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.

Rocky Braat went to India as a disillusioned tourist. When he met a group of children with AIDS, he decided to stay. He never could have imagined the obstacles he would face, or the love he would find.

Dr Adam Rutherford shows how decoding the genome has led us to begin to understand the very process by which our DNA makes us different; how it makes each one of us on earth unique and influences who we are and the traits we have. He reveals how, as we try and understand the relationship between who we are and our genes, we stand at the beginning of the most exciting scientific journey of all time.

For centuries scientists have been attempting to come up with an elixir of youth. Now remarkable discoveries are suggesting that ageing is something flexible that can ultimately be manipulated. The scientists are attempting to piece together why we age and more vitally for all of us, what we can do to prevent it. But which theory will prevail? Does the real key to controlling how we age lie with a five-year-old boy with an extraordinary ageing disease or with a self-experimenting Harvard professor? Could one of these breakthroughs really see our lives extend past 120 years?

The world stood in fear of an emerging new disease that threatened to kill millions. A new flu variant H1N1 had arrived. If this latest pandemic has taught anything, it is just how little is understood about the invisible world of viruses. But that has not stopped scientists trying. Horizon follows the leading researchers from across the world, who are attempting to unravel the many secrets of viruses to understand when and why they kill.

The world is affected by an obesity epidemic, but why is it that not everyone is succumbing? Medical science has been obsessed with this subject and is coming up with some unexpected answers. As it turns out, it is not all about exercise and diet. At the centre of this programme is a controversial overeating experiment that aims to identify exactly what it is about some people that makes it hard for them to bulk up.