The Role of Selenium in Preventing Disease Remains a Subject of Study
In 1817, while attempting to isolate an entirely different element, Jons Jakob Berzelius accidentally discovered selenium. Although 30 years ago it was shown to be essential for animals, selenium’s importance in human nutrition has attracted wide attention only in recent times.
Despite this relatively short period of study, claims are flying about the mineral’s ability to prevent cancer and prolong life. In reality, more research is needed to illuminate the role of selenium in health and disease.
The first suggestion that it might prevent tumor formation surfaced toward the end of the 1940s. Since then, numerous animal studies have documented selenium’s ability to inhibit tumor formation at a variety of sites, including skin, liver and colon. How these findings relate to humans is unclear.
Because of differences in soil concentration of the mineral, geographic location helps determine selenium status. Yet no detailed studies have been conducted relating soil selenium levels and cancer incidence worldwide. The data that do exist are a mosaic of inconsistencies. In New Zealand, where the soil is selenium-deficient and serum-selenium levels are low, statistics indicate that, except perhaps for colon cancer, inhabitants are no more susceptible to other forms of the disease.
Researchers have compared serum-selenium levels in cancer patients to those of individuals without the illness. A study of cancer patients in New Zealand found that their serum levels, although low, were no different from those without cancer who lived in the same area. A recent investigation in Finland, however, revealed significantly lower selenium levels in women with uterus and cervical cancers.
It can be argued that the selenium depletion was an effect of the disease rather than a cause. But, the investigators counter that the serum levels did not vary with the stage of cancer, as might be expected if depletion occurred over the course of tumor development. And conversely, elevated selenium levels have been reported in other types of cancer, leukemia for one.
Study Needed to Answer Question
What is needed to help answer the question is a study in which selenium levels of a large group of people are monitored over many years and compared to the incidence of cancer development.
Attention also has focused on the relationship between selenium and cardiovascular disease. The interest stems primarily from reports in the late 1970s describing the first example of a selenium-responsive disease in humans. Named Keshan’s syndrome after the province in China where it was originally identified (a region extremely low in selenium), it is characterized by a degeneration of the heart muscle, or cardiomyopathy.
Because the occurrence of the disease shows a seasonal variation that cannot be explained by changes in selenium levels of the food supply or in the individuals themselves, it probably involves some other factor, like a virus, as well. A treatment program of sodium selenite, however, has dramatically reduced the incidence of the disease.
A similar cardiomyopathy has been observed in selenium-deficient animals, along with abnormal heart rhythms and blood-pressure changes.
Although a number of studies have linked low serum-selenium levels to coronary heart disease in humans, a recent report from Great Britain could find no correlation between selenium status and such coronary heart disease risk factors as cholesterol, triglyceride levels and blood pressure. The investigators did note a decrease in selenium in the serum of smokers, a group known to be at greater risk for coronary heart disease. It is still a mystery how this correlation relates to the disease itself.
Any Biochemical Basis?
Is there any biochemical basis for all the speculation about selenium? Most researchers point to the selenium-containing enzyme, glutathione peroxidase, which protects cell walls and the genetic material within from damage caused by the breakdown products of fat metabolism. This “antioxidant” property is similar to that of Vitamin E, and the two nutrients are thought to complement and, in some cases, to substitute for one another if need be. The ability of glutathione peroxidase to prevent cell damage is responsible for the exaggerated claims that selenium can ward off or retard the aging process.
Here in the United States, there is little cause for alarm about selenium intake. Unlike isolated areas of the world, regional differences in soil content are of limited concern in this country because we eat foods produced in many places with varying amounts of selenium in the earth. While as yet there is insufficient information to establish a Recommended Dietary Allowance, the Food and Nutrition Board suggests a “safe and adequate” intake of between 50 and 200 micrograms a day.
Good sources of the mineral include seafood, kidney, liver, meat and grain products. As for selenium supplements, too much can be toxic, so we strongly advise depending on a varied diet to meet your day’s needs.
Breast-Feeding Babies May Lower the Incidence of Ear Infections
Question: I have heard that breast-feeding protects against middle-ear infections. Is that true?
Answer: There is some evidence for this, although the area deserves further study. It has been suggested that milk is more likely to drain into the middle ear and that middle-ear infection (otitis media) is more likely to occur in bottle-fed babies who are fed in a relatively prone position--especially in those whose bottle is “propped.”
Several studies investigated the difference in middle-ear infections in the two groups and most found fewer infections in those who were breast-fed. One, a Finnish study reported a couple of years ago, followed a group of healthy infants for three years. The infants were divided into three categories: long-term breast-feeding (six months or more with breast milk), a second group (breast-fed for two to six months) and a third group, which consumed breast milk for less than two months.
At the end of one year, 220 (87%) of the original 256 babies were still available for examination. At that time 6% of those in the long-term breast-fed group had had a middle-ear infection compared to 19% of those in the groups that were breast-fed minimally or not at all.
That difference persisted up to three years, but, unfortunately, by that time only a little more than 50% of the children were available for study. Middle-ear infections at less than 6 months of age and recurrent ear infections were also more common in those breast-fed for less than two months.
Interestingly, the frequency of respiratory infections, which are known to precede ear infections, did not differ among the groups. That led the authors to speculate that bottle feeding while lying down was the culprit. But since they did not collect data on positioning, this remains a theory.
A recent review of the evidence by the Task Force on Infant Feeding Practices of the American Academy of Pediatrics pointed out that while a number of other ear studies also found benefit in breast-feeding, there were serious limitations in the studies’ design, such as looking at ear disease many years after infancy and relating it to early feeding history. This type of recall is subject to large error. Moreover, one study reviewed failed to find a difference in infection rates.
Thus, the Task Force concluded that further investigation is necessary before a relationship can be firmly established.
Q: By now I am well aware that alcohol abuse and pregnancy just don’t mix. But so far, everything I’ve read seems to give less than satisfactory answers to the question of whether moderate amounts of alcohol are dangerous. Why is that?
A: There are several reasons why this is a difficult question to answer. First, most studies are based on self-reporting of drinking, and thus there is a strong possibility of underestimation. Second, most results are expressed as average daily intakes, revealing nothing about consumption patterns, or differences between those women who drink a consistent amount regularly and those who binge. Finally, there is no single definition of just what the term “moderate” really means.
The studies reported in the literature have defined the term in various, sometimes opposite, ways. For example, some studies merged heavy and moderate drinkers. Another chose to pair the so-called moderate drinkers with those who abstained. Data from such studies obviously cannot be compared.
In other words, what we now have is a series of observations that do not permit us to draw any firm conclusions about safe levels of alcohol intake during pregnancy. Future research may determine whether there is a level of alcohol intake below which there are no effects on the unborn fetus. Until such information is available and while that issue is still in doubt, prudence would clearly indicate abstinence.
Q: I have two questions about alcohol. First, since it is derived from foods which are mainly carbohydrates, I had always assumed that the body handled it as it would carbohydrates. Recently I read something which made me think that I was misinformed. Second, I have read different and conflicting information about how much alcohol the liver is able to handle in a given period. Can you straighten me out?
A: While alcohol may derive from carbohydrate sources, it is actually metabolized by the body like fat. And it cannot be converted to glucose or to amino acids. Alcohol is either burned for energy or, if the calories it provides are in excess of those needed to meet immediate energy demands, converted to fat.
As for your second question, the liver can metabolize between 7 and 10 grams of ethyl alcohol--that’s equivalent to about one ounce of hard liquor--per hour. When the body gets more than it can break down, the excess passes into the general circulation. And while it travels to all body organs, it has a special affinity for the brain.
