What's worse for us: sugar or fat? Twin doctors Chris and Xand Van Tulleken go on month-long high-fat and high-sugar diets to find out.

What's worse for us: sugar or fat?

To answer the hottest question in nutrition, twin doctors Chris and Xand Van Tulleken go on month-long high-fat and high-sugar diets. The effects on their bodies are shocking and surprising.

But they also discover that in the debate about fat and sugar, the real enemy might have been hiding in plain sight.