Chapter 3: Biological MoleculesIssues in Biology |
Some of the most important decisions we make concerning our health involve our diets. Unfortunately, in our efforts to consume a healthy balance of foods, it can be difficult to sort out good information from that which is questionable or dangerous. This confusion does not only arise from "snake oil"-style advertisements that claim massive doses of vitamins can cure long lists of diseases. Scientists can also add to the confusion as their recommendations change to reflect new studies and new data. To illustrate the difficulty in judging information, we’ll consider the case of vitamins—particularly vitamin A.
Humans have known for hundreds of years that a lack of fresh fruits and vegetables can cause serious, potentially fatal diseases such as scurvy. However, until the early years of this century, it was thought that fruits and vegetables prevented disease by removing a toxic by-product produced by the body itself. Such ideas made bowel "cleansing" and other purging methods popular as a means of preventing disease, a concept that continues to be promoted. Experiments on the diets of rats in the early 1900s demonstrated that, instead of removing toxins, foods actually provide substances that are needed for health. These substances were called vitamin A and B, although the chemical nature of these molecules was not deciphered until much later. Vitamin C was subsequently discovered and was the first vitamin to be purified and whose chemical structure was revealed, earning a Nobel Prize for Albert Szent-Gyorgyi Von Nagyrapolt in 1937. Subsequent studies on human nutrition, usually using animal models, revealed that humans require a total of 13 vitamins to remain healthy.
All vitamins are organic molecules that our bodies need in small amounts in order to perform essential enzyme reactions or to protect against damage caused by normal metabolic reactions. Because our cells lack the necessary enzymes to produce vitamins, we must consume them by eating the cells (or products of cells) from other organisms that can manufacture them. When we eat a carrot, we are eating carrot cells packed with carotene, which can be easily converted to vitamin A. Clearly, a molecule that is a vitamin for us is not a vitamin for all other organisms. For example, dogs do not need to consume vitamin C because their cells contain the necessary enzymes to manufacture it for themselves. And certainly, carrot plants do not need an external source of vitamin A.
Vitamin A is actually a member of a complex group of molecules that include retinoids and carotenoids. Carotenoids, which include beta-carotene, are produced by plants (and many other organisms) and give certain fruits or vegetables their distinctive orange or yellow colors. Some carotenoids are needed for photosynthesis. In the human body the functions of vitamin A, a retinoid, are less well understood. One clear role of vitamin A is to produce retinal, which is the light receptor that binds to the protein rhodopsin and enables vision. Its role in vision explains one of the major symptoms of vitamin A deficiency: night blindness. Other symptoms of Vitamin A deficiency include xerophthalmia, or "dry eye," which is an inability to properly produce lubricating mucus in tears. This condition is currently the leading nonaccidental cause of blindness in the world. Vitamin A is also needed for proper function of the skin and other epithelial tissues as well as for proper growth and development. The biochemical nature of these functions of vitamin A are not known but are thought to result from its effects on gene expression.
What is so confusing about vitamin A? There appears to be a true consensus that eating high amounts of fruits and vegetables helps to lower the risk of cancers in humans. One reasonable hypothesis was that the protective molecules in fruits and vegetables were "antioxidant" vitamins such as vitamins A, C, and E. The CARET (Carotene and Retinol Efficacy Test) study was designed to test this idea by supplementing the diets of thousands of smokers with extra beta-carotene and vitamin A. As in all good epidemiological studies, the CARET study included controls. In this case some of the individuals were given supplements that appeared the same as the vitamin-containing capsules but actually lacked vitamins. The expectation was that individuals taking beta-carotene/vitamin A would develop cancers less frequently than the controls who were not taking additional beta-carotene/vitamin A. The CARET study was halted nearly 2 years early when it was discovered that individuals taking the supplements actually had a higher risk than controls of developing a cancer. Results of similar studies performed elsewhere were consistent with an enhancing role of beta-carotene in developing cancer. This new data forced scientists to do an apparent about-face on the advisability of taking beta-carotene supplements. Now, instead of supporting vitamin A supplements, many scientists urge against taking additional vitamin A at all and instead advise patients to obtain this vitamin directly from food.
Where does this leave the average consumer in the United States? First of all, we need to realize that we are all in a situation of "Let the Buyer Beware." The U.S. Food and Drug Administration (FDA) is charged with proving that a pharmaceutical company’s claims for a particular drug are actually valid. However, the FDA does not test claims for the effects of dietary supplements. The bottom line is that, when it comes to vitamins, minerals, amino acids, herbs, bee pollen, and so on, suppliers do not have to provide evidence that their claims for longer life, higher energy, better outlook on life, more hair (or whatever) are true. All they are required to do is show that the items do not cause harm. However, when taken at high doses, even vitamin A can cause serious problems, including birth defects. Simply because a substance occurs naturally does not make it safe under all conditions.
Second, in order to make informed decisions about dietary supplements, it can do no harm—and potentially great good—to delve deeper into the claims of scientists and salespeople alike. Approach major changes to your diet or health treatments with skepticism and caution. Judge whether the source of the information you are evaluating is likely to be trustworthy. Consider whether the information is designed to entice you to purchase the item. Read and evaluate multiple points of view.