Tag Archives: dehydration

Carbohydrate ingestion during exercise of relatively short duration and high intensity

Intermittent high intensity exercise and carbohydrate ingestion

High Intensity: During-Exercise Nutrition
Fig. 1- During-Exercise Nutrition

Carbohydrate ingestion during intermittent high intensity or prolonged (>90 min) sub-maximal exercise can:

  • increase exercise capacity;
  • improve exercise performance;
  • postpone fatigue.

The intake of very small amounts of carbohydrates or carbohydrate mouth rinsing (for example with a 6% maltodextrin solution) may improve exercise performance by 2-3% when the exercise is of relatively short duration (<1 h) and high intensity (>75% VO2max), that is, an exercise not limited by the availability of muscle glycogen stores, given adequate diet.
The underlying mechanisms for the ergogenic effect of carbohydrates during this type of activity are not metabolic but may reside in the central nervous system: it seems that carbohydrates are detected in the oral cavity by unidentified receptors, promoting an enhanced sense of well-being and improving pacing.
These effects are independent of taste or sweet and non-sweet of carbohydrates but are specific to carbohydrates.

It should be noted that performance effects with drink ingestion are similar to the mouth rinse; therefore athletes, when they don’t complain of gastrointestinal distress when ingesting too much fluid, may have an advantage taking the drink (in endurance sports, dehydration and carbohydrate depletion are the most likely contributors to fatigue).

Conclusion

It seems that during exercise of relatively short duration (<1 h) and high intensity (>75% VO2max) it is not necessary to ingest large amounts of carbohydrates: a carbohydrate mouth rinsing or the intake of very small amounts of carbohydrates may be sufficient to obtain a performance benefit.

References

Hydration before endurance sports

Dehydration and endurance sports

Pre-hydration
Fig. 1 – Pre-hydration

In endurance sports, like Ironman, open water swimming, road cycling, marathon, or cross-country skiing, the most likely contributors to fatigue are dehydration and carbohydrate (especially liver and muscle glycogen) depletion.

Pre-hydration

Due to sweat loss needed to dissipate the heat generated during exercise, dehydration can compromise exercise performance.
It is important to start exercising in a euhydrated state, with normal plasma electrolyte levels, and attempt to maintain this state during any activity.
When an adequate amount of beverages with meals are consumed and a protracted recovery period (8-12 hours) has elapsed since the last exercise, the athlete should be euhydrated.
However, if s/he has not had adequate time or fluids/electrolytes volume to re-establish euhydration, a pre-hydration program may be useful to correct any previously incurred fluid-electrolyte deficit prior to initiating the next exercise.

Pre-hydration program

If during exercise the nutritional target is to reduce sweat loss to less than 2–3% of body weight, prior to exercise the athlete should drink beverages at least 4 hours before the start of the activity, for example, about 5-7 mL/kg body weight.
But if the urine is still dark (highly concentrated) and/or is minimal, s/he should slowly drink more beverages, for example, another 3-5 mL/kg body weight, about 2 hours before the start of activity so that urine output normalizes before starting the event.

It is advisable to consume small amounts of sodium-containing foods or salted snacks and/or beverages with sodium that help to stimulate thirst and retain the consumed fluids.
Moreover, palatability of the ingested beverages is important to promote fluid consumption before, during, and after exercise. Fluid palatability is influenced by several factors, such as:

  • temperature, often between 15 and 21 °C;
  • sodium content;
  • flavoring.

And hyper-hydration?

Hyper-hydration, especially in the heat, could improve thermoregulation and exercise performance, therefore, it might be useful for those who lose body water at high rates, as during exercise in hot conditions or who have difficulty drinking sufficient amounts of fluid during exercise.
However there are several risks:

  • fluids that expand the intra- and extra-cellular spaces (e.g. glycerol solutions plus water) greatly increase the risk of having to void during exercise;
  • hyper-hydration may dilute and lower plasma sodium which increases the risk of dilutional hyponatraemia, if during exercise, fluids are replaced aggressively.

Finally, it must be noted that plasma expanders or hyper-hydrating agents are banned by the World Anti-Doping Agency (WADA).

Conclusion

“Pre-hydrating with beverages, if needed, should be initiated at least several hours before the exercise task to enable fluid absorption and allow urine output to return toward normal levels. Consuming beverages with sodium and/or salted snacks or small meals with beverages can help stimulate thirst and retain needed fluids” (Sawka et al., 2007).

References

Endurance sports and nutrition

What are endurance sports?

Endurance Sports
Fig. 1 – Endurance Sports

In the last years endurance sports, defined in the PASSCLAIM document of the European Commission as those lasting 30 min or more, are increasing in popularity and competitions as half marathons, marathons, even ultramarathons, half Ironmans, or Ironman competitions attract more and more people.
They are competitions which can last hours, or days in the more extreme case of ultramarathons.
Athletes at all levels should take care of training and nutrition to optimize performance and to avoid potential health threats.
In endurance sports the most likely contributors to fatigue are dehydration and carbohydrate depletion (especially liver and muscle glycogen).

Dehydration and endurance sports

Dehydration is due to sweat losses needed to dissipate the heat that is generated during exercise. To prevent the onset of fatigue from this cause, the nutritional target is to reduce sweat losses to less than 2–3% of body weight; it is equally important to avoid drinking in excess of sweating rate, especially low sodium drinks, to prevent hyponatraemia (low serum sodium levels).

Glycogen depletion and endurance sports

Muscle glycogen and blood glucose are the most important substrates from which muscle obtains the energy needed for contraction.
Fatigue during prolonged exercise is often associated with reduced blood glucose levels and muscle glycogen depletion; therefore, it is essential starting exercise/competition with high pre-exercise muscle and liver glycogen concentrations, the last one for the maintaining of normal blood glucose levels.

Other problems which reduce performance and can be an health threat of the athlete, especially in long-distance races, are gastrointestinal problems, hyperthermia and hyponatraemia.
Hyponatraemia has occasionally been reported, especially among slower competitors with very high intakes of low sodium drinks.
Gastrointestinal problems occur frequently, especially in long-distance races; both genetic predisposition and the intake of highly concentrated carbohydrate solutions, hyperosmotic drinks, as well as the intake of fibre, fat, and protein seem to be important in their occurrence.

References