High sodium drinks

High Sodium Drinks: Sodium, plasma and performance

A high-sodium drink boosts blood plasma volume and endurance performance without compromising the body’s heat regulation systems, according to a new US study.

Preservation of blood volume is a key factor in sustaining optimal endurance exercise, since it is critical for maintaining an adequate supply of oxygen and nutrients to working muscles while facilitating the removal of metabolic waste. And the need to maintain plasma volume becomes more crucial as significant quantities of metabolic heat are generated, causing competition for blood flow between skin and muscles.

One way to boost plasma volume is by loading up with sodium, although this may have the additional – and undesirable – effect of inhibiting sweating, so impairing thermal regulation during exercise.

The researchers in this study set out to discover whether giving healthy young men an ‘immediate pre-exercise orally-ingested, low fluid-volume sodium load (IPOSL)’ would increase plasma volume and improve performance during a 15-minute time trial without compromising thermal regulation, as measured by sweat rate and core temperature.

Fourteen 23-32-year-old moderately fit recreational cyclists took part in two identical exercise trials, involving 45 minutes of submaximal cycling exercise followed by a 15-minute maximal effort time trial against a constant resistance after each of the following conditions:

  • A high-sodium drink (IPOSL) with a total volume of 10ml per kg of body weight, divided into three equal portions and consumed at 15-minute intervals;
  • A no-sodium drink (placebo) with the same total volume, consumed in the same way.

The contrasting drink regimes led to significant differences in pre-exercise resting plasma volume, with IPOSL leading to a 3.1% expansion and placebo a 4.7% reduction in volume.

After beginning exercise, participants in both conditions lost a significant percentage of plasma volume, which persisted throughout the exercise phase. IPOSL maintained plasma volume during exercise to a greater extent than placebo at 15 and 30 minutes, but the effect was lost at 45 minutes.

Nevertheless, participants performed significantly more work during the 15-minute time trial in the IPOSL condition, riding just under 1k further than after placebo. And no between-groups differences were observed for heart rate, core temperature, rate of perceived exertion or total body sweat rate, suggesting no ill effects of the high-sodium drink on heat regulation.

The most curious aspect of these results was the fact that the cyclists performed better in the time trial after IPOSL even though the effects on plasma volume had worn off by then.

The researchers point out that this difference in performance may have been caused by the reduction in baseline plasma volume in the placebo trial rather than the increase in the IPOSL trial. And they call for further research to elucidate the true reason for their results.

Int J Sports Med 2005; 26: 182-187

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