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Though a bane for dieters, a slower metabolism may actually be a good thing. We’ve known for more than a century that calorie restriction can increase the lifespan of animals, and the metabolic slowdown may be the mechanism. That could be why the tortoise lives 10 times longer than the hare. Rabbits can live 10 to 20 years, whereas “Harriet,” a tortoise evidently collected from the Galapagos by none other than Charles Darwin himself in the 1830s, lived until 2006. Slow and steady may win the race.

One of the ways your body lowers your resting metabolic rate is by creating cleaner-burning, more efficient mitochondria, the power plants that fuel our cells. It’s like your body passes its own fuel-efficiency standards. These new mitochondria create the same energy with less oxygen, and produce less free radical “exhaust.” After all, your body is afraid famine is afoot, and so, it is trying to conserve as much energy as it can.

The largest caloric restriction trial to date indeed found both metabolic slowing and a reduction in free radical-induced oxidative stress—both of which may slow the rate of aging. The flame that burns twice as bright burns half as long. But whether this will result in greater human longevity is an unanswered question. Caloric restriction is often said to extend the lifespan of “every species studied.” But that isn’t even true of all strains within a single species. Some scientists don’t think calorie restriction will improve human longevity at all; others suggest a 20 percent calorie restriction starting at age 25 and sustained for 52 years could add 5 years onto your life. Either way, the reduced oxidative stress would be expected to improve our healthspan.

Members of the Calorie Restriction Society, self-styled CRONies (for Calorie-Restricted Optimal Nutrition), appear to be in excellent health, but they’re a rather unique self-selected bunch of individuals. You don’t really know until you put it to the test. Enter the CALERIE study, the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy, the first clinical trial to test the effects of caloric restriction.

Hundreds of non-obese men and women were randomized to two years of 25 percent calorie restriction. They only ended up achieving half that but lost about 18 pounds and three inches off their waists, wiping out more than half of their visceral abdominal fat. That translated into significant improvements in cholesterol levels, triglycerides, insulin sensitivity, and blood pressures. Eighty percent of those who were overweight when they started were normal weight by the end, compared to a 27-percent increase in those who became overweight in the control group.

In the famous Minnesota Starvation Study that used conscientious objectors as guinea pigs during World War II, the study subjects suffered both physically and psychologically, experiencing depression, irritability, and loss of libido. The subjects started out lean, though, and had their calorie intake cut in half. The CALERIE study ended up being four times less restrictive, only about 12 percent below baseline calorie intake, and enrolled normal-weight individuals, which in the U.S. these days means overweight, on average. As such, the CALERIE subjects experienced nothing but positive quality-of-life benefits, with significant improvements in mood, general health, sex drive, and sleep. They only ended up eating about 300 fewer calories than they were eating at baseline. So, they got all these benefits—the physiological benefits, the psychological benefits—all from only cutting about a snack-sized bag of chips worth of calories from their daily diets.

What happened at the end of the trial, though? In the Minnesota Starvation Study and calorie deprivation experiments done on Army Rangers, as soon as subjects were released from restriction, they tended to rapidly regain the weight, and sometimes even more. The leaner they started out, the more their bodies seemed to drive them to overeat to pack back on the extra body fat. In contrast, after the completion of the CALERIE study, even though their metabolism was slowed, they retained about 50 percent of the weight loss two years later. They must have acquired new eating attitudes and behaviors that allowed them to keep their weight down. After extended calorie restriction, for example, cravings for sugary, fatty, and junky foods may actually go down.

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