Like the weight of racquets and length of shorts, nutrition is subject to trends. Here are ? ve of the biggest today, and how they can impact your game.

Functional Foods

THE SKINNY: Nutrient-enhanced food is nothing new—adding vitamins A and D to milk or folic acid to ? our started years ago. But lately a new wave of engineered foods has hit shelves with promises ranging from preventing heart disease to improving stomach health.

FOOD FOR THOUGHT: Tennis players can bene? t from many new offerings, says Leslie Bonci, director of sports-medicine nutrition at the University of Pittsburgh Medical Center. Yogurt and cheese with probiotics can improve digestion and boost immune function, a plus if you’re training a lot. Foods like pasta or eggs forti? ed with omega-3 fatty acids can have an anti-in? ammatory effect to reduce soreness. Calcium in orange juice can improve bone health.

EATER BEWARE: While forti? ed foods ? ll in some nutritional gaps, they’re not a catch-all for a healthy diet. To stay on top of your game, eat a variety of vegetables, fruits, whole grains, and lean proteins.

Glycemic Index

THE SKINNY: Originally created to help diabetics control their blood-sugar levels, the glycemic index has become a hot weight-loss trend, says Lona Sandon, assistant professor of clinical nutrition at University of Texas Southwestern Medical Center and a spokesperson for the American Dietetic Association. The GI rates carbs based on how quickly they raise your blood sugar. Foods with high values cause spikes in blood sugar; those on the low end are metabolized more slowly, causing gradual increases. To ?nd out a food’s GI, go to glycemicindex.com.

FOOD FOR THOUGHT: The GI can help you plan meals around training. Three hours before a workout or match, fuel up with foods that have low GI values, like whole-wheat pasta. But an hour before play, choose a high-GI food such as a banana.

EATER BEWARE: Many nutritionists dismiss the GI as a weight-loss tool because most foods are not consumed alone, and eat-ing more than one type of food alters its GI value. Plus, not all low-GI foods are healthy: Potato chips and Snickers bars both qualify as low or moderate GI while carrots have a high rating.

Grain Alternatives

THE SKINNY: Because an estimated one out of every 133 people is sensitive to gluten (a protein found in wheat, rye, and barley), leading to abdominal pain, grains like quinoa, amaranth, spelt, and buckwheat are gaining popularity.

FOOD FOR THOUGHT: In addition to providing variety, some of the grains are more nutritious. One cup of cooked quinoa, for example, contains 9 grams of protein, which is about the same amount as a glass of fat-free milk.

EATER BEWARE: Including new grains in your diet can break the monotony, but adding too much can lead to weight gain. Find out how many servings of whole grains you need each day at www.mypyramid.gov.

Flexitarianism

THE SKINNY: If you eat mostly plant-based foods like grains, veggies, and fruits with the occasional meat, ? sh, poultry, or dairy dish thrown in, you’re following a ? exitarian diet. Roughly 25 percent of Americans have a meatless meal at least four days a week, according to the ADA.

FOOD FOR THOUGHT: “Eating mostly plant-based foods is great if you’re consuming a wide variety of fruits, vegetables, grains, beans, and legumes,” Sandon says.

EATER BEWARE: Tennis players need to eat enough protein for recovery after a tough session on court, Sandon says. If you’re skimping on dairy, make sure you get enough B12, vitamin D, and calcium through supplements or other foods.

Local Foods

THE SKINNY: From farmers’ markets to community agriculture programs to restaurants that feature only local fare, consumers are increasingly seeking out foods that come from nearby farms.

FOOD FOR THOUGHT: Locally grown produce is often picked and served within a couple of days, as opposed to factory-farmed foods, which may travel thousands of miles over a week. In addition to supporting local farmers and being more environmentally friendly (the less transport, the fewer carbon emissions), some fresh foods may be more nutritious, since water-soluble vitamins like vitamin C diminish over time.

EATER BEWARE: Not all locally grown food is organic. And if you’re limiting yourself only to local fare, you might have to be more creative in your cooking depending on the season.