``What goes in'' is everything you eat and drink. Humans, like bears and raccoons, are omnivores--we can eat just about anything, and as long as we get a reasonable variety, we'll be O.K. You don't see a 'coon stalking away from an overturned garbage can because the contents are low in calcium, nor a bear turning up his nose on finding the sandwiches in your picnic basket aren't made with the latest trendy low-sodium lecithin-enriched oat bran bread. There's no reason you should be obsessive about food either. Since we're efficient food processing machines, it's possible to reduce all the complexity of food to a single number that gives the total energy the body can extract from it--the calorie. The essential thing you need to know about what goes in is the total number of calories you eat in a day. All the rest are minor details.