Nutrition

Fluid physiology 101

Jun 16, 2020

Runner rehydrating

To help you understand the importance of balancing fluids correctly in your sports diet, here are some of the key points on fluid replacement for the physically active.

Fluid and Electrolyte Requirements

Fluid needs vary greatly from person to person, so it’s hard to make a one-size-fits-all recommendation. Sweat rates commonly range between 500ml to 2 litres per hour, depending on your sport, body size, intensity of exercise, and clothing; the weather conditions (hot or cold); whether you are heat acclimatised; and how well trained you are. Sweat rates for a 50 kg slow runner might be 1 pound (480 ml) of sweat per hour, whereas a 91 kg fast runner might lose about 4 pounds (2 litres) per hour. Even swimmers sweat—almost a pound (0.5 kg) per hour during hard training. Football players wearing full equipment in the summer heat might lose more than 16 pounds (8 litres) of sweat in a day.

  • Water in saliva and gastric secretions helps digest food.
  • Water in sweat dissipates heat through the skin.
  • During exercise water absorbs heat from your muscles, dissipates it through sweat, and regulates body temperature.
  • Water in blood transports glucose, oxygen, and fats to working muscles and carries away metabolic by-products such as carbon dioxide and lactic acid.
  • Water in urine eliminates metabolic waste products. The darker the urine, the more concentrated the wastes.
  • Water throughout the body lubricates joints and cushions organs and tissues.

 

On a daily basis, the simplest way to tell whether you are adequately replacing the fluid lost in sweat is to check the colour and quantity of your urine. For specific colours, search “urine colour chart” on the Internet. If your urine is very dark and scanty, it is concentrated with metabolic wastes, and you need to drink more fluids or eat more foods with a high water content such as prepared oatmeal, yogurt, and fruit. (Most people get 20 to 30 percent of their fluids from foods; some people actually eat all their daily water requirement.) When your morning urine is pale yellow, your body has returned to water balance. Your urine may be dark if you are taking vitamin supplements; in that case, volume is a better indicator than colour.

In addition to monitoring urine and weight loss, you should also pay attention to how you feel. If you feel chronically fatigued, headachy, or lethargic, you may be chronically dehydrated. This is most likely to happen during long hot spells in the summertime. Dehydration can be cumulative.

Sweat contains more than just water; it has electrically charged particles (electrolytes, more commonly called minerals) that help keep water in the right balance inside and outside of your cells. The amount of electrolytes you lose via your sweat depends on how much you sweat, your genetics, your diet, and how well you are acclimatised. The table below shows the electrolyte loss that can occur with sweating—and how the losses can be replaced with food.

Electrolyte Loss in Sweat

Electrolyte Average amount lost

in 2 lb (1 L, 1 qt) of sweat

Food comparison
Sodium 1,000 mg (range 200-1,600) 1 small packet salt = 590 mg sodium

1 qt (1 L) Gatorade = 440 mg sodium

Potassium 200 mg (range 120-600) 1 medium banana = 450 mg potassium
Calcium 20 mg (range 6-40) 8 oz (230 g) yogurt = 300 mg calcium
Magnesium 10 mg (range 2-18) 2 tbsp peanut butter = 50 mg magnesium

 

Muscle cramps might sometimes be associated with dehydration and electrolyte deficits, but the current hypothesis points to muscle fatigue as the key trigger. Yet, if you sweat profusely, are left caked with salt, and experience cramps, take extra care to drink plenty of sodium-containing fluids before and during exercise. If your diet has a high salt content, you can likely replace sodium losses after exercise with standard post-exercise meals. Consuming extra salt on your food if you have sweated heavily can be a smart way to enhance recovery, retain fluid, and stimulate thirst.


Reproduced by permission from N. Clark, Nancy Clark’s Sports Nutrition Guidebook, 6th ed. (Champaign, IL: Human Kinetics, 2020).

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