Narrator: This is Science Today. Researchers at the Lawrence Berkeley National Laboratory put 28 pairs of identical male twins on two, six-week diets. One diet was high in fat, the other low. One twin was a marathon runner, the other was not. Study leader, Paul Williams, says the goal was to see whether or not the twins' exercise status would affect their response to diet.

Williams: We looked at their cholesterol levels at the end of the first diet and at the end of the second diet. Our initial hypothesis that being an exerciser would protect you to some degree against eating a high fat diet – we didn't find that to be true.

Narrator: Instead, Williams found that it was the twins' genes that were affecting the cholesterol response to the diet. The next step, is conducting a larger identical twin study that includes women.

Narrator: Finding identical twins, is quite challenging because we also require other things of them. We'd like them both to be non-smokers; we'd like the women to be similar in their use of oral contraceptives. And of course the hardest part is usually when you identify a runner who has an identical twin – their twin is also running!

Narrator: For Science Today, I'm Larissa Branin.