Calorie cutting is usually associated with weight loss, but reducing the amount of calories we eat may have a range of unseen benefits. According to the results of a historic two-year trial published Thursday, even healthy people at a normal weight could reap additional benefits from cutting about 300 calories per day.
The trial, published in The Lancet Diabetes and Endocrinology, is among the first to demonstrate that restricting calories can lead to a series of positive health effects in healthy humans. Study co-authors William Kraus, Ph.D., and Carl Pieper, Dr.P.H., both at Duke University, report that when healthy adults cut roughly 300 calories from their diets, their cholesterol, blood pressure, and blood sugar levels improved and biomarkers of inflammation decreased. The results held true even after accounting for weight loss.
“We were surprised at the magnitude of the effect in relatively young and normal weight individuals,” Kraus tells Inverse.
Those findings, explains Pieper, suggest that the benefits of controlling calorie intake extend far beyond weight loss. Cutting back on calories may impact heart health on a molecular level.
“It is our hope that these results can be useful in informing people that caloric restriction can have a beneficial effect across a variety of health indicators — including cardiovascular functioning,” Pieper tells Inverse.
What Happens When You Restrict Calories?
These results come from 218 participants who were enrolled in the second phase of the multi-institution CALERIE study, a two-year investigation into the effects of caloric restriction. When phase one (a smaller, pilot phase) began in 2006, it was the first to specifically investigate caloric restriction in humans, though restricting calories has been studied extensively in animals.
Two of the most famous animal studies began in the ‘80s, when the University of Wisconsin and the National Institute on Aging both began studying rhesus monkeys. After 20 years, the University of Wisconsin researchers found that monkeys on calorie-restricted diets tended to live longer and had lower rates of cancer, cardiovascular disease, and insulin resistance, though the NIA study showed no effects.
Naturally, those conflicting results from two major studies caused some confusion. But in 2017, the teams came back together and reanalyzed the studies, from the data, all the way down to their methods. That cleared up the picture: Caloric restriction did extend life in monkeys.
"We were surprised at the magnitude of the effect in relatively young and normal weight individuals."
In the new study, the team digs deep into exactly what happens when humans cut calories out of their diet, zooming in to see what happens in the body when calories are scarcer than usual.
All of the markers for cardiometabolic disease that the team measured improved significantly when participants restricted their calories, including levels of LDL (bad cholesterol) and blood pressure. But they also noted a reduction in the level of high-sensitivity C-reactive protein, a marker of heart disease risk that also may be linked to cognitive impairment and aging.
After two years, participants who restricted their diets were able to reduce their levels of C-reactive protein to levels that, according to American Heart Association guidelines, would reduce their risk of heart disease by 50 percent.
In the paper, they add that these findings have “profound public health relevance,” though they’re still not sure exactly why caloric restriction is driving them, Kraus said in a statement on Thursday.
“There’s something about caloric restriction, some mechanism we don’t yet understand that results in these improvements,” he said.
How Hard Is It to Cut Out 300 Calories?
To put this research into perspective, these 300 calories amounted to 11.9 percent of the total calories in the study participants’ diets — that means that on average, they went from eating 2467 calories per day to eating 2170 calories per day. That’s the equivalent of cutting out a dessert like a Starbucks vanilla frappuccino or six Oreos. It’s also roughly the equivalent of a slice of pizza.
"Some caloric restriction is easily attainable."
That level of calorie cutting, though it may sound small, does take some getting used to. Initially, the authors aimed for a 25 percent reduction in calories, but people only achieved 11.9 percent of that goal over the course of their experiment.
“Some caloric restriction is easily attainable. It is more difficult to attain and sustain 25% caloric restriction such as was prescribed in this trial,” Pieper says.
But overall, the benefits of calorie restriction weren’t limited to people who were able to make significant cutbacks. “However, any level of restriction was associated with a lowering of these markers,” Pieper says.
Regardless of whether 300 seems like an attainable number, this trial suggests that it may be a sacrifice worth working towards — even for adults who aren’t looking to lose weight.
“These findings are of substantial public health importance even when started in people who are healthy, young and middle-aged, and not obese,” the team writes.
Methods: Methods CALERIE was a phase 2, multicentre, randomised controlled trial in young and middle-aged (21–50 years), healthy non-obese (BMI 22∙0–27∙9 kg/m2) men and women done in three clinical centres in the USA. Participants were randomly assigned (2:1) to a 25% calorie restriction diet or an ad libitum control diet. Exploratory cardiometabolic risk factor responses to a prescribed 25% calorie restriction diet for 2 years were evaluated (systolic, diastolic, and mean blood pressure; plasma lipids; high-sensitivity C-reactive protein; metabolic syndrome score; and glucose homoeostasis measures of fasting insulin, glucose, insulin resistance, and 2-h glucose, area-under-the curve for glucose, and insulin from an oral glucose tolerance test) analysed in the intention-to-treat population. This study is registered with ClinicalTrials.gov, number NCT00427193.
Findings: From May 8, 2007, to Feb 26, 2010, of 238 participants that were assessed, 218 were randomly assigned to and started a 25% calorie restriction diet (n=143, 66%) or an ad libitum control diet (n=75, 34%). Individuals in the calorie restriction group achieved a mean reduction in calorie intake of 11·9% (SE 0·7; from 2467 kcal to 2170 kcal) versus 0·8% (1·0) in the control group, and a sustained mean weight reduction of 7·5 kg (SE 0·4) versus an increase of 0·1 kg (0·5) in the control group, of which 71% (mean change in fat mass 5·3 kg [SE 0·3] divided by mean change in weight 7·5 kg [0·4]) was fat mass loss. Calorie restriction caused a persistent and significant reduction from baseline to 2 years of all measured conventional cardiometabolic risk factors, including change scores for LDL- cholesterol (p<0·0001), total cholesterol to HDL-cholesterol ratio (p<0·0001), and systolic (p<0·0011) and diastolic (p<0·0001) blood pressure. In addition, calorie restriction resulted in a significant improvement at 2 years in C-reactive protein (p=0·012), insulin sensitivity index (p<0·0001), and metabolic syndrome score (p<0·0001) relative to control.
Interpretation: 2 years of moderate calorie restriction significantly reduced multiple cardiometabolic risk factors in young, non-obese adults. These findings suggest the potential for a substantial advantage for cardiovascular health of practicing moderate calorie restriction in young and middle-aged healthy individuals, and they offer promise for pronounced long-term population health benefits