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[Personaltrainer]

Dehydration And Performance

Even on cool days, dehydration can seriously damage your performance. Here's a guide to avoiding it

Many athletes dehydrate during competitive events, especially long
ones, even when it's not particularly hot. You can't rely on
feeling thirsty as a reminder to replace fluid lost through
sweating - one of nature's dirty tricks is that exercise suppresses
thirst. Dehydration impairs both physical and mental performance in
all types and levels of sport, yet it can be avoided (or at least
minimised) by appropriate drinking strategies. Before we explain
what those are, here's a bit of background physiology.

Exercise produces heat. Prevention of overheating occurs by
transfer of heat to the skin by vasodilation of the cutaneous
circulation, and by the cooling effect of evaporation of sweat.
Exercise causes body fluid losses from moisture in exhaled air as
well as from sweating. Although sweat rates are highest under
conditions of high-intensity exercise in heat and high humidity,
total fluid losses can be appreciable in very prolonged events,
whatever the conditions. Unless fluid losses are replaced by
drinks, sweating causes progressive depletion of circulating blood
volume, leading to hypohydration (commonly called dehydration) and
a thickening of blood. This places a strain on the cardiovascular
system, with a rise in heart rate in order to maintain adequate
blood flow to exercising muscles and vital organs. As blood volume
depletes, blood flow to the skin is reduced. As a result, sweating
decreases and heat dissipation from the skin is impaired, causing
body core temperature to rise, potentially leading to heat stress,
collapse and even death.

Even low levels of dehydration have physiological consequences. A
loss of 2% bodyweight (just 1kg for a 50kg person) causes an
increase in perceived effort and is claimed to reduce performance
by 10-20% A fluid loss exceeding 3-5% bodyweight reduces aerobic
exercise performance noticeably and impairs reaction time,
judgement, concentration and decision making - vital elements in
all sports, from pole-vaulting to football. A particular issue for
boxers is that dehydration increases risk of brain injury.

The two main factors influencing early fatigue and impaired
performance (both physical and mental) in all types of sports and
exercise are depletion of body levels of carbohydrate (CHO) and/or
fluid. Maintaining adequate CHO and fluid intake optimises training
benefit by enabling athletes to train harder and for longer, and
can make a difference between winning and losing. The opportunity
and ability to eat and drink during training and competition
depends on the sport; sports drinks may not be the only feasible
option but most of them do provide a convenient supply of CHO and
fluid simultaneously, are well tolerated if used appropriately, and
can be adapted to suit individual needs in varying circumstances.

There are guidelines for fluid and CHO requirements of various
activities, but deciding what, when and how much an individual
athlete should consume is not always straightforward. In
particular, sweat rates vary among individuals subjected to the
same exercise conditions. Probably the most important feature of
any sports drink is PALATABILITY (in large volumes, not just a
sip). The perfect drink has no value if it tastes awful, and what
suits one person may not suit another.