Some carbohydrates are good for the heart

Relation

Martínez-Gonzáles MA, Fernandez-Lazaro CI, Toledo E, et al. Changes in carbohydrate quality and concomitant changes in cardiovascular risk factors: a longitudinal analysis in the randomized PREDIMED-Plus trial.Am J Clin Nutr. 2020;111(2):291-306.

Draft

Prospective analysis. Multicenter, randomized primary prevention trial (the PREDIMED-PLUS trial).

Participant

A cohort of the PREDIMED study of 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 55 to 75 years old and the women were 60 to 75 years old.

Study parameters assessed

Researchers randomly assigned participants to a control group or an intervention group. The control group received instructions and advice on adhering to a Mediterranean diet. The intervention group received instructions and advice on adhering to an energy-restricted Mediterranean diet that focused on reducing caloric intake by 500 to 1,000 kcal/day and limiting refined carbohydrate sources. Researchers administered a dietary frequency questionnaire to participants at baseline, 6 months, and 12 months; from this they calculated the energy and nutrient intake for each subject.

For each participant, researchers calculated a carbohydrate quality index (CQI) based on fiber intake, the glycemic index of foods consumed, the ratio of whole-grain to whole-grain carbohydrates, and the ratio of solid carbohydrates to solid + liquid carbohydrates.

At 6 and 12 months, researchers also assessed participants in the intervention arm on a 17-point scale of adherence to their given diet.

Primary outcome measures

The primary endpoint of this study was weight change at 6 and 12 months. Secondary endpoints included changes in waist circumference, blood pressure, blood glucose, hemoglobin A1c and blood lipid levels.

Key insights

Overall, participants with the greatest change in their CQI had the greatest reductions in weight, waist circumference, and blood pressure. After 6 months, this group also had more significant changes in triglyceride levels, blood sugar and hemoglobin A1c. After 12 months, these improvements were sustained, with additional improvements in the ratio of total cholesterol to high-density lipoprotein (HDL) cholesterol. In general, participants in the intervention group had a greater change in CQI than those in the control group.

Practice implications

Many studies have established the Mediterranean diet as a superior plan for long-term weight loss and cardiovascular health. A few large meta-analyses have been published this year 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/exercise diets. The review authors noted that “the most consistent evidence has been reported for the Mediterranean diet, with evidence of improvement in weight, BMI, total cholesterol, glucose and blood pressure.”¹A further comparison of 14 popular, named diet programs for weight loss and cardiovascular health found that although the Mediterranean diet showed slightly smaller effects on weight loss and cardiovascular parameters after 6 months, it was the only diet plan that showed sustained effects after 12 months.²

What sets the study currently under review apart is that it compares two versions of a Mediterranean diet and examines the data through the lens of changes in the quality, rather than quantity, of carbohydrates consumed by participants. Researchers advised all participants in the study to eat a version of the Mediterranean diet, with the control group using a more traditional format and the intervention group using a "restricted energy" version. The differences between the two are: The energy-restricted diet has restricted upper limits on the consumption of white bread, pasta, white rice and alcohol; no added sugar to drinks; a minimum recommendation for whole grains and pasta; and reduced limits on consumption of red meat, pork, butter and cream.^3.4

While we all have an innate feeling that refined carbohydrates are nutritionally inferior to whole grains, fruits, and vegetables, the CQI helps us quantify how choosing one carbohydrate source 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 fiber, glycemic index, the ratio of whole grains to refined grains or grain products, and the ratio of solid to liquid carbohydrates.⁵

What sets the study currently under review apart is that it compares two versions of a Mediterranean diet and examines the data through the lens of changes in the quality, rather than quantity, of carbohydrates consumed by participants.

This study does not specify the differences between the lowest and highest quintiles of CQI, but there are several studies that make use of it. For example: A study of Ghanaian women found that those whose diets were in the highest quintile of the CQI had lower rates 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 examined health markers based on participants' CQI at a specific point in time, this study examined the extent of change in CQI from baseline to 6 and 12 months. So the data is not based on the absolute amount of consumption by each participant; They are based on how much the participants' eating habits changed during the study. Those in the highest CQI quintile increased consumption of fruits, vegetables, whole grain bread, fish, legumes, fiber, and nuts and decreased consumption of refined grains, white bread, sweetened beverages, red meat and pork, and dairy products. Those who were least adherent to a Mediterranean diet at the start of the study generally showed the greatest improvements in CQI. Additionally, the intervention group had 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 thelancetpublished a meta-analysis that found fiber and whole grain consumption, but not glycemic index, had the greatest 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 fiber consumers with the lowest consumers....Risk reduction...was greatest when daily fiber intake was between 25 g and 29 g. Similar results were observed for whole grain intake."⁷

Similarly, a Korean study published in 2020 looked at both protein and carbohydrates. The Korean study compared adults who ate moderate and high-carb diets, further dividing them into those who ate primarily plant vs. animal proteins. They found that those who ate a low-carb diet with a high intake of plant protein had the lowest cardiovascular risk factors.⁸

Overall, the data supports the guidelines we have adopted as clinicians for ages: eat whole foods, maximize fresh fruits and vegetables, minimize refined starches and sugars, and consume animal products in moderation. Clinically, we can look at the criteria used to determine CQI and advise patients on how to optimize their diet to support a healthy weight and cardiovascular health. Systematic classification of carbohydrate quality can help patients visualize what their intake should look like: According to this study, one should eat 25 grams or more of fiber per day, choose whole grains over refined grains and sugar, and minimize sweetened drinks. If patients need additional guidance and/or need to focus more on glycemic control, using the glycemic index as a guide may also be helpful.

But how do we individualize plans for our patients? An interesting question that this study raises for me is whether a regionally adapted diet also has an impact on health. As we learn more about how our genetics and epigenetics impact us as individuals, I expect we will find that eating a diet consistent 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 guidelines for the prevention of atherosclerotic cardiovascular disease are similar to the energy-restricted Mediterranean diet, except for the recommendations to increase the intake of seaweed and soy products, as well as the recommendation to eat only moderately 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 world. Do we advise patients to eat based on where their family is from or what grows well in their region, or is there a universal plan that will make everyone healthy? I'm eagerly awaiting the next round of information.