When you exercise regularly, what you eat is important. Calories are your body's fuel for energy, so when training for a half marathon, you need to get your calorie intake right to ensure maximum performance in training -- and on the day itself. It's not simply a matter of guessing -- eat too few calories, and you'll be tired, lethargic and will underperform, but consume too many, and you'll gain unwanted fat.
General Calorie Guidelines
The United States Department of Agriculture recommends that women and men should consume between 1,800 and 2,400 and 2,000 and 3,000 calories per day respectively to maintain weight. While these guidelines are useful, they are more geared to the general non-exerciser. You could base your daily caloric intake around them by starting in the mid-point and adjusting in accordance with your weight loss or gain, your energy levels and performance, but a more scientific, individual approach would be better.
BMR is your basal metabolic rate, and is the number of calories your body burns every day at rest. There are two BMR calculations for either gender. For women, multiply your weight in pounds by 4.35 and your height in inches by 4.7 and add these together. Then multiply your age by 4.7 and take this away from the first figure. Add 655 to the number you're left with, and this is your BMR. Men should do the same calculation, but multiply weight by 6.23, height by 12.7 and age by 6.8, and add this to 66. An example for a 40-year old, 150-pound, 65-inch-tall female would be:
BMR = 655 + ( 4.35 x 150 ) + ( 4.7 x 65 ) - ( 4.7 x 40) = 1332.5 calories.
And the calculation for a 30-year-old, 200-pound, 72-inch-tall male would be:
BMR = 66 + (6.23 x 200) + (12.7 x 72) - (6.8 x 30) = 2022.4 calories.
Once you have your BMR, you can tailor your calorie intake to your needs. In training for a half marathon, you're going to need extra calories above your BMR to fuel your training. Use the Harris Benedict Equation to do this. Take the result of your BMR calculation, and multiply it by 1.55 if you're starting to train lightly, by 1.725 if you're running intensely six or seven times per week and by 1.9 if you're in the peak of training, or running twice per day.
You can change your calorie intake during training if you feel it is necessary. If you're gaining weight, try eating 200 calories less per day. Likewise, if your training is suffering and you lack energy, you're probably not eating enough, so add in an extra 200 calories each day. The running website MarathonRookie recommends getting around 65 percent of your calories from carbs like rice, pasta, fruit and sweet potatoes, 10 percent from proteins such as meat, fish and dairy products and a further 25 percent from fats like nuts, seeds, olive oil and avocado.