What is an easy way to quickly estimate water loss and fluid needs following physical activity?

Good nutrition and hydration are essential to help you perform at your best when exercising. For anyone who will be exercising for an extended period, it is important to plan what you eat and drink before, during and after exercise. This is especially important for anyone involved in sport where optimal nutrition and hydration can make a significant difference to your performance. For anyone participating in exercise at a more moderate level, a healthy diet that includes plenty of fruit, vegetables and water will usually suffice.

Fuel for exercise

The body’s main sources of fuel during exercise are fat and carbohydrate, and the one you need to focus on is carbohydrate. This is because we all have ample stores of fat to undertake even the longest bouts of exercise (unless in a state of starvation). However, our carbohydrate stores (in the form of muscle glycogen) are more limited and can become significantly depleted during vigorous exercise in excess of 90 minutes or moderate exercise of a longer duration (several hours).

Carbohydrate stores can also become depleted over the course of several exercise sessions, if not replenished through appropriate nutrition between times. Depleted muscle glycogen will impair your capacity to exercise, limiting how fast you can run, cycle or swim. This is clearly evident in some people at the end of a marathon, in which their muscles have become depleted of glycogen and they struggle to maintain a speed faster than a slow jog.

Fluid for exercise

Maintaining your body in a fully hydrated state is essential for both your health and performance when exercising. If you are dehydrated you will have a reduced blood volume and less fluid available to form sweat. Dehydration will reduce your capacity to deliver oxygen to your muscles and your ability to prevent your body from overheating, both of which will adversely affect your exercise performance. So it’s important to be fully hydrated when you start exercising, and to maintain a regular intake of fluids while exercising to prevent dehydration.

Food and fluid before exercise

In most circumstances, most of us will have enough stored fat and carbohydrate to fuel our next exercise session without needing to make special arrangements (as exemplified by those who walk or jog before breakfast). However, if you are undertaking a more prolonged or vigorous bout of exercise, you may wish to maximise your glycogen stores before you exercise, and you can do this by eating a meal between one and 4 hours beforehand. This allows enough time for the meal to be digested. Ideally, you should eat a small amount of food that is low in fibre for easy digestion.

To increase your body’s fuel supply, the meal should be predominantly carbohydrate — approximately 2 to 5 grams of carbohydrate per kilogram of bodyweight. Meals based around rice, pasta, bread or potatoes are often advocated by sports dietitians, and you’ll need to try different ones to see what suits you best. The meal should also be low in fat and protein, to minimise any gastrointestinal discomfort.

For most sports and types of exercise, it is recommended that you drink 400-600 mL of fluid one to 2 hours before the activity, and another 200-400 mL 15 minutes before exercising. Water is usually a suitable choice of fluid to drink before exercising.

Food and fluid during exercise

Fluid loss can impair performance and can affect your body’s ability to control its own temperature. If you are exercising for less than 60 minutes, you should drink approximately 200 mL of fluid every 15-20 minutes. Water is appropriate in this situation. In longer duration activities where there is a risk of glycogen depletion, such as more than 60 minutes of vigorous exercise, a sports drink containing glucose and electrolytes can be most effective. And for activities lasting several hours these sports drinks can be supplemented with energy bars.

Post-exercise food and fluid

After exercise it is important to restore your body’s fuel and fluid stores to normal levels. For most people this is easily achieved by following a healthy diet that includes plenty of fruit and vegetables, and plenty of non-alcoholic, non-caffeinated fluids.

Replacing fluid stores largely depends on how much fluid was lost during exercise. This can be calculated by comparing your pre- and post-exercise bodyweight. A simpler method is to check your urine. You need to drink sufficient fluid until you are passing clear, dilute urine.

The amount of fluid that you'll need to drink will depend upon how much you’ve sweated and the temperature of the environment. So on hot days after a vigorous bout of exercise, you may need to drink several litres. In doing so, be wary of consuming it in the form of sugary drinks, as you may take in more calories than you’ve burned off during the exercise. This may not be a problem if you are a highly trained sports person with a good body composition, but will be contrary to your goals if you are trying to lose weight.

To replenish your glycogen stores after exercising vigorously, you need to eat 1-1.5 grams of carbohydrate per kilogram of bodyweight within the first couple of hours after exercise. Ideally, this should be in the form of high GI foods, such as sports drinks, muffins or white bread. Over the 24 hours after exercise, a total of 7 to 10 grams of carbohydrate per kilogram of bodyweight should be ingested to maximise the glycogen stores again, thereby preparing you for your next bout of exercise.

Note of caution

People with diabetes, people with metabolic disorders and those on special diets should consult their specialist and dietician for advice on how to modify food and fluid intake in accordance with exercise.

1. Australian Institute of Sport [website]. Fluid: who needs it? (updated 2009, July). Available at: https://www.ausport.gov.au/ais/nutrition/factsheets/hydration2/fluid_-_who_needs_it (accessed 2010, Feb 9)2. Australian Institute of Sport [website]. Eating before exercise (updated 2009, July). Available at: https://www.ausport.gov.au/ais/nutrition/factsheets/competition_and_training2/eating_before_exercise (accessed 2010, Feb 15)

3. Australian Institute of Sport [website]. Recovery nutrition (updated 2009, July). Available at: https://www.ausport.gov.au/ais/nutrition/factsheets/competition_and_training2/recovery_nutrition (accessed 2010, Feb 15)

Water is essential to life. It constitutes the medium in which chemical reactions occur and is crucial to normal function of the cardiovascular system. Water constitutes about 70 percent of body weight in the normal adult. It decreases from 75 percent at birth to 50 percent in old age and is the largest component of the body. Adipose tissue contains less water than lean tissue; thus women have slightly less body water than men. The effects of dehydration occur with as little water loss as 1 percent of body weight and become life threatening at 10 percent (Adolph et al., 1947). Humans cannot adapt to a chronic water deficit, so fluid losses must be replaced if physiological function is to continue unimpaired.

The purpose of this chapter is to review the water requirements of soldiers exercising in the heat. The Desert Shield and Desert Storm operations in 1990 and 1991 made us acutely aware of the importance of military maneuvers in severe heat. Military missions are often 4 to 6 hours in duration and require mild to heavy exercise. This discussion will examine the range of these work loads. Furthermore, chronic water intake is a concern because inadequate water intake over days can lead to water depletion and heat exhaustion.

The requirement for water in the heat is dependent on fluid lost, which in turn depends on such factors as exercise intensity, exercise duration, environmental conditions, state of training and heat acclimatization, gender, and age. Selected studies are used to illustrate the influence of these different factors rather than to review the literature. Finally, the prediction of sweat losses under a variety of conditions is discussed, as well as the calculation of water requirements under these circumstances.

Total body water constitutes about 70 percent of lean body mass and is most simply divided into two major compartments: (a) intracellular water, which represents 50 percent of body weight or 35 liters in a 70-kg man, and (b) extracellular water, which represents 20 percent of body weight or 14 liters. The latter compartment is subdivided into plasma volume (5 percent body weight) and interstitial fluid volume (15 percent body weight). Intracellular water is not readily measured. It is calculated from measurements of total body water and extracellular fluid volume.

Table 5-1 gives normal values for daily water intake and output in a healthy adult. However, these values are subject to marked variation. For example, respiratory water loss can range from 200 ml per day when breathing humidified air to 1500 ml per day when exercising at high altitude. Water loss from cutaneous evaporation could range from 500 ml per day at rest in a cool environment to 10 liters per day during exercise in the heat. Fecal losses could range from 100 ml per day when on a mixed diet to 32 liters per day or more in a patient with diarrhea. Obligatory urine volume is limited by the concentrating power of the kidneys, but it can vary from 250 to 1400 ml per day depending on diet. Urine volume is usually 700 ml per day, but a high-protein diet demands more obligatory water to excrete the osmotically active products of protein metabolism.

Water requirements during exercise in the heat primarily depend on evaporative cooling. Metabolism and environmental heat exchange determine the required evaporative cooling (E req) to achieve thermal balance. Because respiratory water loss contributes little to evaporative cooling in warm or hot environments, cooling must come primarily from cutaneous sweat secretion. The rate of sweating and its regulation are determined by core and skin temperatures, skin wettedness, heat storage, metabolism, and the set point.

The U.S. military deploys troops to tropical and desert climates, and therefore military men and women are exposed to both wet and dry heat. Figure 5-1 shows the sweat responses as well as the mean changes in rectal temperature, heart rate, and metabolic rate of four distance runners walking 5.6 km per hour in dry heat, in wet heat, and in a cool environment. Experiments were performed 4 to 5 weeks apart and consisted of 4 hours of continuous walking, lunch (30 minutes), followed by another 2 to 3 hours of walking. Water was ingested ad libitum, but the subjects were constantly informed of their weight loss and were successful in maintaining fluid balance. All men walked 6 hours in the neutral and hot-dry environments except one subject who stopped walking after 5.5 hours in dry heat with a rectal temperature of 39°C and a heart rate of 136 beats per minute (bpm). Another subject walked for 7 hours in dry heat and finished with a rectal temperature of 38.3°C and a heart rate of 132 bpm. Sweat rate in the desert environment averaged 1210 ± 56 (x ± SE) ml per hour (Table 5-2).

In the hot-wet environment, sweat rate averaged only 716 ± 56 (x ± SE) ml per hour, which resulted in higher rectal temperatures (39.3°C) and heart rates (132 bpm). The reduced rate of sweating in this environment was associated with sweat gland fatigue (Brown and Sargent, 1965; Hertig et al., 1961; Kerslake, 1972; Nadel and Stolwijk, 1973; Robinson and Gerking, 1947). The mechanism responsible for this phenomenon is not clear, but evidence suggests that it is related to excessive wetting of the skin (Brebner and Kerslake, 1964; Collins and Weiner, 1962; Nadel and Stolwijk, 1973). These subjects were highly trained and essentially heat acclimatized as a result of their training. Untrained or unacclimatized subjects would have considerably lower sweat rates and would experience much more physiological strain than was shown by these men.

Under constant environmental conditions, skin sweating is a linear function of heat production or exercise intensity (Nielsen, 1969). Training in a neutral environment that results in a significant elevation in maximal oxygen uptake

Maximal sweating capacity can rise from 1.5 liters per hour in a healthy unacclimatized man to as much as 2 to 3 liters per hour in a highly trained acclimatized soldier (Wenger, 1988). One of the highest sweat rates ever observed was recorded on Alberto Salazar during the 1984 Olympic Marathon. Salazar was running at 85 percent of

Military personnel range in age from 18 to 50 years and comprise 14 percent women. The effects of age and gender on thermoregulation, particularly the sweating response to exercise and thermal stress, have been elegantly reviewed by Drinkwater (1986). Contrary to popular opinion, differences in physiological responses to thermal stress cannot be attributed to differences in gender or age. When differences do appear among subjects of different age and gender, they are primarily due to differences in aerobic power or heat acclimatization.

In the 1960s, studies of temperature regulation at rest and during exercise in the heat concluded that women were less tolerant of exercise in the heat than were men (Morimoto et al., 1967; Wyndham et al., 1965); however, these investigators did not match their subjects for aerobic power or body weight-to-mass ratio. Weinman et al. (1967) were the first to suggest that gender differences could be explained by differences in physical fitness.

With regard to the effects of age on exercise-heat tolerance, it is well accepted that the aged are more susceptible to thermal injury than their younger counterparts during heat waves. This apparent heat intolerance among the aged has been attributed to a reduction in sweating capacity, a decline in aerobic fitness, or a combination of the two. In a recent review of the effects of exercise and age on thermoregulation, Kenney and Gisolfi (1986) found no evidence that men or women up to 50 to 60 years of age had any impairment in temperature regulation that could be attributed to age per se. This conclusion is also supported by the review of Drinkwater (1986). Robinson et al. (1986) found a decrement in sweating capacity in men 44 to 60 years of age, but this decline in sweating had no adverse effect on the ability of these men to acclimatize to work in a hot-dry (50°C) environment. The decline in heat tolerance associated with men and women 50 to 60 years of age can be readily attributed to reductions in cardiovascular fitness, lack of heat acclimation, or both.

Sweat rate can be predicted from a measure of the overall heat load (Ereq) and the maximal evaporative cooling capacity of the environment (Emax) (Shapiro et al., 1982). The advantage of the latter prediction is that sweat rate (and therefore water required) can be determined from environmental conditions, exercise intensity, and the type of clothing worn without making any physiological measurements (Shapiro et al., 1982). The formula for calculating sweat loss in g per m2 per hour is

sweat loss = 27.9 × E req(E max)-0.455

PARTICIPANT: It is a little unclear. I thought you said that men sweat more but if you expressed it as amount of sweat, provided surface area was the same, but then it looked like in one of the slides it was different.

DR. GISOLFI: No, they are not the same. Even if you express it as percent body surface area, women still sweat less. But the important point is, women are able to maintain the same core body temperature as men when they are at the same relative work load.

PARTICIPANT: And was that formula applicable for both men and women?

DR. GISOLFI: No, the formula was based on men.

PARTICIPANT: Is there any effect from body mass?

DR. GISOLFI: Body fat is going to impede heat loss, certainly, and if you evaluate the impact of body weight to surface area ratio, the heavier person has a greater metabolic heat load and has a smaller surface area to dissipate that heat. These individuals will have more trouble dissipating heat when exposed to a warm environment exercising at the same intensity as an individual who is not carrying that much weight.

PARTICIPANT: Does it affect sweating?

DR. GISOLFI: Not to my knowledge, just having an increased subcutaneous layer of fat does not influence the sweating response.

PARTICIPANT: I have another question about age. Do you have any data on general range?

DR. GISOLFI: There doesn't seem to be a difference in the sweating response up to about 50 or 55. When you get over 60 and it is more clear over 70 years of age, then there is a decrement in the sweating response.

The individuals that Robinson studied (Robinson et al., 1986) were over 60. I think the mean age was something like 61 or 62 years. There was a decrement in the sweating response, but it wasn't reflected in their ability to regulate their body temperature which is, again, the more important point.

PARTICIPANT: Carl, in 1980 Dimri published a review of 55 papers in which he looked at this last point that you mentioned here, the increase in sweating rate during a 7-to 10-day period of heat acclimation (Dimri et al., 1980).

He found in those 55 papers that 15 of them showed no increase in the sweating rate. I know you published a paper at least once that showed no increase in the sweating rate during the deacclimation of 7 to 10 days. Could you comment on that for us?

DR. GISOLFI: I think it depends on the level of fitness of the subject. If you are dealing with a relatively fit individual that you then heat acclimatize, you probably see little change in the sweating response.

If you are dealing with people who are terribly unfit and you heat acclimatize them, you will see a rather substantial elevation in the sweating response.

PARTICIPANT: And also I know you mentioned that in dry environments when you published these studies, for hot, dry and wet, dry environments, there was less of an increase in the sweating range.

PARTICIPANT: At the initiation of exercise, there is an immediate drop or increase in plasma osmolality that doesn't seem to be such in the sweating. That is, there seems to be a movement of fluid from the blood volume to intracellular volume.

I am wondering whether that is due to being acclimatized and unacclimatized and to what extent does that shift in plasma volume affect the sweat rate.

DR. GISOLFI: Initially, I don't think plasma volume and plasma tonicity have a marked influence on sweat rate. The sweating response is being driven primarily by an increase in core body temperature and secondarily by changes in skin temperature.

There are influences from increments in plasma volume and tonicity but compared to an elevation in core body temperature, they are rather small.

DR. GISOLFI: I wanted to make a comment back on sweating, though, on how sweating changes with acclimatization. You must be careful in your interpretation of the literature because if you just look at total body sweating, you get very misleading results.

The magnitude of sweating is related to the rise in body temperature. So if you heat acclimatize a soldier, you will observe increases in sweating early in the process. However, at the end of acclimatization, if you are looking at just total body sweating, you are actually looking at a small rise in body temperature and the same sweating response. The important point is that for a given rise in core temperature, you do have more sweating.

PARTICIPANT: I would like to go back to this prediction equation briefly. That did not take into account obesity or any kind of differences in adiposity amongst individuals; is that right?

DR. GISOLFI: That's correct. The heat required term is based on metabolic rate, which takes body weight into account. Adiposity is not specifically addressed by this equation.

PARTICIPANT: This was done in a military population?

DR. GISOLFI: This was done in the military population, to my knowledge. It is only body weight that is taken into consideration.

PARTICIPANT: So this is a specialized group, then, so it would not necessarily fit across the board; is that what you are saying then?

DR. GISOLFI: Yes. I would also say that I am not familiar with any literature that has indicated that an increase in adiposity reduces the sweating response.

PARTICIPANT: In fact, you showed your men had half the body fat of women and yet their ability to lose heat was equal so adiposity may or may not make any difference, probably not.

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Carl v. Gisolfi, Department of Exercise Science, The University of Iowa, Iowa City, IA 52242