ALL MAMMALS, from field mice to Jack Black, are hardwired to gain weight for winter. But where our forebears fattened up to survive, today's athletes need no such padding. (Thank you, central heating.) If you plan on skiing all day in cold weather, however, you need more calories than the average gym rat, so eat right or risk packing on pounds like an evolutionary throwback.

Your body turns food into heat through a process called thermogenesis, and fuel for the furnace is what you need to maintain essential muscle and internal warmth—and perform well—while exercising in cold temps. The key is getting those extra calories from healthy sources that will keep you trim and goose your immune system during cold-and-flu season. With no farmers' market to browse and an overabundance of rich stews and processed sweets, winter can be a nutritional minefield. Here's how to navigate it.

EAT FOR WARMTH
Not long after you step out into the cold, the blood vessels in the periphery of your body become restricted. In other words, your hands and feet cool as your body instinctively regulates the difference between external and internal temperature, thus creating a demand for more fuel than the workout alone requires. "Athletes tend to blow through their carbohydrate reserves faster in winter than in warmer months," says Declan Connolly, director of the Human Performance Laboratory at the University of Vermont.

Add long days outside, like skiing first to last chair, and your body, which stores only about 1,500 to 2,000 carb calories at a time, has trouble keeping up with the demands for energy and warmth. "That's why when you're out skiing or snowshoeing, the smell of heavy, carb-rich food floating up from the lodge is so good," says Connolly, who works with winter athletes to develop seasonal training and diet plans. "We actually get hungrier faster when we burn more carbohydrates." Replenish carbs at every meal by going heavy on whole-grain breads or pancakes, pastas, and fruit, and choose snacks that are 60 to 70 percent carbs.

THE MENU
You can do better than a lift-line bagel and nachos in the lodge. You'll hammer the slopes all day with the carbs, calories, and nutrition in this winter diet plan.
BREAKFAST: Whole-grain toast with nut butter and jam, and a serving of yogurt; or whole-grain pancakes, eggs, and fruit.
LUNCH: A bowl of vegetable stew with whole-grain crackers or bread; or a turkey sandwich on whole-grain bread with lettuce, avocado, tomato, sprouts, and mustard, plus pretzels on the side.
DINNER: Baked chicken or fish with whole-grain pasta and salad; or a stir-fry with brown rice, vegetables, and lean meat.
DESSERT: Pumpkin pie is loaded with antioxidants.
SNACKS: Trail mix, string cheese, Snickers, peanut-butter crackers, fruit, and high-carb energy bars are all good options every couple of hours between meals.
DRINK: In cold weather, it's not always enough to merely stay hydrated. To boost warmth, drink beverages that include spices like cinnamon, cloves, and nutmeg, which stimulate digestive enzymes and thus help you generate more heat after eating.