About 30% of people in the study group had a genetic signature that, in theory, should have pointed to success on the low-fat diet, while 40% had a low-carb "profile".
The lead author of the study, Professor Christopher Gardner, said that the findings proved that between the two diets, the fundamental strategies for losing weight remain the same. "It's because we're all very different, and we're just starting to understand the reasons for this diversity".
"We made sure to tell everybody, regardless of which diet they were on, to go to the farmer's market, and don't buy processed convenience food". About half were men and half were women.
U.S. academics were attempting to end the long-running controversy over the merits of low-fat versus low-carbohydrate diets.
"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 in a statement.
During the first two months, dieters in each group were told to limit carbohydrates or fats to 20 grams daily, about the amount that's in 1 1/2 slices of whole wheat bread and a handful of nuts respectively.
After the second month, they were instructed to make incremental adjustments as needed, adding back 5-15 grams of fat or carbs gradually and aiming to reach a balance they believed could be maintained for the rest of their lives.
By the end of the study, individuals in both groups lost weight, on average about 13 pounds (almost 6kg). Similarly, a few women whose DNA did not "match" went through a divorce or other upheaval, ate for emotional comfort, gained weight, and made the mismatched group look bad - a reminder that so many emotional, economic, metabolic, social, and other forces affect someone's chance of losing weight that the effect of genes gets lost in the noise.
Even though the previous study used the same genotype patterns that were tested in the new study, researchers were not able to confirm the results in the larger study, Gardner said.
Further, vast weight loss variability was observed among the participants of both the groups. Researchers then randomly assigned them to either a low-fat or low-carb diet. For example, a fizzy drink may be low-fat, but it's certainly not healthy. He said some people are more satiated on whole grains and some people are more satiated with healthy fats.
At the end of the 12 months, those on low-fat diets reported a daily average fat intake of 57 grams.
After a year the results came in, and they may prove frustrating for those seeking an objective victor.
"It would have been sweet to say we have a simple clinical test that will point out whether you're insulin resistant or not and whether you should eat more or less carbs", he added.
Dr Gardener's study also found that a person's genetics or insulin metabolism is not necessarily a discriminating factor when it comes to a diet's effectiveness, despite the advent and popularity of DNA diets in the weight loss industry now.
Dr. Walter Willett, chairman of the nutrition department at the Harvard T.H. Chan School of Public Health, said the study did not support a "precision medicine" approach to nutrition, but that future studies would be likely to look at many other genetic factors that could be significant. I feel we owe it to people to be smarter than to just say "eat less".