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/ Source: TODAY
By Madelyn Fernstrom

Is the key to a longer life as simple as just cutting back on your daily calories? Some Americans seem to think so and some scientific studies suggest they might be right: Cutting calories promotes longevity, independent of weight loss.

The key to longevity remains elusive and is linked to an unpredictable variety of both biological and behavioral factors. Some animal studies conducted in rodents and monkeys as far back as the 1930s (and still in progress) show eating 25-40 percent fewer calories every day could extend an animals’ lifespan by up to 50 percent. Sounds impressive, but is this true for people? It’s time to take a closer look.

What is the calorie-restriction diet?

The CR diet is a balanced eating plan of all nutrients that limits the total number of calories consumed daily to roughly 25–40 percent fewer calories than needed to maintain a stable body weight. It’s not designed primarily for weight loss, although weight loss is a main side effect.

This eating plan takes a lot of mental focus and discipline to maintain sufficient nutrient intake for good health. More recently, variations in the daily calorie cut-backs have developed because of the difficulty of the daily diet. The variations include alternating day restrictions (cutting back 30 percent of calories every other day); the 5:2 plan (cutting back 30 percent of calories two days a week); a week of calorie cutting every month or two.

What does calorie-cutting look like?

To figure out the daily calories of a CR plan, you need to do a little math. If you’re a 5-foot-5-inch woman weighing a healthy 145 pounds, you need to eat around 2,200 calories daily to maintain your weight. Switching to a CR plan drops this to around 1,600 calories a day, for a 25 percent reduction. For a 40 percent reduction, you’re down to 1,320 calories. There’s plenty to eat, with a variety of low-calorie fruits and vegetables, fiber rich starch, lean plant and animal proteins, and heart-healthy fats, but the bigger the restriction in calories, the tougher it is to stick with it.

Does it work?

There’s no solid evidence of why the CR plan might contribute to longevity, independent of weight loss, but there are multiple proposed theories: It lowers your core body temperature (perhaps an adaptive response to reduced energy output), lowers metabolic rates, reduces the production of free radicals (cellular-damaging particles released from cells), reduces DNA damage and its hormonal effects support cellular repair. It also results in “hormesis," providing a low-intensity biological stress creating a “defensive state,” better preparing the person to fight off infection and survive adversity.

While animal studies consistently show longevity with a long-term CR diet, only short-term studies exist in humans that attempt to address this, with one study showing that after two years on a CR diet, this lowered the risk for heart attack, stroke and diabetes. The participants lost about 10 percent of their weight, so it’s unclear how much weight loss contributed to the observed effects.

Interestingly, while people in this study were asked to cut back by 25 percent of calories, they were able to cut back only to about 12 percent over the two-year period. While the reasons for this are unknown, it demonstrates that such a severe eating plan is not easy to stick with.

Is it worth a try?

It’s important to know scientists look at the CR diet and its effects as a research paradigm — to look at the aging process in a broad sense. This is not an extreme weight-loss regimen, nor are there sufficient data to recommend this as a definitive step towards longevity.

National guidelines still maintain a moderate, balanced eating plan is the best way to optimize good health and longevity, in part because it’s easy to stick with for life. While adequate nutrients can be still chosen to maintain good health with a CR diet, the degree of restriction is high, and it takes a great deal of mental focus and discipline. There are no definitive data documenting that this eating plan, alone, boosts longevity.

Always remember to check with your doctor before making any significant changes in your diet, especially if you take prescription or over the counter medications.

Madelyn Fernstrom , PhD is NBC News Health and Nutrition Editor. Follow her on Twitter.