In the ongoing nutritional war between adherents of low-fat and low-carb diets, the skeptical cardiologist has generally weighed in on the side of lower carbs for weight loss and cardiovascular health.
The science in nutrition is gradually advancing and the DIETFITS study recently published in JAMA is a welcome addition.
DIETFITS is a really well done study which provides important insights into three huge questions about optimal diet:
- Should we choose a low-fat or a low-carb diet for weight loss and cardiovascular health?
- Do baseline insulin dynamics predict who will respond to low-fat versus low-carb diet?
- Can we predict who will respond to low-fat versus low-carb by genetic testing?
The Details Of DIETFITS
Stanford investigators recruited 609 San Francisco area individuals between the ages of 18 to 50 years with BMI of 28 to 40 and randomized them to a “healthy” low-fat diet or a “healthy” low-carb diet.
During the first 8 weeks of the study, low-fat participants were instructed to reduce fat consumption to <20 gm/ day while the low carb participants were instructed to reduce digestible carbohydrate to <20 gms/day.
Then individuals were allowed to add back fats or carbs back to their diets in increments of 5 to 15 g/d per week until “they reached the lowest level of intake they believed could be maintained indefinitely.” Importantly no explicit instructions for energy restriction were given.
The “healthy” instructions for both groups were as follows
- maximize vegetable intake
- minimize intake of added sugar, refined flours and trans-fats
- focus on whole foods that are minimally processed, nutrient dense and prepared at home whenever possible
Dietfits Outcomes-Diet And Weight
- Total energy intake was reduced by 500-600 kcal/d for both groups
- The low-fat vs the low-carb intake at 12 months was 48% versus 30% for carbs, 29 vs 43% for fat and 21 vs 23% for protein.
- Mean 12 months weight change was -5.3 kg for low-fat vs 6-6.0 kg for low-carb which was not significantly different
- There was no difference between groups in body fat percentage or waist circumference
- Both diets improved lipid profiles and lowered blood pressure, insulin and glucose levels
- LDL (bad cholesterol) declined more in the low-fat group whereas HDL (good cholesterol) increased more and triglycerides declined more in the low-carb group.
Thus both diets were successful for weight loss and both improved risk markers for cardiovascular disease after a year.
DIETFITS- Can Genes and Insulin resistance Predict Best Diet?
Surprisingly, the study found no significant diet-genotype interaction and no diet-insulin secretion interaction with weight loss.
This means that they could not predict (as many believed based on earlier studies) who will benefit from a low carb diet based on either currently available genetic testing or a generally accepted measure of insulin resistance.
As the authors point out, these findings “highlight the importance of conducting large, appropriately powered trials such as DIETFITS for validating early exploratory analyses.”
As you can imagine this study has led to quite an uproar and backlash from dedicated combatants in the macronutrient wars.
A reasoned summary and response from Andreas Eenfeldt, a low carb proponent can be found on his excellent low carb/keto Diet Doctor site here.
If I’m allowed to speculate, the reason that we did not see any major additional benefit from low carb in this study is that the groups ended up so similar when it came to bad carbs. The low-fat group ended up eating fewer carbs too (!) and significantly less sugar, while the low-carb group ended with a somewhat weak low-carb diet, reporting 130 grams of carbs per day.
Eenfeldt emphasizes that low-fat diets never “win” these macronutrient dietary skirmishes:
On the whole, this study adds to the 57 earlier studies (RCTs) comparing low carb and low fat for weight loss.
From a standing of 29 wins for low carb, zero for low fat and 28 draws, we now have 29 wins for low carb and 29 draws. The wins for low fat stay at zero.
Larry Husten at Cardiobrief.org in his analysis of the study quotes a number of experts including Gary Taubes, the low carb pioneering journalist
Taubes speculates “that the weight loss may have been similar not because any diet works if you stick with it and cut calories (one possible interpretation) but because of what these diets had in common — avoid sugar, refined grains, processed foods. Whether the low-carb arm would have done even better had Gardner kept their carbohydrates low is something this study can’t say. (And Ornish [low-fat diet proponent] would probably say the same thing about fat consumption.)”
The low-fat or vegan disciples seem to have had a muted response to this study. I can’t find anything from John McDougal , Dean Ornish, Caldwell Esselstyn or Joel Fuhrman.
Readers feel free to leave comments which link to relevant analysis from the low-fat proponents.
Dietfits-Perspective Of The Participants
Julia Volluz at Vox wrote a fascinating piece recently which involved interviewing some of the participants in this study.
She points out that although the average DIETFITS participant lost over 10 pounds, “Some people lost more than 60 pounds, and others gained more than 20 during the year.”
She obtained permission from the lead author, Christopher Gardner and interviewed “Dawn, Denis, Elizabeth*, and Todd — two low-fat dieters and two low-carb dieters — about their experiences of succeeding or faltering in trying to slim down”
I highly recommend reading the entire article for details but Volluz concludes
And that leads us to one of the burning mysteries of diets: how to explain why some people fail where others succeed — or the extreme variation in responses. Right now, science doesn’t have compelling answers, but the unifying theme from the four study participants should be instructive: The particulars of their diets — how many carbs or how much fat they were eating — were almost afterthoughts. Instead, it was their jobs, life circumstances, and where they lived that nudged them toward better health or crashing.
DIETFITS-Importance of “Healthy” Diet
Most likely the success of both of these diets is due to the instruction that both groups received on following a “healthy” diet. This guidance is remarkably similar to what I advocate and is something that combatants in the diet wars ranging from paleo to vegan can agree on.
The JAMA paper only provides the description I listed above but Volluz adds that participants were instructed to:
… focus on whole, real foods that were mostly prepared at home when possible, and specifically included as many vegetables as possible, every day … choose lean grass-fed and pasture-raised animal foods as well as sustainable fish ... eliminate, as much as possible, processed food products, including those with added sugars, refined white flour products, or trans-fats … prepare as much of their own food as possible. …
Indeed, if you want to see a very detailed description of the instructional process for participants check out the very detailed description of the methods here.
Yours in Health,
N.B. I was searching for a reasoned response to this study from the low fat camp and to my surprise came across this fascinating video featuring the lead author of the study, Christopher Gardner, on (no fat/vegan) John McDougal’s YouTube site. Gardner is clearly on the side of sustainable, local , ethical food consumption but to his credit, his research , publications and comments on DIETFITS don’t reveal this.