College students are a generally healthy lot. But as they grow heavier each year, reflecting the national rise in obesity rates, campus officials are trying to promote healthier habits--and, they hope, mitigate the impact of a sobering trend. More than one-third of college students have a body-mass index, the main measurement for obesity, that classifies them as overweight or obese, according to the American College Health Association. And childhood-obesity rates keep going up. Given those realities, officials on many campuses are deliberating their responsibility to do more than buy classroom furniture that accommodates larger bodies, as many have. Even with modest changes, say in menus or fitness offerings, colleges confront broader questions about their obligation to take on a complex public-health problem. Warnings about the risk of disease and other far-off consequences often fall on deaf ears, health practitioners say, as young people don't tend to contemplate mortality. Letting students experience the benefits of exercise and balanced meals can often be the hook. And colleges are well positioned to give such nudges. Colleges can shape the environment where students live, work, and play so that good choices are easy. It's about setting up healthy behaviors they'll continue throughout their life. Tactics for that vary. Some colleges have removed trays from dining halls to limit the temptation to load up on food. Others label dishes with calorie and nutrition information, build mobile apps for students to research meal options, or start conversations about nutrition and exercise during medical visits for backaches or the flu. Mindful of sensitivities about body image, officials approach the matter with broad strokes, and try not to single out certain students. Still, some health centers will even write prescriptions for exercise in an attempt to drive home the importance of physical activity.