Martínez-Gonzáles MA, Fernandez-Lazaro CI, Toledo E, et al. Carbohydrate quality changes and concurrent changes in cardiovascular risk factors: a longitudinal analysis in the PREDIMED-Plus randomized trial. Am J Clin Nutr. 2020;111(2):291-306.
Prospective analysis. Multicenter, randomized primary-prevention trial (the PREDIMED-PLUS study).
A cohort of the PREDIMED study that included 5,373 overweight/obese Spanish adults (body mass index [BMI] 27-40 kg/m2) with metabolic syndrome and no history of cardiovascular disease. The men in the study were aged 55 to 75 years and the women 60 to 75 years.
Study Parameters Assessed
The researchers randomly assigned participants to a control group or an intervention group. The control group received instructions and counseling for adherence to a Mediterranean diet. The intervention group received instructions and counseling for adherence to an energy-reduced Mediterranean diet that focused on reducing caloric intake by 500 to 1,000 kcal/day and limiting refined carbohydrate sources. Researchers gave participants a food frequency questionnaire at baseline, 6 months, and 12 months; from each of these, they calculated energy and nutrient intakes for each subject.
For each participant, the researchers calculated a carbohydrate quality index (CQI) based on their intake of dietary fiber, the glycemic index of foods eaten, ratio of whole-grain to total-grain carbohydrates, and ratio of solid carbohydrates to solid+liquid carbohydrates.
At 6 and 12 months, researchers also evaluated participants in the intervention arm on a 17-point scale of adherence to their given diet.
Primary Outcome Measures
The primary outcome in this study was weight change at 6 and 12 months. Secondary outcomes included changes in waist circumference, blood pressure, blood glucose, hemoglobin A1c, and blood lipid parameters.
Overall, the participants who had the greatest change in their CQI had the greatest reduction in weight, waist circumference, and blood pressure. After 6 months, this group also had more significant changes in triglyceride levels, blood glucose, and hemoglobin A1c. At 12 months, these improvements persisted with the addition of an improvement in the ratio of total cholesterol to high-density lipoprotein (HDL) cholesterol. Generally, the participants in the intervention group had a greater change in CQI than those in the control group.
Many studies have established the Mediterranean diet as a superior plan for long-term weight reduction and cardiovascular health. This year, a couple of large meta-analyses have been released comparing weight loss and changes in cardiovascular markers for many of the more common weight-loss plans. One review compared Mediterranean, paleolithic, intermittent energy restriction, Nordic, vegetarian, DASH (Dietary Approaches to Stop Hypertension), portfolio, low-carbohydrate, high-protein, low-fat, and low-glycemic index/load diets. The review's authors stated that “the most consistent evidence was reported for the Mediterranean diet, with suggestive evidence of an improvement in weight, BMI, total cholesterol, glucose, and blood pressure.”1 Another comparison of 14 popular, named dietary programs for weight loss and cardiovascular health found that, while the Mediterranean diet showed slightly lower effects in weight loss and cardiovascular parameters at 6 months, it was the only diet plan that showed sustained effects at 12 months and beyond.2
What distinguishes the current study under review is that it compares 2 versions of a Mediterranean diet and reviews the data through the lens of changes in the quality, rather than quantity, of carbohydrates participants consumed. The researchers counseled everyone in the study to consume 1 version of the Mediterranean diet, with the control group using a more traditional format and the intervention group using an “energy-reduced” version. The differences between the 2 are: In the energy-reduced diet, there are restricted upper limits for consumption of white bread, pasta, white rice, and alcohol; no added sugars to beverages; a minimum recommendation for whole-grain cereals and pasta; and decreased upper limits for red meat, pork, butter, and cream consumption.3,4
While we all have the innate sense that refined carbohydrates are nutritionally inferior to whole grains, fruits, and vegetables, the CQI helps us quantify how the choice of 1 source of carbohydrate over another actually impacts health. The CQI is calculated based on 4 components, each on a scale of 1 to 5, with 1 being the least optimal and 5 being the most optimal. The 4 components include dietary fiber, glycemic index, ratio of whole grains to refined grains or grain products, and the ratio of solid to liquid carbohydrates.5
What distinguishes the current study under review is that it compares 2 versions of a Mediterranean diet and reviews the data through the lens of changes in the quality, rather than quantity, of carbohydrates consumed by participants.
This study doesn’t specify the differences between the lowest and highest quintiles of the CQI, but there are several trials that make use of it. For example: A study on Ghanaian women found that those whose diets were in the highest quintile of the CQI had lower incidence of general and abdominal obesity. For reference, in this study in Ghana, the differences between the highest and lowest quintiles for each parameter were: glycemic index Q1=66.3 +/- 3.1, Q5=63.5 +/- 5.9; solid carbohydrates/total carbohydrates Q1=0.900 +/- 0.046, Q5=0.967 +/- 0.019; total fiber (g/day) Q1=17.3 +/- 3.6, Q5=25.5 +/- 8.4; and whole grain/total grain Q1=0.003 +/- 0.038, Q5=0.191+/- 0.166.6
While other studies have looked at health markers based on participants’ CQI at 1 point in time, this study looked at the amount of change in the CQI from baseline to 6 and 12 months. So the data are not based on the absolute quantity of each participant’s consumption; they are based on how much participants' eating habits changed throughout the study. Those in the highest CQI quintile significantly increased consumption of fruits, vegetables, whole-grain bread, fish, legumes, fiber, and nuts and decreased consumption of refined cereals, white bread, sweetened drinks, red meat and pork, and dairy. Those who adhered least to a Mediterranean diet at the onset of the study generally showed the greatest improvements in CQI. Also, the intervention group had a greater overall change in CQI and better improvements in biomarkers of cardiovascular health.
Other studies have shown that high-quality carbohydrates, in the form of fiber, are associated with better cardiovascular health. In 2019, the Lancet published a meta-analysis that found fiber and whole-grain consumption, but not glycemic index, made the most profound impact on weight and cardiovascular health. The authors stated, “Observational data suggest a 15-30% decrease in all-cause and cardiovascular related mortality ... when comparing the highest dietary fiber consumers with the lowest consumers. ... Risk reduction ... was greatest when daily intake of dietary fibre was between 25g and 29g. Similar findings for whole grain intake were observed.”7
In a similar vein, a Korean study published in 2020 looked at protein as well as carbohydrates. The Korean study compared adults who ate moderate- vs high- carbohydrate diets and separated them further into those who ate primarily plant-based vs animal-based proteins. They found that those who ate a moderate-carbohydrate diet with a high intake of plant-based protein had the lowest cardiovascular risk factors.8
Overall, the data substantiate the guidance we as clinicians have embraced for ages: Eat whole foods, maximize fresh fruits and vegetables, minimize refined starches and sugars, and moderate intake of animal products. Clinically, we can look at the criteria used to determine the CQI and advise patients on how to optimize their diets to support a healthy weight and cardiovascular health. Systematically classifying carbohydrate quality can help patients visualize what their intake should look like: According to this study, one should consume 25 grams or more of dietary fiber per day, choose whole grains over refined grains and sugars, and minimize sweetened beverages. If patients need additional guidance and/or more focus on blood sugar control, using glycemic index as a guide may be useful as well.
But how do we individualize plans for our patients? One interesting question this study brings up in my mind is whether eating a regionally appropriate diet also impacts health. As we find out more about how our genetics and epigenetics impact us as individuals, I expect that we will find that eating in line with our heritage will become a critical component in personalizing each patient’s optimal diet. This study shows that the less-refined version of the Mediterranean diet works well for people of Mediterranean heritage. For comparison, the Japan Atherosclerosis Society’s Guidelines for Prevention of Atherosclerotic Cardiovascular Diseases are similar to the energy-reduced Mediterranean diet, with the exception of the recommendations to increase intake of seaweed and soy products, plus the recommendation for only moderate consumption of low-carbohydrate fruits.9
In the United States, we have a mix of cultures, native foods, products that grow well here, and access to products from around the globe. Do we advise patients to eat according to where their family originated from or according to what grows well in their local region, or is there 1 universal plan that makes everyone healthy? I eagerly await the next round of information.