milk.6 These children also gained a little more weight, but not height. The reported extra bone density could not be validated by any changes in the blood enzyme markers that typically reflect bone growth and bone re-absorption. Insulin-like growth factor was also found to be higher in the extra milk group.

Several experts wrote replies to this study, which had been paid for by the U.K. dairy industry. One expert, doing his own math from the study, found the milk group to have a slightly lower average total bone mineral content at the end of the study.7

A U.S. study on child twins found some increases in bone density in the arms and spine with calcium supplements (not milk).8 Once puberty began, however, calcium provided no benefits.

While dairy promotions also include praise for the protein of milk as well, this protein may be more of a problem in osteoporosis than calcium could be a solution. The animal proteins of meat and dairy products cause calcium loss.9The level of calcium needed in the diet depends greatly on the animal protein intake.10 For many of the high animal protein diets of Americans, it may not be possible to consume enough calcium in the diet to compensate for the amount lost to these high-acid proteins.11 For this reason, Americans have among the highest osteoporosis rates in the world, while their dairy intake is also among the highest. Doubled animal protein causes 50 percent more calcium loss. Yet, when a high protein intake is soy-based, a positive calcium balance can be maintained with only 450 mg of calcium per day.12

The chief concern over bone density is that it gradually reduces with maturity. At a certain point of bone loss, the term osteoporosis is used. This is a level where low-trauma bone fractures become more frequent. Spinal fractures are a problem, as are hip and arm fractures, which are easier to measure for research purposes. The highest level of bone density attained in young adulthood correlates with bone density maintained in later decades. What is not entirely understood is how much impact dietary factors have on these events. Some studies suggest that childhood calcium intake before puberty may have some slight positive effect. At the same time that diabetes, cancer, and other concerns may limit the amount of dairy that should be given to a child, it also appears that bone protection is no reason to promote dairy consumption beyond childhood.

In 1986 a Harvard researcher produced a graph that demonstrated a nearly direct relationship between calcium intake and hip fractures - the more calcium, the more fractures.13,14

A 1987 study of 106 adult women suggested that calcium intake between 500 and 1,400 mg per day led to no difference in bone mineral densities.15

A larger Italian study found that in women who consumed between 440 and 1,025 mg of calcium per day, a slightly increased number of hip fractures occurred with higher milk intakes.16