One example of a molecule whose "handedness" is important to its function is alpha-tocopherol. As most health-conscious folks know, the alpha-tocopherol found in natural foods is "d-alpha tocopherol." But the synthetic form found in many drug-store supplements is "dl-alpha." The molecules in d- and dl-alpha-tocopherol are made up of the same atoms, put together in the same order; the difference is that the atoms in the molecules of the dl- mixture are twisted in different directions in space, just like you can twist the shafts in a tinker-toy chain to get many different patterns from the same basic building blocks. These different "handednesses" are referred to as different "isomers" (EYE-so-mers) of the molecule.
Your body is designed in such a way that it can often make much better use of the natural isomer of a molecule than it can do with that molecule's synthetic look-alikes. Again, alpha-tocopherol is a great example: milligram for milligram, the d- form of the molecule is much better used and retained by the body than is the dl- mixture. Still, in the case of dl-alpha-tocopherol, there's nothing particularly dangerous about having those extra, synthetic isomers in your supplement: they're just weaker, less effective imitations of the real thing. In other words, the dl- form is not so much an "evil twin" as a geeky kid sibling.
In other cases, however, putting the wrong isomer into your body can actually harm you 8. An example that's becoming better and better known is trans-fatty acids. Again, most health-conscious people have heard of trans- fats, but few people understand what they are, or why they're dangerous. Found in large quantities in most margarines, but also sprinkled throughout the processed-food universe, synthetic trans-fatty acids are actually just unnatural isomers of natural polyunsaturated fats. When you expose the natural "cis-" isomer of a polyunsaturated fat to a great deal of heat and pressure (as is done in the partial hydrogenation of vegetable oils), you can literally twist its structure, rearranging the molecule's orientation in space. Thus the synthetic trans- isomer is born.
Changing a fatty acid from the cis- to the trans- form is a matter of a simple rotation of orientation … yet it completely alters the effects of the fat on your body. The crucial difference between the natural cis-isomers of polyunsaturated fats and their synthetic trans-forms was shown in a large study of the diet and health of over 80 000 female nurses in the United States 9. This study found that the total amount of fat in the nurses' diets did not affect their odds of heart attack or of heart disease death. Instead, it found that it's the kind of fat that a person puts into his or her body that affects heart health.
When a woman in the study took 5% of the carbohydrate Calories in her diet, and replaced them with Calories from polyunsaturated fats in their natural cis- form, she cut her odds of a heart attack or a heart-disease death by an impressive 38%. But when a woman replaced just 2% of her carbohydrate Calories with trans-fatty acid isomers, she nearly doubled her risk of heart attack or death from heart disease (a 93% higher relative risk)! 9 Frighteningly, scientists studying the same group of women have found that a similar pattern exists for risk of diabetes: the natural isomer is protective, while the artificial isomer puts a person at greater risk.
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