The metabolic effects of not eating enough for your workouts can be disastrous. When you place your body in an extreme calorie deficit, a survival mechanism kicks in that slows down your metabolism. Your body begins to conserve energy and burn calories at a much slower rate because it thinks it's starving. This reduction in metabolism can make it extremely difficult to lose fat. It can also make your energy levels plummet, causing the intensity of your workouts to suffer.
Your body has complex nutritional needs that are met with a balanced diet of carbohydrates, fats and proteins from whole food sources. An insufficient diet can cause deficiencies in the vitamins and minerals your body needs for healthy cellular function. Inadequate consumption of macronutrients can be particularly damaging when you're working out. For example, if you're not eating enough protein, heavy training can cause your body to break down your muscle tissue for energy. Inadequate carbs can poorly impact brain function and reduce the glycogen your body needs to perform activities. Very low carbohydrates combined with exercise can also result in potentially dangerous hypoglycemia.
In addition to the physical effects, an inadequate diet coupled with intense exercise can take a psychological toll. Most people who restrict calories severely do so for weight loss. When you do this, in addition to exercise, you can begin to feel ravenously hungry and end up binging. This is where so many people trip up with dieting -- not only do they replace all the calories they burn with exercise, but they usually consume excess calories when their metabolism is in a reduced state. This can ultimately result in weight gain. The starvation and binge cycle can take an emotional toll that results in feelings of failure and depression.
Minimum Calorie Requirements
Your daily calorie requirements will vary depending on how much you weigh now, your gender and your overall fitness level. In general, daily calorie needs for adult women range from 1,600 to 2,400 calories, and 2,000 to 3,000 calories for adult men, according to the U.S. Department of Agriculture's Dietary Guidelines for Americans 2010. To lose weight, create a safe deficit by reducing daily calorie intake by about 500 calories, the guidelines recommend. A healthy weight loss is no more than 2 pounds per week.