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

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It turns out neither low carb nor low fat is good.

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.

Moderation is good.

this is the sort of dieting we tried in high-school.  it sucked!  we just kept yo-yoing all over the place with our bodies.

and ketosis is a terrible state to be in that does indeed make your breath stink.

moderation.  yes.  unless it involves la croix or blueberries... then you can go crazy!!

It sounds like ketosis also makes it hard to think.

Why can you have all the La Croix and blueberries you want? Low calories?

ketosis sucks for a lot of reasons.  and it's hard on your kidneys.

la croix is just water with bubbles and maybe some flavor, so yes, no calories, no sweetener.

blueberries are 100% good for you!  i'm pretty sure you couldn't eat too many blueberries, ever.

Those are good to know. So your mainstays are bubbles, blueberries, and kefir?

and sushi, bananas, string cheese and cereal.  okay, and haagen dazs.

Haha, sweet. What kind of cereal?

The brain needs carbs and exercise needs carbs.

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.

We started with £100,000 of fake money and he almost tripled what I earned over an hour. 

The same was true for my physical performance. We spent a day with Nigel Mitchell, the head of nutrition at Team Sky Cycling. 

Over a series of tests - all of which involved needles and long sessions of uphill cycling - he put us through our paces. Again Chris thrashed me in every test.

So, even though I seemed to be losing more weight, everything became harder to do.

Any good diet needs to be sustainable and avoid processed sugar mixed with fat:

For any diet to work you have to be able to keep it up for the rest of your life. I thought I would stick to low carbs after we finished, but having my first meal with carbs  - and the boost in energy and alertness it gave me - reminded me that for a month I had been under-performing in all areas of my life, and I'd felt dreadful.

The diet industry is polarised around simple debates such as fat vs sugar because there are huge amounts of money at stake.

Farmers, food manufacturers, lobbyists, scientists and authors of diet books need to defend one or other side. Fortunately, you don't need to worry about any of that. 

What we discovered is that 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. 

We interviewed some amazing scientists who showed us that a combination of fat and sugar (such as in milk chocolate or ice cream) has a similar effect on your brain to cocaine. 

Remove either and your tub of ice cream will be a lot less appetising and a lot less addictive. It'll have fewer calories, too.

What we relish is fat/sugar combinations - chocolate, ice cream, French fries. To see the effect of these combinations on the brain - and why they might be addictive like cocaine - after the diet part of our experiment finished, I had an eye scan done by Jennifer Nasser, a nutrition professor at Drexel University in the U.S. 

This scan effectively detects dopamine, a neurotransmitter that activates the reward centres of the brain. I was scanned while I ate fatty, sugary foods: sure enough a huge surge of dopamine. 

So, what were our conclusions? If you want to lose weight it will be much easier if you avoid processed foods made with sugar and fat. These foods affect your brain in a completely different way from natural foods and it's hard for anyone to resist eating too much.

And any diet that eliminates fat or sugar will be unpalatable, hard to sustain and probably be bad for your health, too.

Most important line:

"For any diet to work you have to be able to keep it up for the rest of your life."

TLDR: The best diet is Cocaine.

Technically that's not food.

Plus cocaine increases probability of dying early:

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