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Estimating your Caloric replacement needs is always a challenge. And as
CHANGE IN WEIGHT (IN LBS) = (CALORIES BURNED - CALORIES CONSUMED)/3500
you will see the results reflected in the bathroom scales.
Regular physical exercise will help to protect your muscles (at the expense of fat) during periods of negative caloric balance so you will not lose significant muscle mass even if you underestimate your calorie needs. However, if you overshoot on the calorie replacement, and especially if you have been exercising at a slow pace (which will preferentially burn fat calories while maintaining muscle glycogen stores),any post ride carbohydrate loading may find muscle glycogen stores already "filled" and any additional carbohydrate calories will be converted directly into fat.
The bottom line
Eat a high carbohydrate diet (60 to 70% carbohydrate, low in fat), the diet that is best for endurance performance . Do weight training to maintain upper body muscle mass. And keep an eye on the bathroom scale to determine if you have estimated replacement needs correctly. With a regular exercise program, a modest weight gain should be in muscle mass and any weight loss from fat.
Fluids
Although water does not provide caloric energy, adequate hydration is at least as important to athletic performance as the food you eat. One of the biggest mistakes of many competitive athletes is failing to replace fluid losses associated with exercise. This is especially the case in cycling as rapid skin evaporation decreases the sense of perspiring and imparts a false sense of only minimal fluid loss when sweat production and loss through the lungs can easily exceed 4 pints per hour. For a successful ride, it is essential that you start off adequately hydrated, begin fluid replacement early, and drink regularly during the ride. In fact, a South African report on two groups of cyclists, one consciously rehydrating, the other not, exercising at 90% of their maximum demonstrated a measurable difference in physical performance as early as 15 minutes into the study.
Total body fluid losses during exercise lead to a diminished plasma volume (the fluid actually circulating within the blood vessels) as well as a lowered muscle water content. As fluid loss progresses, there is a direct effect on physiological function and athletic performance. An un-replaced water loss equal to 2% of base line body weight will impact heat regulation, at 3% there is a measurable effect on muscle cell contraction times, and when fluid loss reaches 4% of body weight there is a measurable 5-10% drop in performance. In addition, one study demonstrated that this performance effect could persist for 4 hours after re-hydration takes place - emphasising the need to anticipate and regularly replace fluid losses. Maintaining plasma volume is one of the hidden keys to optimal physical performance. So make it a point to weigh yourself both before and after the ride - most of your weight loss will be fluid, and 2 pounds is equal to 1 quart. A drop of a pound or two won't impair performance, but a greater drop indicates the need to reassess your on the bike program. And use the post ride period to begin replacement of any excess losses. If you do so, you will be well rewarded the next time out.
But as a word of warning to those who practice the philosophy of "if a little is good, a lot is better", there are also risks with overcorrecting the water losses of exercise. There have been reports of hyponatremia (low blood sodium concentration) with seizures in marathon runners who have over replaced sweat losses (salt and water) with pure water. This risk increases for longer events (more than 5 hours).
Weighing yourself regularly will help you tailor YOUR OWN PERSONAL replacement program. A weight gain of more that 1 or 2 pounds will indicate that you are overcorrecting your water losses and may be placing yourself at risk for this unusual metabolic condition.
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