Does eating carbohydrates last during a meal improve blood glucose levels? Original paper

In this randomized controlled trial in participants with prediabetes, eating protein and nonstarchy vegetables before starchy carbohydrates at each meal did not meaningfully affect glycemic control or cardiometabolic risk factors.

The effect of a carbohydrate-last food order dietary intervention on cardiometabolic risk factors in people with prediabetes.

The primary outcome was the change in glucose tolerance, defined as the number of participants who achieved a reduction in blood glucose of at least 15% during a 75-gram oral glucose tolerance test. The secondary outcomes were HbA1c, HOMA-IR, beta-cell function (measured using the disposition index), blood lipids, body weight, and dietary intake.

39 adults (average age of 60; 56% women and 44% men) with prediabetes and overweight or obesity.

In this 16-week randomized controlled trial, the participants received standard nutrition counseling (the control) or standard nutritional counseling plus carbohydrate-last food order counseling. The carbohydrate-last food order counseling involved advising the participants to consume nonstarchy vegetables and protein-rich foods before starchy carbohydrates at each meal. If this was not feasible for a meal (e.g., when consuming a sandwich), the participants were instructed to start the meal with nonstarchy vegetables, such as a salad.

To assess changes in dietary intake, the participants were instructed to complete a 3-day food diary at baseline and the end of the study. Physical activity levels were assessed at baseline and the end of the study using a questionnaire.

Handout given to participants outlining the carbohydrate-last food order intervention

The above graphic is a recreation of the handout used in the study.

The only statistically significant difference between groups was that protein intake increased in the intervention group, compared to the control group (intervention group: +10 grams per day; control group: –15 grams per day).

Compared to baseline, body weight and HbA1c decreased and vegetable intake increased in the intervention group compared to baseline, but not compared to the control group. In the control group, intake of calories, fat, protein, grain, and dairy decreased compared to baseline, but not compared to the intervention group.

Numerous studies have tested whether consuming certain foods and nutrients before others during a meal improves the postprandial (postmeal) glycemic response. The evidence from acute studies — studies that examined the glycemic response to a single meal — demonstrates that a carbohydrate-last food order generally attenuates the postprandial rise in blood glucose.^[1] Moreover, this effect has been documented in people with healthy blood glucose levels, as well as people with prediabetes or type 2 diabetes (T2D).

However, a notable limitation of the evidence from acute trials is the total duration in which the postprandial glycemic response is assessed. Virtually all of the studies in this area of research measured the glycemic response over 2 to 3 hours. To this point, 2 separate studies in participants with T2D reported that a carbohydrate-last food order resulted in lower blood glucose levels at 30 to 90 minutes after meal consumption, compared to a carbohydrate-first food order, but the blood glucose level later on (i.e., at 180–240 minutes) was lower with the carbohydrate-first meal order.^[2][3]

If a carbohydrate-last food order intervention improves blood glucose levels within a brief window of time, but doesn’t translate into improvements in the average 24-hour blood glucose level or HbA1c, then it’s of little utility for improving long-term health outcomes. Therefore, it’s important to examine whether the results from longer-term studies match up with the data from acute studies. Unfortunately, very few long-term studies have been conducted.

There have been 2 other long-term randomized controlled trials involving carbohydrate-last interventions. The first, which lasted 2 years, assigned the participants to receive either standard healthy eating advice for T2D or instructions to consume nonstarchy vegetables before starchy carbohydrates at every meal.^[4] HbA1c decreased in both groups compared to baseline, but the decrease was greater in the carbohydrate-last food order group.

When interpreting these findings, it’s important to consider the potential influence of differences in nutrient intake. At the end of the study, the carbohydrate-last food order group had higher intakes of protein (73 vs. 61 grams per day), fiber (18.4 vs. 14.5 grams per day), and green vegetables (+112 vs. +49 grams per day, compared to baseline). In accordance with these results, the summarized study also reported a higher protein intake in the carbohydrate-last food order group, compared to the control group, and an increase in vegetable intake compared to baseline.

As such, it’s possible that the beneficial effects of a carbohydrate-last food order intervention are largely derived from changes in nutrient intake, specifically an increase in the intake of protein, fiber, and vegetables. The proposed mechanism of action is that consuming protein and/or fat enhances the secretion of GLP-1,^[5] a hormone that stimulates insulin secretion, delays gastric emptying, and suppresses appetite.^[6] Fiber also seems to delay gastric emptying, presumably by increasing the viscosity of food mass in the gastrointestinal tract.^[5][7] Fiber may also improve postprandial glucose levels acutely by reducing intestinal absorption of carbohydrates and fats, while it may facilitate benefits in the long term by altering gut microbiota composition.^[7]

In the second study involving a carbohydrate-last intervention,^[8] which was 8 weeks long, 2 groups were instructed to consume a calorie-restricted (daily energy deficit of 200 kcal), standard diet for T2D. The only difference between groups was the order in which foods were consumed at lunch and dinner. Although measures of glycemic control improved compared to baseline in the carbohydrate-last food order group only, there were no clear differences between groups for changes in the examined outcomes, with the exception of coefficient of variation (a measure of glycemic variability, i.e., how much blood glucose levels fluctuate from the average 24-hour blood glucose level), which decreased (improved) in the carbohydrate-last food order group, compared to the control group.

As demonstrated by the above, there is relatively weak evidence to indicate that a carbohydrate-last food order intervention can improve glycemic control in participants with T2D. This is why the summarized study is notable, as it examined the long-term effects of a carbohydrate-last food order intervention in participants with prediabetes. Even more so than the limited evidence in participants with T2D, there did not appear to be any benefit of a carbohydrate-last food order intervention. There are some possible reasons that could explain the lack of an effect.

It’s important to consider the intervention used in the control group. Both the intervention and control groups received standard nutritional counseling from a registered dietitian, which consisted of education on portion sizes and macronutrients, healthy beverages, and limiting salt intake. It’s possible that if the control group did not undergo any sort of dietary intervention, meaning that they did not receive nutritional counseling and continued with their usual diet, there would have been more significant differences between groups in the examined outcomes, as there were a number of changes in dietary intake reported in the control group compared to baseline.

Additionally, according to evidence from acute trials, the effect of a carbohydrate-last food order intervention is larger when it is compared to a carbohydrate-first food order intervention, as opposed to when all the components of the meal are consumed simultaneously.^[1] In the summarized study, it was not reported that any specific instructions were given to the control group with respect to which foods to consume first or last during each meal.

Although these factors can overshadow the potential benefits of a carbohydrate-last food order intervention, the intervention used in the control group in the summarized study was similar to those used in the studies on participants with T2D, and these studies still reported beneficial effects of a carbohydrate-last food order intervention. As such, it’s possible that a carbohydrate-last food order intervention just doesn’t confer clinically meaningful effects on glycemic control in people with prediabetes.

A final factor that may have contributed to the lack of significant differences between groups is adherence to the carbohydrate-last food order intervention. Data from exit surveys completed by the participants in the intervention group indicated that 61.1% of participants ate vegetables/protein before carbohydrates at meals “more than half the time”, whereas 33.3% answered “nearly all the time”, and 5.6% (1 participant) answered “half the time”. So, adherence to the intervention wasn’t perfect, and this could have contributed to the lack of differences between groups.

In summary, a carbohydrate-last food order intervention has been shown to improve the postprandial glycemic response in acute trials in participants with or without impaired glycemic control, but there is a lack of evidence from randomized controlled trials to indicate that this strategy improves measures of glycemic control in the long term. There may be a small benefit of a carbohydrate-last food order intervention in people with T2D, but the present study failed to replicate these findings in people with prediabetes. Further randomized controlled trials are needed to determine whether a carbohydrate-last food order intervention can improve markers of glycemic control over the long term in people with prediabetes.