Whether it's for a wedding, high school reunion or just because you need to, it is possible to lose 40 pounds in 100 days. It won't be easy, however. A combination of diet and exercise is the healthiest and most reliable way to lose weight, and this volume of loss will require quite a bit of both. For the next 100 days, you will need to make significant lifestyle changes, but the weight loss is possible if you stick to your plan.
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Calculate how many calories you need to cut from your diet and/or burn from exercise to reach your goal. Losing 40 pounds in 100 days means losing about 2.8 pounds per week. According to the U.S. Department of Health and Human Services, one pound is equivalent to 3,500 calories, which requires calorie deficit of 9,800 per week -- or 1,400 per day -- in order to lose the weight. This means burning 1,400 more calories than you take in each day. You can achieve this through any combination of diet and exercise; for example, eat 800 fewer calories per day and burn 600 through exercise.
Design a diet and exercise plan to meet your goals. In the above example -- eating 800 fewer calories and burning 600 calories -- you will probably need to do moderate-to-vigorous cardiovascular exercise such as brisk walking, jogging or swimming for about an hour each day and eat significantly smaller meals as well as fewer snacks. The National Institutes of Health advises avoiding foods high in fat and sugar, excess alcohol, and stress or boredom, all of which can sabotage your weight-loss goals. Eat plenty of lean protein and fresh fruits and vegetables; they help you fill up on fewer calories. Drink only water, unsweetened coffee or tea or other calorie-free beverages. Write out a meal plan for each day and stick to it.
Follow your diet and exercise program for 100 days. Reduce situations in which you might be tempted to cheat on your diet, such as going out to eat or to the movies. Weigh yourself once a week to track your progress. Your weight may begin to plateau after a few weeks, when your body begins to get used to your exercise routine and reduced-calorie diet. Avoid this problem by making your workout a little harder or adding some weight training to your program. Weight-training possibilities include using dumbbells or weight machines, or performing squats, pushups and situps.