Vitamin D: too much of a useful thing.

The adage that too much of a good thing can prove hazardous was underscored this week by a scientific report linking a spate of vitamin D poisonings to milk. A second report suggests that manufacturers routinely add too much or too little vitamin D to milk and infant formula.

Vitamin D is essential for the formation of strong bones. In children, a deficiency of this nutrient causes rickets, a deforming disease of the bone. In the United States, milk has been fortified with vitamin D since the 1930s, a policy that has greatly reduced the incidence of rickets. Two new reports in the April 30 NEW ENGLAND AND JOURNAL OF MEDICINE suggest this public health policy has a downside.

In the first report, endocrinologist Ellen W. Seely of the Brigham and Women's Hospital in Boston and her colleagues identified seven adults and a 15-month-old girl with unexplained vitamin D poisoning. Too much vitamin D results in undesirably high concentrations of the mineral calcium in the blood, which can cause fatigue, weight loss and, in severe cases, irreversible kidney and cardiovascular damage.

After reviewing the medical records of these eight people, the researchers sent them a questionnaire asking about their intake of fortified foods such as milk and cereals. None reported taking supplemental vitamin D. After some sleuthing, the scientists traced the problem to milk produced by a local dairy. All eight people were customers of the dairy and drank from one-half to three cups of milk per day.

Analysis of the dairy's milk revealed a wide range of vitamin D concentrations. The Food and Drug Administration recommends that milk contain 400 international units of vitamin D per quart. However, at least one batch had 232,565 international units per quart.

A more extensive analysis of the Boston cases by the Centers for Disease Control (CDC) in Atlanta revealed an association between drinking the milk produced by this dairy and the occurrence of vitamin D toxicity. CDC identified 11 additional cases of vitamin D toxicity that were not included in the study led by Seely. However, the vast majority of people who drank milk from the Boston-area dairy showed no sign of ill health caused by vitamin D, says CDC's Thomas Sinks. CDC released preliminary findings from its unpublished study on April 28.

A second study reported in the same journal set out to determine the extent of the fortified-milk problem. Michael F. Holick of Boston University's School of Medicine and his colleagues purchased 42 containers of milk and 10 cans of infant formulas from supermarkets in Massachusetts, Virginia, New Jersey, Vermont and New Hampshire.

They discovered that milk and baby formula rarely contained the amount of vitamin D stated on the label. Ten percent of the milk samples and all of the infant formula samples contained excessive amounts of vitamin D. However, most dairies added too little rather than too much of this nutrient to their milk.

Health officials believe the massive addition of vitamin D to milk probably doesn't happen often. Nancy Ridley of the Massachusetts Department of Public Health calls the Boston dairy "a rare, rare exception." She notes that this dairy added vitamin D by hand, a process than could have led to the error.

Nonetheless, such reports may lead to greater federal and state regulation of the dairy industry. "Both reports strongly indicate the need for routine, specific analyses of the vitamin D content of fortified foods by regulatory agencies," comments John G. Haddad of the University of Pennsylvania in Philadelphia.