About the Study
The purpose of the experiment was to compare the effects of a low-carbohydrate diet with a low-fat diet both on body weight and heart health. 150 racially-diverse participants, all of whom were obese, were randomly divided into 2 groups: a low-carb group and a low-fat group. The low-carb group was directed to eat 40 grams or less of carbohydrates per day while the low-fat group was told to take in less than 30% of their calories from fat, which aligns with the recommendations from the 2010 Dietary Guidelines. However the participants were not given calories goals to adhere to, though the two groups had a comparable average calorie intake at the end of the trial.
The weight loss results seem pretty clear cut. After 12 months, approximately 80% of participants had completed the trial. Participants following the low-carb diet saw significant decreases in weight loss, body fat, and certain markers associated with cardiovascular risk compared to those on the low-fat diet.
Of course, before you decided to drastically reduce carbs to around 40 we should take a look into some deeper details that weren't included in the abstract.
The low carbs weren't nearly as low as we think. For one, 40 carbs seems a bit drastic (something that is obviously not for athletes). As mentioned above, the low-carb dieters were instructed to eat less than 40 grams of carbs per day; the actual average carb intake for the entire group ranged from 93 grams per day at the six month mark to 127 grams at 12 months, which is over 300% more carbs than the target amount assigned at the beginning of the study.
Study participants had an unfair advantage. Both groups received either meal replacement bars or shakes and routine counseling from a nutritionist for the duration of the study. These two factors likely helped participants adhere to their diets and stick with the year-long trial. This makes the applicability of the study less for the general population.
Participants in the low-carb group ate significantly more protein. Something had to take the place of all the cut carbs, and in this case protein seemed to fill that spot. Protein has been shown to have a positive impact on fat loss, by increasing satiety and maintaining calorie-burning muscle. While cutting carbs may play a big roll in fat loss, it's hard to tell exactly how much came from low-carbs and how much came from high-protein.
They didn't use the best method for measuring fat loss. Researchers essentially used a fancy scale that measures body fat using bioelectrical impedance, which works by measuring total body water. All accuracy issues aside, low-carb diets reduce water weight quickly which, in this case, could have possible been misinterpreted as additional fat loss.
Waist circumference would likely have been one of the better ways to measure fat loss, however the differences between the two groups in this area were negligible. The low-carb group had smaller waists than the low-fat group for the first half of the study however, both groups had similar decreases in waist circumference at the conclusion of the trial. Based on waist circumference measurements, it's hard to support that low-carb is superior to low-fat.
The study doesn't actually prove that low-carb diets lead to greater weight loss compared to low-fat diets. Instead, it shows consuming fewer carbohydrates may increase protein consumption, and something about that combination seems to enhance weight loss - though the cause isn't crystal clear.
What is clear is that adhering to a diet of 40 grams of carbohydrates per day for any amount of time is incredibly difficult, even with low-carb meal supplements and nutritional counseling. The upside? The results from tho study does show low-carb paired with high-protein has results.
So what do I personally think? I think it all depends on the person. Clearly, eating low-carbs is not for athletes in season and particularly not for endurance athletes. Before you start a low-carb lifestyle or short-term 'diet', be sure to take all the precautions regarding your health.