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Nutrition is essential to life; as a minimum, it is necessary to eat enough to fuel basic energy needs. But eating is also pleasurable, and the popularity of diet books and food preparation television shows suggests that many people are fascinated, perhaps even preoccupied, with dietary choices and dietary indulgences. Food options are ubiquitous as well; even hardware stores and gasoline stations offer food options, and many other nonrestaurant or food markets offer food for purchase. Dietary advice is widely sought, and often new diet studies garner wide media attention. As an example, a 2019 article published in JAMA on the association of egg intake with incident cardiovascular disease and mortality had collected 13 citations, 42 000 views, and an Altmetric score higher than 3500 within 4 months of publication.1 With all of this attention, there is still great disagreement and controversy about what are the healthiest diet choices.
Some of the root causes of the disagreement about the healthiest diet choices have been discussed in Editorials and Viewpoints in JAMA. It is acknowledged that many studies are potentially biased because of industry funding and study design.2-4 Many studies are observational, and measures of diet exposure are often inconsistent, incomplete, or inaccurate.2 The typical criterion standard—the randomized clinical trial—is potentially feasible with diet interventions, but long-term studies are notoriously difficult in large part because of poor long-term adherence to prescribed diets.
In this issue of JAMA, a large-scale clinical trial from Spain, conducted by a collaborative group known as the PREDIMED-Plus consortium, reports interim results of a long-term study of patients with metabolic syndrome.5 The trial includes 6874 participants. Those randomized to the study intervention group were encouraged to consume an energy-reduced Mediterranean diet and to engage in physical activity, while the control comparison group was given advice to follow an energy-unrestricted Mediterranean diet. The intervention is planned to last at least 6 years and the median follow-up time for the clinical end points is expected to be 8 years. Because of the crucial importance of diet adherence in a long-term diet intervention study, the authors examined participant-reported dietary changes, body weight changes, and other risk factors after the first 12 months. The primary outcome was the reported adherence to an energy-restricted Mediterranean diet. At 12 months, the intervention group achieved greater adherence to the energy-restricted Mediterranean diet than the control group. Importantly, these are interim results, and the long-term outcomes of the effect of the dietary intervention on major clinical end points (as per the study design) will be of importance. However, because of the essential importance of diet to population health, these preliminary results are published in JAMA.
These interim results are meaningful in several ways. First, the greater diet adherence in the intervention group provides reassurance that the intervention is having a measurable effect on diet and body weight. This is an important intermediate step but not yet sufficient to inform new dietary recommendations. Second, the authors have demonstrated in this study and previous research6 that large-scale dietary intervention studies are practical if carefully conducted. The long-term main end point results are eagerly awaited.
Corresponding Author: Philip Greenland, MD, Departments of Preventive Medicine and Medicine, Northwestern University Feinberg School of Medicine, 680 N Lake Shore Dr, Ste 1400, Chicago, IL 60611 (firstname.lastname@example.org).
Conflict of Interest Disclosures: None reported.
Greenland P. Dietary Adherence in a Clinical Trial of a Nutritional and Behavioral Intervention. JAMA. 2019;322(15):1500. doi:10.1001/jama.2019.15855
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