Some colleagues had broken out data from a 2008 diet study that seemed to indicate that people might lose more weight on low-fat or low-carbohydrate diets based on their genes.

It didn’t include very many people, however, so Gardner and colleagues recruited more volunteers and tried a more rigorous approach.

"We've all heard stories of a friend who went on one diet, it worked great, and then another friend tried the same diet and it didn't work at all," Gardner said.

"It's because we're all very different, and we're just starting to understand the reasons for this diversity. Maybe we shouldn't be asking what's the best diet, but what's the best diet for whom?"

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So that’s what Gardner and his colleagues tried.

They ended up with 600 overweight volunteers and looked at differences in three genes closely linked with metabolism: PPARG, ADRB2, and FABP2. PPARG is involved with how the body metabolizes fats. Studies on various versions of ADRB2 suggest it can affect weight loss because of its role in burning fat. FABP2 plays a role in metabolism and helps control how the body uses cholesterol and triglycerides.

The researchers also tested how people respond to a big dose of sugar. The theory is that people whose bodies respond in a more extreme way to a giant slug of sweetness might do better on a low-carb diet.

The 600 volunteers were randomly assigned to either a low-fat or low-processed-carbohydrate diet. They were also given common-sense advice about eating more vegetables and whole grains and less fatty and sugary junk food.

“We told everybody they should buy whole foods. Nobody should have foods with refined sugar. They suck,” Gardner said.

“Also, we advised them to diet in a way that didn't make them feel hungry or deprived — otherwise it's hard to maintain the diet in the long run."

“If it says low-fat on the package, think: They’re brownies!"

Before the study started, the volunteers ate an average of 87 grams of fat and 247 grams of carbs a day. A hamburger patty has about 17 grams of fat and a tablespoon of butter has 12 grams; one-third of a cup of pasta has 15 grams of carbohydrate while a 12 ounce cola soft drink has 39 grams of carbs.

By the end of the study volunteers on the low-carb diet got daily carbs down to about 132 grams and those on the low-fat diets were down to 57 grams of fat, the team reported in the Journal of the American Medical Association.

After a year, 200 of the volunteers had dropped out. The rest had lost an average of 13 pounds. Some lost a lot more weight than others — 60 pounds or more, while others actually gained weight.

But who lost weight and how much had almost nothing to do with their genetic pattern or which diet they were on, Gardner and colleagues found.

There were also no differences in blood pressure, insulin levels, blood sugar levels or cholesterol readings, except that the volunteers in the low-carb group on average saw increases in their LDL (“bad” cholesterol) levels.

“It was disappointing, but this is how science works,” Gardner said.

The people who lost the most weight said it wasn’t so much what they ate, as how they thought about food. They said they thought carefully about each meal.

Gardner said it makes sense.

“What we are trying to tell everybody is that they can’t game the system,” he said. “If it says low-fat on the package, think: They’re brownies! And don’t buy low-fat chips, no matter what it says on the package. They’re chips!”

This kind of study is expensive, time-consuming and difficult to do, Gardner said.

There are more than 850,000 genetic changes called single-nucleotide polymorphisms (SNPs) that his team could test to see if any of them affect dieting success.

“I don’t think I want to look at 850,000. That’s too many,” Gardner said.