Hot dogs, ice cream, burgers and other junk-food classics can instantly become healthy treats if they're made with soybeans, right? Not so fast. Soy protein may help lower cholesterol, but it's questionable whether or not it fights cancer, as once believed. In fact, some researchers think superhigh soy intake might trigger some cancers. Most Japanese eat only about 8 to 10 grams of soy protein a day, whereas American soy fanatics get 40 grams. Healthy limits: Scientists agree that two to three servings (about 25 grams) of soy foods, such as soy milk, soy burgers and tofu, are safe. But if you're pouring soy milk over soy cereal and stuffing a soy-based energy bar into your pocket to eat later, you may be overdoing it.
At some point recreational athletes began worrying about dehydration so much they started drinking themselves to death. Hyponatremia, dangerously low salt levels that lead to swelling of the brain, is a rare but growing concern at endurance events where racers OD on H2O—sometimes drinking more than a liter (32 ounces) per hour for four or five hours. Performance-wise, less can be more. In a recent study, South African researchers asked runners to drink at three different rates: according to thirst (13 ounces per hour); at a moderate rate (4 ounces every 15 to 20 minutes) or at a high rate (10 ounces every 15 to 20 minutes). After two hours of exercise, there was no difference among the groups in core body temperature or finishing times, but some high-rate guzzlers had stomach distress. Healthy limits: bicycling advises drinking at a moderate rate (about one 16-ounce bottle an hour). But let your thirst be your guide. If it's a hot day, drink a little more. Just don't force down gallons of liquid because you think you have to. For rides lasting more than an hour, pack a sports drink; the electrolytes help prevent hyponatremia.
Many athletes think protein is the wonder nutrient, so they eat protein-fortified foods, says Piattoly. "But your body can process only about 4 to 5 grams of protein per hour, so the 50 grams you got from that protein bar and shake will take about 10 hours to digest and absorb." Your body needs protein to repair and build muscle, but eating excessive amounts won't do the job better. Some people fill up on protein and don't eat enough carbs, says Piattoly. Some experts believe excessive protein also stresses your kidneys. Healthy limits: Active cyclists need one-half to three-quarters of a gram of protein a day per pound of body weight, typically about 90 to 135 grams a day for a 180-pound man and 70 to 105 grams for a 140-pound woman. A three-ounce serving of chicken delivers 25 grams and a glass of milk another 10 grams, so it's easy to get enough through a healthy diet.
Though fish contains healthy omega-3 fatty acids that help fight heart disease, and maybe even Alzheimer's, it also contains heavy-metal toxins such as mercury (especially in large fish, such as tuna, swordfish and shark), which when eaten regularly can remain in your system, and in large amounts can lead to serious health and cognitive problems, including heart disease and reduced memory and concentration. Healthy limits: Choose cold-water fish known to contain less mercury, such as Alaska wild salmon, herring and sardines, which are high in omega-3s. Or, limit yourself to two meals of fish per week.
Most Americans eat about 12 grams of fiber a day, but we should be topping out at around 25 to 30. High-fiber foods lower cholesterol, help manage blood sugar and digest slowly so you feel full longer and don't overeat. When you're fueling for a big ride or race, though, timing is everything. "Two bowls of high-fiber cereal, such as Kashi or Raisin Bran, right before a ride might not sit right--or might sit longer than you want," says Piattoly. A big dose of fiber before exercise can lead to nausea, constipation and other GI upset. Healthy limits: About an hour or two before exercise, eat easy-to-digest lower-fiber carbohydrates such as bagels, bananas and energy bars. Avoid foods that have more than 5 grams of fiber per serving.