Weight gain and obesity can be explained by lack of willpower and self-control or by genetic and hormonal factors.But when you have one thin twin and one fat twin, it's hard to blame any of those things.These topics fall between the cracks at medical school.
But which diets work, why we eat too much and why losing weight is so hard don't sit within any medical speciality.
For years it was thought fat was bad for you: it made you get fat, so low-fat food was good.
But the 'fat is bad' dogma is being widely challenged.
Carbohydrates, including sugar, are increasingly viewed as the evil, fattening, toxic ingredient.
We were allowed to eat as much as we wanted, except I couldn't have carbohydrates and Chris was allowed only the barest amount of fat - you need some fat to survive, so he restricted himself to food with less than 2 per cent fat.
But take away carbohydrates and the joy goes out of meals.
And remove all fruit and veg - they all have carbs - and you get constipated.
Though I was never hungry, I felt slow and tired, and my breath was terrible.
There was one saving grace: each of the diets was easy to follow because they have just one simple rule.
And I also had a pretty good reason to persevere: I really thought my low-carb diet would work and I'd end up slim and healthy a month later.
This can lead to what's called metabolic syndrome, a combination of abdominal obesity, high blood pressure and raised cholesterol and other fats in your blood.