Sugar v Fat

One twin gave up sugar, the other gave up fat. Their experiment could change YOUR life

"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.

But which really is worse for you? In a unique experiment for BBC's Horizon, Chris and I set out to find the answer by going on different diets for a month. Identical twins like us are extremely useful in experiments because we have exactly the same genes. This means any changes we observed would be due to the diets and not genetics.

I went on a no-carbohydrate diet - essentially no sugar - and Chris went on an extremely low-fat diet...

Let me tell you straight up that both of these diets were miserable. I thought I'd got the better deal: I could eat meat, fish, eggs and cheese. 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.

Chris on his low-fat diet didn't fare much better. He never felt full, so was constantly snacking, and like me he found that all the pleasure had gone out of meals: pasta without olive oil is boring.

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. That's because the logic underpinning low-carb diets seems pretty convincing. The thinking is that carbohydrates raise your blood sugar and stimulate your body to produce insulin...

Well-respected scientists will tell you that if you cut out carbohydrates (thus lowering your insulin levels), it's almost impossible to gain weight. These scientists believe reducing our sugar intake is the only way to solve the obesity epidemic. But, as our results show, it's a bit more complicated than this...

One of the words you hear a lot when people talk about very low-carb diets is ketosis. This is where your body makes chemicals called ketones, which can act as fuel for the brain, which can't use fat.

But they're not great brain food. While I wasn't distracted by hunger for the month, I felt thick-headed, and this was most evident in a stock trading competition with Chris... The same was true for my physical performance... even though I seemed to be losing more weight, everything became harder to do.

And the tests we did to assess our levels of blood fats and risk of diabetes at the end of the diets revealed an astonishing and concerning truth about how my body had been fuelling itself in the absence of carbs.

While it was getting some energy from the protein in my diet, some was probably also coming from breaking down my own muscle.

Our experiment showed that you can lose a lot of weight, as I did, on a low-carb diet, but that isn't necessarily good for you.

You can lose weight on a low-fat diet, as Chris did - but over the long term unregulated consumption of sugar may also have negative health consequences.

The most interesting thing we found was that we were asking the wrong question. It's not which is worse for you, fat or sugar, but rather which foods are making so many of us gain weight and why?

The insulin hypothesis sounds scientific, but it doesn't explain what large, independent research studies over long periods of time have shown: low-carb diets don't work for everyone or even a majority of people. For any diet to work you have to be able to keep it up for the rest of your life...

The real reason we're all getting fatter isn't fat or sugar. Furthermore, sugar alone isn't very addictive - only horses snack on sugar cubes and very few people gorge on boiled sweets or dry toast. And fat isn't really addictive either: when did you last sneak a spoonful of butter from the fridge late at night? The modern processed food industry knows this and that's why you're rarely sold the two separately - what is addictive is the combination."