Calorie restriction helped improve all factors of metabolic syndrome in people without obesity, according to the phase II CALERIE trial.
Adults who restricted their calories by 12% — about 300 fewer calories a day — saw significant improvement in metabolic syndrome score compared with adults on a control diet after 12 months, and this improvement was maintained through 24 months, William Kraus, MD, of Duke University School of Medicine in Durham, North Carolina, and colleagues wrote in The Lancet Diabetes & Endocrinology.
Compared with the control dieters, calorie restrictors saw significant improvements in multiple cardiovascular risk factors from baseline through two years of the diet:
- Systolic BP: +2.15 mm Hg (control) vs -2.20 mm Hg (calorie restriction)
- Diastolic BP: +1.55 mm Hg vs -3.40 mm Hg
- LDL-cholesterol: +0.03 mmol/L vs -0.23 mmol/L
- Total cholesterol to HDL-cholesterol ratio: -0.047 vs -0.532
- Triglycerides: -0.03 mmol/L vs -0.27 mmol/L
Calling the trial “groundbreaking” in an accompanying commentary, Frank Hu, MD, PhD, of Harvard TH Chan School of Public Health in Boston, praised the study for being the “first long-term calorie restriction intervention in non-obese young and middle-aged participants with a large sample size.”
Hu also pointed out that the high retention and compliance rate was a particular strength of the trial, which was “higher than a typical weight loss trial.”
Regardless, the hurdle to this type of intervention is adherence to a calorie-restricted diet long-term in the general population. This was represented by the 18% dropout rate in the calorie-restricted group versus only a 5% dropout rate in those on the control diet.
The benefits of a calorie-restricted diet likely span beyond cardiovascular health, Hu noted, highlighting how this diet strategy is also being studied for slowing the aging process and extending lifespan. Despite these benefits, Hu seemed less then convinced that this diet strategy could actually put a dent in the global obesity epidemic.
“We are living in an increasingly obesogenic environment with an abundance of energy-dense, nutrient-poor foods, which are cheap, readily available, accessible, and heavily marketed,” he wrote, adding that “[b]ecause individual food choices are shaped by the food environment, the long-term sustainability of calorie restriction and its benefits on body weight can be easily undermined.”
Kraus and colleagues also noted some other significant benefits after two years of a calorie-restricted diet, including an improvement in fasting insulin (-1.71 μIU/mL), fasting glucose (-0.05 mg/dL), C-reactive protein (-0.068 nmol/L), and insulin sensitivity (+0.099).
Although not surprised by the many health benefits of a calorie-restricted diet, Kraus told MedPage Today his group was “surprised at the magnitude of the effect in already healthy, relatively young, normal weight individuals. And these effects were above and beyond those due to the weight loss alone.”
More than advocating solely for calorie restriction, Kraus pointed out how “maintaining a healthy weight and getting adequate amounts of physical activity are important health goals,” which should be advocated to patients. However, he also added that it’s “entirely possible that some degree of caloric restriction should be included with these other health behaviors due to its beneficial effects on health parameters.”
The Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) trial included 143 adults on the calorie-restricted diet and 75 adults on an ad libitum control diet with a baseline BMI from 22 to 28, the majority of whom were women. Those on the diet were instructed to restrict their calories by 25%, but few achieved this level of restriction.
During the initial 6 months on the diet, the group averaged a 19.5% decrease in energy intake — accounting for about 480 kcal less per day — which later dropped to an average of 9.1% fewer calories after six months. During the two-year study period, adults on the diet intervention averaged an 11.9% reduction in energy intake per day, or 216 fewer calories a day. Those in the control diet saw a slight, but non-significant 0.8% decrease in mean calorie intake.
After two years, those who restricted their calories saw a 16.5 lb (7.5 kg) drop in body weight and a 2.6 point drop in BMI. The majority of body weight lost was also fat mass — 11.7 lb (5.3 kg) of total weight lost — with only a 4.9 lb (2.2 kg) drop in fat-free mass. They saw a 4.6% reduction in body fat, as well. On the other hand, the control dieters experienced a small 0.2 lb (0.1 kg) increase in body weight after two years.
In a sensitivity analysis controlled for reduction in body weight, Kraus’ group still determined a “substantial residual and significant dose–response effects of calorie restriction on cardiometabolic risk factors,” they reported.
The study was funded by the National Institute on Aging and National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health. Kraus and co-authors reported no disclosures.
Hu reported grants from the National Institutes of Health and has received research support from the California Walnut Commission and honoraria for lectures from Metagenics and Standard Process and honoraria from Diet Quality Photo Navigation.