Scientists have long known of a fairly reliable way to extend the lives of rodents and other lab animals: cutting the number of calories they eat by 10 to 40 percent.
The strategy, known as calorie restriction, has been shown to extend the lives of various organisms and reduce their chances of developing cancer and other age-related diseases. Whether it can do the same for humans is debatable. But an intriguing new study suggests that long-term calorie restriction in young adults can have an impact on their health.
In the new study, funded by the National Institutes of Health and published this month in the journal Lancet Diabetes & Endocrinology, researchers looked at a group of 143 healthy men and women between the ages of 21 and 50. They followed instructions to restrict calories for two years. They could eat what they wanted if they reduced the amount of food they ate, with the goal of reducing the number of calories they ate 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%。
A lot of people don't reach that goal. On average, dieters were able to cut their total calories by about 12 percent, or roughly 300 calories a day -- the equivalent of a large bagel, a few chocolate chip cookies or a small cup of starbucks mocha frappuccino. But many in this group had improved cardiovascular and metabolic health, even though they were already in the normal range.
They lost weight and body fat. Their cholesterol levels improved, their blood pressure dropped slightly, their blood sugar control improved and their inflammation decreased. Meanwhile, the 75 healthy people in the control group who were not on calorie restriction showed no improvement in these measures.
The calorie-restricted group benefited in part from significant weight loss, an average of 16 pounds over the two-year study period. But their metabolic health improved more than expected, suggesting that caloric restriction may have some unique biological effect on the body's disease pathways, said William Kraus, the study's lead author and a professor of medicine and cardiology at duke university.
We were not surprised by the change, he said. "But the magnitude of the change is really striking. "In a sick population, no combination of five drugs can produce that much improvement."
Calorie restriction may be a useful way to improve Health and lose weight, but it is unclear whether the changes revealed by the new study will ultimately translate into longer lives and fewer chronic diseases, said Frank Hu, chairman of the nutrition department at the Harvard T.H. Chan School of Public Health, who was not involved in the study.
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 says the modest improvements experienced by participants must be sustained over time to produce long-term benefits. But he questioned whether caloric restriction was practical for most people, given that "we live in a fat-inducing environment where there are a lot of energy-dense but nutrient-poor foods that are cheap, easy to get and sell like hot cakes."
But he added that some people may find caloric restriction feasible if they combine it with popular dietary strategies like the Mediterranean diet, intermittent fasting or cutting back on carbohydrates.
The new study confirms how difficult it is to limit calories. Study participants underwent intensive training programs to learn how to cook low-calorie meals, participated in group discussions and regularly consulted nutrition experts. But they still don't meet half of their 25% calorie reduction target. As anyone who has ever dieted knows, maintaining weight loss over the long term is the hardest part.
But the new research is groundbreaking in several ways. The study, funded entirely by the NIH and costing $55 million, was called "Calerie" -- Comprehensive Assessment of long-term Effects of Reducing Intake of Energy. This is the first major clinical trial to examine the effects of calorie restriction in young adults of normal weight or slightly overweight but not obese. The trial was designed to see if caloric restriction could affect healthy aging and disease status.
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 they set strict calorie targets, they also gave the subjects some flexibility to eat what they wanted. They found that the subjects' protein intake did not change, but they ate significantly less fat and carbohydrates. The subjects also consumed more micronutrients such as vitamins A, K and magnesium, indicating A significant increase in fruit and vegetable intake, said Susan b. Roberts, one of the study's authors and A senior scientist at Tufts University's USDA Human Nutrition Research Center on Aging.
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 eat healthier, she said. "Things like nuts, whole grains, green vegetables and beans."
Dieting can be an unpleasant experience for many people. But the researchers found that, to the surprise of many subjects, caloric restriction was not intolerable. Although more people dropped out of the diet group than the control group, overall, the study had a high retention rate. The researchers looked at measures of quality of life and found that the calorie-restricted group slept better, had more energy and were in better mood. There was also no significant increase in hunger or food cravings compared to the control group, Roberts said.
We didn't measure anything that said they were in bad shape, she said.
One question the study doesn't answer is whether caloric restriction extends human life as it does in other animals. To test this, researchers can only keep people on such diets and keep following them for decades.
But in the end, caloric restriction did have a beneficial effect on a range of risk factors for diabetes and heart disease. Both diseases cause the death and disability of millions of americans, especially older people.
Duke's krauss said he hopes to study the participants again in 10 years to see if the benefits of caloric restriction can be sustained. This phenomenon, known as the legacy effect, exists in high-intensity exercise training and other health interventions. But for now, he said, the public lesson from the experiment is that people can benefit from cutting 300 calories from their daily diet.
It's not hard to reach this caloric restriction, he said. "It's basically the size of an after-dinner snack."