Q:  It’s summertime and I’m a lot more active — biking, swimming, playing sports — than I am in winter. What is good vegetarian fuel for when I’m being really active? What do I need and where can I get it?

A:  If you’re aiming to power up your athletic activities, you’ll find an endless array of nutritional products — and a cacophony of conflicting advice on what to eat and when. But a look inside those exercising muscles of yours will tell you a lot about what you really need.

When you start to exercise, your muscles get their fuel from glycogen. This long, branching molecule quickly disintegrates during exercise to free up glucose  simple sugar  which is what your muscles use for quick energy.

Glycogen is the stuff marathon runners try to load into their muscles and livers before a race. Glycogen is made from carbohydrates, and the more carbs in your diet, the more your body builds up its glycogen stores. The most healthful carb-rich foods are also the cheapest: Whole grains (such as brown rice), beans and starchy vegetables (such as sweet potatoes) provide the best carbohydrates.

Avoid carbs that are mainly simple sugars, such as sodas and candy, and refined grains (think white bread and white rice). Don’t forget water. You lose a lot of it in hot weather so be sure to stay hydrated. During a prolonged workout, drink about two cups of water (16 oz.) two hours before you start and another one-half to one cup every 20 minutes.