Diet Studies: Another Round on Carbs, Fat, and Protein

Are we ready for another round on carbs, fat, and protein? It sure seems like it. Popular interest in low-carb diets keeps growing steadily. A more extreme version – keto – generates a lot of buzz right now. And yet, research keeps telling us that playing around with these macronutrients doesn’t make a huge difference. A new study in Nutrition Journal tells us this once again.

Carefully Controlled Study of Carbs and Protein

Madeline Gibson and colleagues conducted a blinded, randomized crossover study of adding protein or carbs to the diet. They fed subjects an extra 20 percent of their calorie needs just before meals in the form of a shake. Those extra calories came from protein in one arm of the study. In the other arm, they came from carbs. After the shake, people could eat whatever they wanted from a buffet.

Neither carbs nor protein made a difference in what people ate. There was no effect on satiety or total calories consumed. As expected, though, people used a little more energy for digesting and metabolizing the protein than the carbs.

If more carbs were problematic, the investigators might have seen an effect on energy balance. But they didn’t. Maybe that’s because this tightly controlled study only lasted five days. Maybe.

But then again, studies like DIETFITS go longer and they don’t find much difference, either. Low carb or low fat, we keep getting the same answer. Results vary from person to person. There is no one righteous and superior diet.

Challenging Standards for Dietary Studies

Adding to the confusion is a simple fact. Dietary studies are messy. They’re hard to do. And often, they’re not very well controlled. Yesterday in JAMA Network Open, David Ludwig, Cara Ebbeling, and Steven Heymsfield pointed this out:

Among top-rated clinical journals, most diet trials would not satisfy an essential criterion for prospective registration, as judged by standards inferred from drug trials.

Expanding on this in a commentary for the New York Times, Ludwig and Heymsfield point out that dietary studies are inherently challenging. Human behavior is complex when it comes to food. In addition, foods are complex mixtures of many substances. Preparations vary. In fact, the variability is so great that it might as well be infinite.

Finding Your Way

So boiling everything down to carbs, fat, and protein might offer nothing more than a distraction. If you have real concerns about a healthful diet, consult a wise dietitian. They are more likely to offer guidance that fits your specific needs. Broad generalizations are less useful. The same goes for pop diet books.

Click here for the study by Gibson et al. For the paper by Ludwig et al click here and then here for the NY Times commentary.

Round and Round, photograph © Michał Koralewski / flickr

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November 14, 2019