Put down that cupcake, pronto. You may have just returned from the gym, but that doesn't necessarily give you an excuse to eat just anything. Although you use more calories on high-intensity exercise days, you don't necessarily need to replace them unless you are fueling long training sessions or trying to gain weight. Calorie burning and consumption don't follow a 24-hour clock. Calorie balance occurs over the course of many days -- a significant deficit, or surplus, on one day doesn't make or break your overall tally.
Your body weight is affected by a simple equation of calories in versus calories out. Consume more calories than you burn and you'll gain weight. Successfully create a calorie deficit in which you eat fewer calories than you burn and you should lose weight. If your goal with exercise is to lose weight, don't eat more calories to make up for those you burn from exercise -- you'll negate the deficit you are creating and hinder weight loss. Remember to eat at least 1,200 calories per day to get all the nutrients you need.
If you are underweight and have been told by your doctor that you need to gain weight to improve your health, you should eat more calories on the days you exercise so you stay in caloric surplus. Exercise can help stimulate your appetite and helps you put on healthy muscle mass, but if you don't eat extra to make up for the calories you burn, you risk maintaining your frail frame or losing more weight. Aim for a calorie surplus of 500 to 1,000 calorie daily to put on at least 1 to 2 pounds per week.
If you're a supermom, training for a marathon or other endurance event, you may exercise for more than 90 minutes at a relatively high intensity several days per week. On these days, you'll need to take in a preworkout snack and some calories when you are exercising to keep your energy up. You may also find your hunger sensors go into overdrive on days you do long workouts, which is your body's way of telling you that you need more fuel. Be cautious of how much extra you take in, however; it's much easier to eat calories than to burn them off. A long workout lasting 90 minutes or more may warrant 200 to 300 more calories total for the day, but not an entire pizza or a box of cookies.
Eating the exact same number of calories day in, day out is nearly impossible, unless you have a private chef, nutritionist and scientist living with you and your family. Chances are, some days, you'll eat slightly more calories than you burn and other days you'll eat slightly fewer. Your body naturally regulates intake to manage your weight. If, however, you start to interfere with it and have extra calories on the days you exercise, you may interrupt this delicate balance and cause weight gain. Unless you have a specific weight goal, trust your hunger sensors. Eat until you feel satisfied, but not stuffed, and make wise, whole-food choices consisting primarily of lean proteins, whole grains and vegetables.