Low-carb beats low-cal (except when it isn’t)

Demonizing carbohydrates continues to be a thing, and now it appears to be moving into diabetic treatment. Why are diabetics so concerned with carbohydrates? Here's a brief, simplified explanation: when you eat carbohydrates, they are broken down in your digestive system into glucose, which enters the bloodstream, raising your blood glucose levels. Your pancreas is then signalled to make insulin, which helps the glucose circulating in your blood to enter your cells so that those cells can make use of the glucose for energy. Diabetics' pancreas don't work as well as they should, and sometimes they don't work at all (I said it would be simplified). When your blood glucose levels fluctuate too much, this can lead to complications of diabetes such as kidney disease, blindness, heart attack or stroke, and more, so it's important for diabetics to monitor their blood sugars and keep them under control.

Fairly recently we've seen research showing that a Mediterranean-style diet, with its emphasis on whole grains, more fruits and vegetables, more fish and shellfish, and less (and leaner) meats, is the best diet for Type 2 diabetics. There's still some discussion (I won't call it controversy), however, over the best type of diet for Type 2 diabetics, especially in non-Western countries.

In Japan, for example, the typical diet is very different from what we are used to here in the United States: a national survey found that the major sources of carbohydrates in the Japanese diet is still rice and processed rice, while in the US the major sources of carbohydrates are soft drinks and soda and breadstuffs (including breads, rolls, pastries, cookies, and the like). Further, the average Japanese diet tends to be about 57% carbohydrates, while in the US the average diet is 49% carbohydrate. With those differences in mind, a team in Japan wondered if a low-carbohydrate diet might be a better fit for Type 2 diabetics - that is, lead to improved control as well as be easier to follow - than the more usual reduced-calorie and macronutrient-balanced diet (Clin Nutr 2017;36:992-1000).

The participants were all overweight, diagnosed Type 2 diabetics who had poor HbA1C scores (a measure of long-term glucose control) and who had already received the standard nutrition counseling (indicating that the standard of care was not working well for them). Half of the participants (32 men and women) were assigned to a reduced-calorie diet with the total number of calories set for each individual based on their ideal body weight. They received training from dietitians on consuming a minimum of 150 grams of carbohydrates per day and between 50% and 60% of their total calories from carbohydrates.

The other half of the participants were trained to follow what the researchers termed a low-carbohydrate diet, with the goal of no more than 130 grams of carbohydrates per day, with one-third of those carbohydrates consumed at each meal as much as possible. The number of calories was not restricted, but the dietitians counseled the patients on choosing unsaturated fats rather than saturated fats.

The participants met with the dietitians regularly, at the start of the study and after 1, 2, 4, and 6 months (when the study ended). At each meeting the participants were weighed, their blood tested for diabetic control as well as for cholesterol and other scores, and they brought in a 3-day food record so that the dietitians could make sure they were adhering to their assigned diet.

At the end of the study, those assigned to the low-carbohydrate diet improved their HbA1C score by about 0.7 of a point (from 8 to 7.3). Those assigned to the low-calorie diet didn't see any change in their HbA1C scores at all. Further, those on the low-carb diet lost more weight (about 1.6 kilograms versus 0.6 kilograms, on average). Cholesterol scores stayed about the same for both groups.

The authors conclude that a low-carbohydrate diet is better for Japanese diabetics, improving glucose control as well as body weight. And if all you looked at were those scores, that would be true.

The problem is when you look at what the participants actually ate. The goal for the low-carbohydrate group was 130 grams of carbohydrates per day. First, this is not a low-carb diet, even by Japanese standards (remember they regularly consume more carbohydrates than those in the US): even the authors admit that "the goal of carbohydrate was moderate-low." Second, the average number of carbohydrates the "low-carb" dieters consumed was 149 grams per day - which is about the minimum recommended as the standard of care.

Worse yet, even though the low-carbohydrate dieters had no limit on the number of calories they were to consume, in practice they actually cut their calories significantly more than those assigned to the low-calorie diet: by about 400 calories per day, on average. The low-calorie dieters, although they were instructed to do so, did not in practice cut their calories by any significant amount.

What this means for you

When the low-carb dieters cut calories more than the low-calorie dieters, you just can't conclude that it's the lower carbohydrates that are the reason for better diabetic control. The only way to prove that lower carbohydrates equals better HbA1C scores is for the number of calories as a whole to be as controlled as the number of carbohydrates (and both controlled better than in this study).

First posted: August 30, 2017