For most people, eating 1,500 calories a day shouldn't lead to weight gain. In fact, this caloric intake is more likely to result in weight loss than anything else. This isn't to say a 1,500-calorie diet won't cause you to gain weight. Your body's caloric need is affected by your age, height, weight and level of physical activity, so you may find under certain circumstances that 1,500 calories is too many calories for you.
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Basal Metabolic Rate
To determine whether or not 1,500 calories is too many calories for your body, you can start off by calculating your basal metabolic rate. Men take (12.7 x height in inches) + (6.23 x weight in pounds) - (6.8 x age in years) + 66 to arrive at their BMR. Women take (4.7 x height in inches) + (4.35 x weight in pounds) - (4.7 x age in years) + 655 to reach this number.
Once you arrive at your basal metabolic rate, multiply this number by your current activity level. Someone who gets little to no exercise should multiply her BMR by 1.2 to arrive at her caloric need. If you get one to three days of light activity each week, multiply BMR by 1.375. Moderate activity levels of three to five days each week can multiply BMR by 1.55, while more active levels of six to seven days a week multiply BMR by 1.725. If, however, you're extremely active more than once a day, multiply BMR by 1.9.
For example, a 70-year-old woman weighing 90 pounds at a height of 5 feet, 2 inches tall has a basal metabolic rate of about 1,010 calories. If she leads a sedentary lifestyle, her caloric need is 1,210 calories. In this situation, 1,500 calories is going to lead to weight gain. If she's moderately active, on the other hand, her caloric need increases to over 1,560 calories, so this caloric intake no longer causes weight gain.
To further grasp the idea of caloric need, let's say this woman is now 110 pounds. Her basal metabolic rate increases to 1,095 calories. A sedentary lifestyle necessitates a caloric intake of just less than 1,315 calories to maintain her current weight, while a moderately active lifestyle requires around 1,697 calories.
At an age of 30, this woman has a basal metabolic rate of 1,283 calories at a weight of 110 pounds. If she was sedentary, her caloric need is 1,540. A moderate level of activity increases her caloric need to 1,988 calories a day.
Although the previous examples may sound somewhat extreme, they can help to illustrate how age, height, weight and physical activity affect your caloric need. As you get older, you tend to lose muscle. Muscle is known to be metabolically active, so its loss reduces your body's caloric need. Both height and weight affect body composition. The larger you are, the more energy it takes to move around, thereby increasing your body's caloric need. The same can be said for physical activity. It really depends on your age, weight, height and physical activity level if 1,500 calories will cause weight gain.