This weeks BBC Horizon special seeks to answer one of the biggest questions in the diet industry and world of nutrition, which is worse for you: sugar or fat?
To find out, the show follows Xand and Chris Van Tulleken as they take on two extreme but contrasting diets. One of the twin doctors (Chris) will spend an entire month on a high-sugar diet, while the other (Xand) spends the same period on a high-fat plan.
What effects will these changes have on the brothers bodies and which diet will prove the least healthy?
Horizon: Sugar v Fat broadcasts at 9:00pm on BBC Two on Wednesday, 29 January, 2014.
To test their diets effects on strength and fitness, we see in this preview Chris and Xand have a cycle race up Box Hill in Surrey, with things not looking good for fat: