Tim Spector studies twins. And now that he is investigating the microbiome (head of the largest investigative project in the UK,) and what our gut bacteria do for us (or not,) twins offer incredible insight into the very personal nature of how this phenomenon may work. He is a Professor of Genetic Epidemiology at King's College London and author of over 700 academic papers, a large proportion of which relate to nutrition and the causes of obesity.
He brings together both case studies and accounts of other research in an insightful yet entertaining account of where our understanding is now and where it is headed. I enjoyed this book very much and recommend it heartily.
The Diet Myth: The Real Science Behind What We Eat