Leyla Weighs In: Breaking down the DIETFITS randomized clinical trial


| By Leyla Muedin MS, RD, CDN

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Breaking down the DIETFITS randomized clinical trial

By now you’ve probably heard/read the latest in JAMA about the trial examining the effect of a low-fat vs low-carb diet after 12 months on weight loss in overweight adults. The trial also looked for any relationship between insulin secretion and genotype pattern with respect to the two diets.

This was a randomized clinical trial – the gold standard of scientific research with regard to reliability and validity of results. This trial included 609 nondiabetic adults between the ages of 18 to 50 years old with a BMI ranging from 28 (overweight) to 40 (very obese). 305 subjects were randomized to 12 months of a healthy low fat diet and 304 to a healthy low carb diet during the same period. The participants took part in small group sessions with health educators emphasizing diet quality within the parameters of their assigned diets. 

At the end of twelve months, researchers found there was no significant difference in weight change between the low-fat and low-carb diets, and neither genotype pattern or baseline insulin secretion was associated with the dietary effects on weight loss. Weight change for the low fat dieters was -5.3 kg (11.6 lbs) and -6.0 kg (13.2 lbs) for the low carb dieters. They found no significant diet-genotype pattern interaction or diet-insulin secretion interaction. 

Now let’s take a closer look at the macronutrient breakdowns of the diets they administered: 

Low fat diet: 
48% carbohydrates
29% fat
21% protein

Low carb diet:
30% carbohydrates
45% fat
23% protein 

Yes you read that right. At 30% carbs, where’s the low carb diet? Based on a 2000 calorie diet, that a whopping 150 grams of carbs a day. Even on 1600 calories a day, it’s still an over-the-limit 120 grams of carbs. In no way, shape or form is that a low carb diet. 

To get an idea what that looks like, that’s a bowl of oatmeal and a banana at breakfast (~50 grams), a sandwich at lunch with a side of potato salad (~45 grams) and a side of quinoa and medium sweet potato with your salmon or rack of lamb at dinner (~40 grams). In no way, shape or form is that a low carb diet. 

Flawed methodology once again is what we’re witnessing here. 

And despite not being a true low carb diet, the low carb group still lost a little more weight than the low fat group. I would’ve loved to see the results of this study done with a true low carb diet that’s 50 grams or less a day. I am certain the results would have been spectacular. 

To your health! 

Leyla Muedin, MS, RD, CDN 


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