By David Kiefer, MD, Editor

Clinical Assistant Professor, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona, Tucson

Dr. Kiefer reports no financial relationships relevant to this field of study.

SUMMARY POINTS

  • This was a randomized, controlled trial investigating the effects of Mediterranean diet enriched with either extra-virgin olive oil or mixed nuts on body weight and central adiposity over five years in people with cardiovascular disease risk.
  • The olive oil group lost 0.43 kg more weight (P = 0.44), and gained 0.56 cm less waist circumference (P = 0.48) than those eating a control diet.
  • The mixed nuts group gained almost a full centimeter less (0.94 cm; P = 0.006) of waist circumference than the control group.
  • There are many possible physiological reasons for the effects seen, including the antioxidant (polyphenol) content of olive oil and the benefits of dietary fat vs. carbohydrate.

SYNOPSIS: The long-term vegetable fat enrichment of a Mediterranean diet leads to less weight gain and less increase of central adiposity than a control diet.

SOURCE: Estruch R, Martinez-Gonzalez MA, Corella D, et al. Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: A prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial. Lancet Diabetes Endocrinol 2016; Jun 6. pii: S2213-8587(16)30085-7. doi: 10.1016/S2213-8587(16)30085-7.

This study is another reworking of the large PREDIMED trial that brought us information about the benefits of the Mediterranean diet on numerous conditions, including cardiovascular disease, breast cancer, and Alzheimer’s disease. PREDIMED, or PREvención con DIeta MEDiterránea, is a five-year, multicenter, randomized, controlled trial that includes 7,447 people at high cardiovascular risk (type 2 diabetes or three or more cardiac risk factors). It has three arms: 1) a Mediterranean diet with no calorie restriction and enriched with extra-virgin olive oil (EVOO) (n = 2,543); 2) a Mediterranean diet with no calorie restriction and enriched with mixed nuts (n = 2,454); or 3) a control diet (participants were advised to avoid all dietary fat; n = 2,450). The primary outcome was the primary prevention of cardiovascular disease (this was proven and received widespread media attention). However, the secondary outcome of interest in the article being reviewed here was the effect of these different diets on body weight and waist circumference.

After randomization, study participants met with trained dietitians approximately every three months to receive individualized dietary advice about the Mediterranean diet (two intervention arms) or about how to limit all dietary fat (the control diet). The goal was to reduce fat to less than 30% of calories (control diet) or increase fat to more than 40% of calories (two intervention arms). The two intervention arms were supplemented, free of cost, with one liter of EVOO weekly or 30 grams of mixed nuts daily. At baseline and annually for five years, trained personnel measured waist circumference to 0.5 cm and body weight on calibrated scales to 0.5 kg. Diet questionnaires closely followed adherence to the different dietary arms.

The median time of follow-up was 4.8 years. At the end of the trial, 1,501 participants in the EVOO group, 1,241 participants in the mixed nuts group, and 1,243 participants in the control group remained in the trial and were assessed. The authors accounted for all study dropouts, and analyzed the data in an intention-to-treat analysis.

The mean age of the study participants was 67 years, 40-46% were male, the range in body weight was 76.6-77.0 kg, and the range of waist circumference was 100.2-100.9 cm, depending on the group. More than 90% of the participants were overweight or obese. Over the five years, predictably, the EVOO group had an increase in consumption of EVOO (and, interestingly, mixed nuts) and the mixed nuts group showed an increase in mixed nuts consumption — no surprise there. Both Mediterranean diet groups ingested more vegetables, legumes, fruits, and fish, and ate fewer sweets, meat, and dairy. After five years, the control group went from an average of 40.0% of calories from fat to 37.4% (P < 0.001), whereas the EVOO and mixed nuts groups increased fat calorie percentage from 40.0% to 41.8% and 40.4% to 42.2%, respectively (P < 0.001).

With respect to the outcomes, all three groups had a slight decrease in body weight and a slight increase in waist circumference (see Table 1). However, when compared to the control diet, the two intervention diets showed some interesting trends (see Table 2). For example, the participants eating a Mediterranean diet with EVOO lost 0.43 kg more weight (P = 0.44), and gained 0.56 cm less waist circumference (P = 0.48) than those eating a control diet. The mixed nuts group had a similar (P = 0.73) loss of body weight as the control group, but gained almost a full centimeter less (0.94 cm; P = 0.006) of waist circumference than the control group.

Table 1: Changes in Body Weight and Waist Circumference in Three Diets after Five Years

 

Body Weight

Waist Circumference

Control diet

-0.6 kg

+1.2 cm

Mediterranean diet with extra-virgin olive oil

-0.9 kg

+0.8 cm

Mediterranean diet with mixed nuts

-0.4 kg

+0.4 cm

Table 2: Difference in Body Weight and Waist Circumference in Two Intervention Diet Arms When Compared to Control Diet

 

Body Weight

Waist Circumference

Mediterranean diet with extra-virgin olive oil

-0.43 kg (P = 0.44)

-0.55 cm (P = 0.048)

Mediterranean diet with mixed nuts

-0.08 kg (P = 0.73)

-0.94 cm (P = 0.006)

COMMENTARY

There are several important takeaways from this study. First is that, as we all know, it is difficult, but not impossible, to change one’s diet. In all three arms, participants were able to shift the percentage of their calories from fat, albeit slightly. It could be argued that this small adjustment to nutrition required, or at least benefitted from, regular meetings with dietitians and the provision of supplemental EVOO or mixed nuts. Could we achieve such results in our patient population? Yes, but we might need to brainstorm with our clinic staff to determine the best way to regularly counsel and support our patients on their road to nutrition and lifestyle change.

Second, changing one’s diet does indeed affect physiology. In this case, perhaps the changes are not striking, but they are there. In the Mediterranean diet groups, the participants were able to halt their midsection growth, and even lose a bit more weight than the control group. There likely are complicated physiological factors at work, given what the scientific community is starting to learn about nutritional (especially carbohydrate) connections to and diabetes risk with central obesity, but at least we can say that this five-year intervention did have an effect. It will have to be left to future studies to determine the how and why. Related to this is the fact that the authors claimed to rebuke the hypothesis that high vegetable-fat diets, such as the two intervention arms here, will promote weight gain and central adiposity. Such concerns were mostly seen in the low-fat dietary push a few decades ago, but nonetheless are still seen in some current clinical recommendations. The growing body of evidence now speaks to the fact that we shouldn’t be afraid of fat, especially EVOO or those from a moderate amount of mixed nuts.

Another interesting conclusion from this study is that some of the results were more pronounced in the EVOO group than the mixed nuts group. Why might that be the case? EVOO contains more than just the important monounsaturated fat, oleic acid. The “catch” in the back of the throat that comes from eating high-quality EVOO is due to a family of antioxidant compounds, polyphenols, which may give an extra positive boost to the physiologic effects seen in this study.

As clinicians try to translate these results into clinical practice, it is important to keep in mind the demographic studied here: older overweight individuals at risk for cardiovascular disease, the majority of whom were women. In these cases, there should be no fear of increasing fat intake from EVOO or mixed nuts; the benefits on body weight and waist circumference may be small, but they are significant.