A new study published in The BMJ claims that there is little if any meaningful difference between various diets for weight loss and blood pressure reduction at six or 12 months, and therefore you should simply choose the diet you like better. Can we trust the data?
The BMJ: Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trialsThe study is a meta-analysis of 121 randomized controlled trials involving 22,000 people. At Diet Doctor we consider this type of study to be the highest quality evidence by our evidence basing policy. However, even with the highest quality evidence, we still need more details than the catchy headline.
The researchers' overall conclusion is that low-carb diets, DASH diets, and other low-fat diets all result in around 8.8 pounds (4 kilos) of weight loss, and a blood pressure reduction of 5 mmHg. Therefore, study authors conclude there is no meaningful difference among the diets. On the surface, since they included Atkins and South Beach diets in the study, that may seem like it could be an accurate conclusion.
Once again, however, the devil is in the details. Study authors define low-carb diets as less than 40% of calories. That means a 2,000-calorie-per-day diet could have 200 grams of carbs and 66 grams of fat. That is a far cry from what we consider a liberal low-carb diet of 100 grams of carbs or a keto diet of less than 20 grams of daily carbs.
Even worse, they go on to say:
The diet was labelled as brand-like when it met the definition of a branded diet but failed to name or reference the brand in the article. For example, dietary programmes that did not refer to Atkins but consisted of less than 40% of kilocalories from carbohydrates per day for the duration of study, or were funded by Atkins, were considered Atkins-like.
Not only did they improperly define low carb, but they muddy the interpretation of branded diets by combining them with "like" diets that aren't truly similar.
Based on these points alone, it is clear that we cannot trust the headlines as they apply to a real low-carb diet.
That doesn't mean we should throw out the study. It turns out there is very interesting data in the study that unfortunately didn't make it into the abstract.
For instance, the South Beach diet had the greatest weight loss of all at almost 22 pounds (10 kilos) compared to 7.9 pounds (3.6 kilos) for DASH and 6.2 pounds (2.8 kilos) for Mediterranean, although the GRADE quality of this evidence was low.
The paleo diet showed the greatest reduction in blood pressure at 14 mmHg compared to 4.6 mmHg for the DASH diet. This was rated as "moderate to high certainty" by the GRADE criteria.
And Atkins had the greatest increase in HDL at 3.4mg/dl compared to a decrease of 1.9mg/dl for the DASH diet.
Unfortunately, these inconsistencies won't stop the headlines from reading "Low-carb diets no better than low-fat." This highlights the important work groups like the Nutrition Coalition and the Low Carb Action Network (LCAN) are doing to make sure the dietary guidelines committee properly defines low-carb diets and accurately analyzes all available quality science.
In fact, Dr. Ted Eytan recently wrote a blog post for LCAN explaining the situation in detail and calling upon all of us to make our voice heard with our local representatives. We encourage you to read his full post.
As we have reported before, 40% carbs isn't even close to low carb. It is inaccurate to conclude that all diets are the same. Instead, we can conclude that this study shows when blended together, high-carb diets of varying carb and fat content are roughly the same for weight loss and blood pressure reduction.
When defined correctly, the data supports that low-carb diets, on average, lead to greater weight loss that low-fat diets. Data also shows that low-carb diets have a greater reduction in blood pressure as well as all of the markers of the metabolic syndrome.
At Diet Doctor, we are clear and consistent in our definitions of low-carb. We call upon the scientific community to do the same.
Thanks for reading,
Bret Scher MD FACC
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