When the dashing genius twin physicians, Chris and Alexander "Xand" van Tulleken, aren’t traveling around the world on humanitarian missions, on far-flung adventures or conducting medical research or filming documentaries, they tackle more mundane challenges.

Case in point: back at home, the brothers went on a diet.

But, as is their nature, it wasn’t just an ordinary diet. The brothers turned what could have been a simple weight loss plan into an intriguing experiment ... and then, as also seems to be their nature, they made a documentary about it.

The brothers, who both realized they didn’t know enough about nutrition, embarked on a mission as human guinea pigs to test the merits of low-fat and low-sugar diets. Chris would go on a super low-fat regimen, Xand would give up sugar. With identical DNA, they became their each other’s control group, of sorts.

The novel idea came about when Xand – at his highest weight ever, 245 pounds – realized he had become the "fat version of his twin" and was a living, breathing "cautionary tale,” he told the Daily Mail.

For the experiment, Chris went super low-fat, allowing only 2 percent of his daily calories to come from fat. Xand went high-protein, Atkins-style, forsaking all sugar and carbohydrates. Their daily activity was otherwise similar and they shared the exact same fitness routines.

In the end, Xand – who was allowed to eat mountains of meat, eggs, and fat – lost a whopping nine pounds. But at a cost. His body went into ketosis, whereby the body burns fat but deprives the brain of the glucose it needs for energy. His breath stank, he became constipated, drained of energy, and mentally foggy.

Chris lost some weight; but without any fat, food became so unsatisfying that he became plagued with constant hunger.

In the end, both brothers declared their diets to be duds. In consultation with nutrition scientists, they concluded that it’s not specifically fat or sugar that is the villain, but the combination of both so prevalent in processed foods.

"We should not vilify a single nutrient," Chris told the Sunday Express. "It is too easy to demonize fat or sugar, but that enables you to let yourself off the hook in other ways. The enemy is right in front of us in the shape of processed foods."

You can watch their trials and tribulations on BBC Two’s "Sugar v Fat," including the clip below:

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'Sugar v fat': Identical twin doctors take on the ultimate diet question
The twin physicians conducted a one-month experiment; one went extreme low-fat, the other extreme low-carb. Who was the victor?