Monitoring body weight is a practical way to assess both energy and fluid balance. Weight stability, particularly during periods of heavy training and competition, provides athletes with a good indicator that overall energy requirements are being met. In addition, monitoring body weight before and after individual exercise training sessions gives athletes the feedback they need to assess hydration status and to take action to prevent dehydration. If body weight loss or weight gain is desired to enhance athletic performance, it is best addressed in the off-season. Weight loss during the competitive season places athletes at risk for inadequate energy, carbohydrate, and fluid intake. Weight gain in the form of body fat compromises endurance, speed, and power.
Theoretically, pregnancy would normally be expected to increase energy requirements. There is no doubt the development of the fetus, the placenta, the increase in breast and uterine tissue as well as in energy reserves, all need extra supplies of energy, and indeed it is possible to calculate with a fair degree of precision what the actual amounts should be. For a healthy mother leading the sort of lifestyle common in an industrialized country, the amount would be about 80 000 kcal. One would therefore expect an increase in energy intake of about this amount in order to maintain energy balance. Far from it! In many instances there is no increase whatever over the pre-pregnant level. There are no indications of any improved metabolic efficiencies: BMR remains at pre-pregnant levels and muscular efficiency stays the same. Although it is difficult to measure, it appears most likely that physical activity is subtly reduced, with an energy saving equivalent to the extra energy needs.
The difference between intake of energy from foods and expenditure on basal metabolism and physical activity. Positive energy balance leads to increased body tissue, the normal process of growth. In adults positive energy balance leads to creation of reserves of fat, resulting in overweight and obesity. Negative energy balance leads to utilization of body reserves of fat and protein, resulting in wasting and undernutrition.
The relationship between energy intake (food and fluid consumption) and energy output (energy expenditure for body maintenance and activity). A balance occurs when energy input equals energy expenditure. If you ignore weight changes due to water retention and loss, you lose weight only if you have a negative energy balance (i.e. your energy expenditure exceeds energy intake) and you gain weight only if you have a positive energy balance (i.e. energy consumption exceeds expenditure). As a general rule of thumb, most women go into a negative energy balance when they consume less than 1200 calories each day, and men when they consume less than 1500 calories each day.
