This is an excerpt from Advanced Sports Nutrition-3rd Edition by Dan Benardot.
Much of the discussion on energy intake focuses on the optimal distribution of the energy substrates. Despite the popular recommendations for high-protein, low-carbohydrate diets, there is no question that focusing on a diet that is relatively high in complex carbohydrates, moderate in high-quality protein, and relatively low in fat is performance enhancing. But this discussion has little meaning in the face of energy intake inadequacy or without knowing how these energy substrates are distributed throughout the day. Put simply, it doesn't matter if high-octane fuel is put in the system if there isn't enough fuel to get you where you want to go at the time you want to get there.
There are recent studies strongly suggesting that assessing within-day energy balance (WIDEB) provides useful performance and health information for athletes. Imagine your car using gas on a long trip--when the gas runs out, the car stops. When humans exhaust readily available fuel and go into a severe energy-balance deficit, we have secondary systems that can keep us running, but often at the cost of the lean mass that we want to preserve. In both male and female athletes, more time in a severe energy-balance deficit, even if 24-hour daily caloric needs are met, is associated with higher cortisol. The cortisol breaks down lean tissue so that the amino acids can be converted to the fuel the body needs to keep running. Providing energy after the body needed it may result in a 24-hour energy-balanced state, but a severe energy-balance deficit would still occur in real time, before the energy is provided. The cortisol production can occur purposefully, as with intermittent fasting that intends to produce a large within-day energy-balance deficit, or it can occur as a result of poor planning, as with an athlete who has a large energy expenditure during training but fails to consume sufficient energy to compensate. These studies have also found that achieving a low energy-balance state at any time during the day results in lower estrogen production in female athletes and lower testosterone-cortisol ratio in male athletes. All of these findings suggest that athletes should be cautious about the within-day fueling strategy because elevated cortisol is associated with lower bone mass and lean mass, lower estrogen is associated with lower bone mass, and lower testosterone-cortisol ratio suggests that it would be difficult for a male athlete to recover or improve muscle mass.
Weight and lean-mass stability are excellent indicators that energy intake matches need. A failure to consume sufficient energy leads to a reduction in weight, with a high proportion of this weight loss coming from a reduction in lean mass as the body tries to compensate for this energy deficit. For most athletes, a lower relative lean mass and higher relative fat mass is not desirable and is a physiological marker associated with decreased performance, even if weight is lowered. This is made worse by the fact that the loss of metabolic mass (i.e., lean mass) is inevitably associated with weight cycling, with progressively lower energy intakes that are contraindicated for assuring optimal athletic performance. Knowing what constituent of weight has changed (more or less fat; more or less muscle) is critically important in knowing if the food intake strategy has been successful. Monitoring weight alone simply fails to give that critical information, and may misguide the athlete into thinking the right strategy is being followed.