No wonder the government recommends at least five servings a day of fruits and vegetables: They're low in calories, full of vitamins, and have been shown to help prevent a variety of diseases. (And they taste good, too.) Some studies have also suggested that people who consume a lot of fruits and veggies gain less weight than those who don't, but other research hasn't found a link. Researchers are trying to suss out exactly what eating from this food group now can or cannot do for your waistline further down the line.

What the researchers wanted to know: Can eating more fruits and vegetables help prevent long-term weight gain and obesity in middle-aged women?

What they did: Researchers at Northwestern University and other institutions examined data from the mammoth Nurses' Health Study, which has tracked the health and habits of a group of 121,700 registered nurses since 1976. The scientists excluded women with a history of certain diseases or who provided incomplete information and came up with a study group of 74,063 women. These women reported how often they ate 16 fruits and 28 vegetables and also reported their body weight and height and were surveyed again 12 years later.

What they found: First, the average study participant was just meeting the government's recommendations, eating about 1.9 servings of fruit and 3.2 of veggies each day. And those who ate more greens were healthier in general; they exercised more and smoked less. But the bottom line was that the more participants increased their intake of fruits and veggies over the next 12 years, the less likely they were to become obese. The women who most boosted their fruit intake over that period had a 25 percent lower chance of ending up obese than the women who decreased their fruit intake the most. Similar trends were found for vegetable intake. Researchers hypothesize that the fiber in these foods may make you feel full and thus eat less—and they're low in calories, so they may replace more energy-rich foods in your diet.

What the study means to you: Go fix yourself a big salad and a veggieburger. Even after adjusting for other habits that accompany a healthful lifestyle, eating more fruits and vegetables seemed to provide protection from a spare tire in middle age. Moreover, unlike drugs, there's pretty much no downside from loading up on this food group (just wash everything well). If you're already eating a lot of fruits and veggies, don't stop, since the study showed that women who cut back see their weight creep up.

Caveats: Women self-reported both their food intake and weight; while this isn't as reliable as having a third party keep a record, this method has been validated in other research from the Nurses' Health Study. And men weren't studied, though there's no reason to think that the same wouldn't hold true for them as well.

Find out more: The nonprofit Produce for Better Health Foundation has a site around its Five a Day campaign, part of a private-public partnership. Good recipes, too.

Read the article: He, K. et al. "Changes in Intake of Fruits and Vegetables in Relation to Risk of Obesity and Weight Gain Among Middle-Aged Women." International Journal of Obesity. December 2004, Vol. 28, pp. 1569–1574. Published online Oct. 5, 2004.

Abstract online: www.nature.com