This strategy, known as caloric restriction, has been shown to increase the life span of various organisms and reduce their rate of cancer and other age-related ailments. Whether it can do the same in people has been an open question. But an intriguing new study suggests that in young and middle-aged adults, chronically restricting calorie intake can have an impact on their health.
In the new study, which was funded by the National Institutes of Health and published this month in the Lancet Diabetes & Endocrinology, researchers looked at a group of 143 healthy men and women who ranged in age from 21 to 50. They were instructed to practice caloric restriction for two years. They could eat the foods they wanted so long as they cut back on the total amount of food they ate, with the aim of cutting the calories they consumed by 25 percent.
这项新研究由美国国家卫生研究院（National Institutes of Health，简称NIH）出资并发表在本月的《柳叶刀糖尿病与内分泌学》(Lancet Diabetes & Endocrinology)期刊上，其中研究人员对一群年龄介乎21岁至50岁的143名健康男性和女性进行了观察。他们按照指示，进行了为期两年的卡路里摄入限制。他们可以食用自己想要的食物，条件是减少摄入食物总量，目标是使他们摄入的热量减少25%。
Many did not achieve that goal. On average, the dieters managed to slash about 12 percent of their total calories, or roughly 300 calories a day, the amount in a large bagel, a few chocolate chip cookies or a small Starbucks Mocha Frappuccino. But the group saw many of their cardiovascular and metabolic health markers improve, even though they were already in the normal range.
They lost weight and body fat. Their cholesterol levels improved, their blood pressure fell slightly, and they had better blood sugar control and less inflammation. At the same time, a control group of 75 healthy people who did not practice caloric restriction saw no improvements in any of these markers.
Some of the benefits in the calorie restricted group stemmed from the fact that they lost a large amount of weight, on average about 16 pounds over the two years of the study. But the extent to which their metabolic health got better was greater than would have been expected from weight loss alone, suggesting that caloric restriction might have some unique biological effects on disease pathways in the body, said William Kraus, the lead author of the study and a professor of medicine and cardiology at Duke University.
“We weren’t surprised that there were changes,” he said. “But the magnitude was rather astounding. In a disease population, there aren’t five drugs in combination that would cause this aggregate of an improvement.”
Calorie restriction may be a useful tool for better health and weight loss, but it’s unclear whether the changes in the new study will ultimately translate into longevity and reductions in chronic disease, said Frank Hu, the chairman of the nutrition department at the Harvard T.H. Chan School of Public Health, who was not involved in the research.
热量限制或许是改善健康和减轻体重的有用途径，但尚不清楚这项新研究所揭示的变化最终能否转化为寿命延长和慢性病的减少，未参与该研究的哈佛大学陈曾熙公共卫生学院(Harvard T.H. Chan School of Public Health)营养学系主任胡炳长(Frank Hu)称。
He said the modest improvements experienced by the participants would have to be sustained over time to produce long-term benefits. But he questioned whether caloric restriction would be practical for most people given that “we are living in an obesogenic environment with an abundance of energy-dense, nutrient-poor foods that are cheap, accessible and heavily marketed.”
He added, however, that some people might find calorie restriction feasible if they combined it with other popular dietary strategies like the Mediterranean diet, intermittent fasting or reduced carbohydrate intake.
The new study provided evidence of just how difficult calorie restriction can be. The study participants went through an intensive training program where they learned to cook low-calorie meals, attended group sessions and had regular check-ins with nutrition experts. Still, they were not able to meet even half the goal of a 25 percent cut in calories. And as anyone who has dieted knows, keeping the weight off long-term can be the hardest part.
Still, the new study was groundbreaking in several ways. Funded entirely by the N.I.H. at a cost of $55 million and called Calerie — for Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy — it was the first major clinical trial to examine the effects of caloric restriction in a group of middle-aged or younger adults who were either normal weight or just slightly overweight but not obese. The goal of the trial was to look at whether caloric restriction could influence healthy aging and disease.
但新研究在多个方面都有开创性。研究完全由NIH资助，耗资5500万美元，全名为“Calerie”——降低能量摄入长期效应综合评估(Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy)。这是以体重正常或轻微超重但不肥胖的青壮年为对象，检验卡路里摄入限制影响的首例主要临床试验。试验旨在查看热量限制能否影响健康老龄化进程和疾病状况。
While the calorie target they set was steep, they did give the subjects some flexibility, allowing them to eat the foods that they wanted. What they found was that the subjects did not change their protein intake, but they did eat significantly less fat and slightly fewer carbohydrates. They also consumed more micronutrients like vitamins A, K and magnesium, indicating a big increase in fruits and vegetables, said Susan B. Roberts, a co-author of the study and a senior scientist at the U.S.D.A. Human Nutrition Research Center on Aging at Tufts University.
虽然他们设定的卡路里目标非常严格，但也给了受试者一些灵活性，让他们可以吃自己想吃的食物。他们发现，研究对象的蛋白质摄入量并没有改变，但他们摄入的脂肪和碳水化合物明显减少。研究报告作者之一、塔夫茨大学USDA人类衰老营养研究中心(Tufts University U.S.D.A. Human Nutrition Research Center on Aging)资深科学家苏珊·B·罗伯茨(Susan B. Roberts)说，受试者还摄入了维生素A、K和镁等更多微量元素，表明水果和蔬菜的摄入量有了大幅增加。
“They were eating more healthy foods,” she said. “Things like nuts, whole grains, green vegetables and legumes.”
For many people, dieting can be an unpleasant experience. But the researchers found that for many of the subjects, caloric restriction was surprisingly not unbearable. While more people dropped out of the diet group than the control group, the study overall had a high retention rate. The researchers looked at measures of quality of life and discovered that the calorie-restricted group reported better sleep, increased energy and improved mood. Compared to the control group, they did not have significant increases in hunger or food cravings either, Dr. Roberts said.
“There was nothing we measured that indicated that they weren’t doing well,” she said.
One question the study could not answer was whether caloric restriction could extend life span in humans the way that it can in other animals. The researchers would have to keep people on the diet and follow them for many decades to test that.
But ultimately, caloric restriction did have a beneficial impact on a wide range of risk factors for diabetes and heart disease, two conditions that cause death and disability for millions of Americans, especially as they get older.
Dr. Kraus at Duke said he hoped to study the participants again in 10 years to see if the benefits of the caloric restriction experiment persist, a phenomenon known as legacy effects, which can occur with intense exercise training and other health interventions. But for now, he said that one takeaway for the public is that people can benefit from shaving as little as 300 calories a day from their diets.
“It’s not that hard to achieve that amount of caloric restriction,” he said. “That’s essentially an after-dinner snack.”
Anahad O’Connor是时报记者，报道健康、科学、营养和其他主题。他还是畅销的消费者健康书籍的作者，其作品如：《别在雷暴中淋雨》(Never Shower in a Thunderstorm)和《你需要吃的十样东西》(The 10 Things You Need to Eat)。