© Borgis - New Medicine 2/2006, s. 39-42
Heat stress responses in men after ingestion of a low-sodium diet
Faculty of Physiological-Medical Sciences, Department of Physiology, Academy of Physical Education, Katowice, Poland
Head of Department: Prof. Józef Langfort, MD, PhD
Aim: The aim of this study was to examine the physiological and metabolical responses to exogenous heat stress after ingestion of a low-sodium diet (580 mg Na/24 h/person).
Material and method: The study included 11 healthy males non-exercising and non-acclimating to heat. The participants were volunteers (aged 21.44±0.91 years, body height 178.3±2.28 cm, body weight 72.54±4.97 kg). The participants took part in two research tasks. Before the experiment, after diet, heat stress and the 24 hour long recovery period, the samples of vein blood were collected. They also assessed the changes in body mass and body composition after receiving the diet and after the heat stress in both tasks (weight, bioimpedance TANITA, Poland) and the changes in water distribution in body water compartments.
Results: The obtained results indicate that the reduction of sodium availability after ingestion of low sodium diet, increases heat accumulation during thermal stress as the result of decrease in water availability to sweating.
Conclusions: Diminution of sodium amount in adult males obstructs the proper rehydration during recovery despite of strong activation of sodium-saving mechanisms.
Physical activity and/or exposure to heat results in some thermal homeostasis disorders. Maintaining a constant internal temperature under such conditions and active defence against the threat of hyperthermia triggers a lot of compensation reactions aiming at recovering the functional balance of the organism. Under a thermal stress affecting the organism its defence against the excess of heat is performed mainly by sweating. Unfortunately, the sweat evaporation from the skin surface involves some loss of water and electrolytes, which can lead (without refilling liquids) to hyperosmotic dehydration. The increase in osmolality and reduction of the systemic liquid volume results in reducing the efficiency of heat dissipation from the body, which can cause hyperthermia. Relatively little is known how effective can be the thermoregulatory mechanism in defence against hyperthermia and dehydration under the conditions of low-sodium diet.
Daily sodium consumption ranges from 700 to 3,600 mg/24h/person. Under the conditions of increased ambient temperature because of the possibility of abundant sweating, the content of this element in food and beverages should be doubled.
The main source of sodium for humans are meat dishes and table salt added to meals and to preserved food during its preparation. Sodium can also come from such sources as: baking soda, local spices or neutralizing agents. The minimum human demand for this element ranges from 500 to 1000 mgNa/24h, though average daily amount we deliver in food ranges from 2,500-5,000 mg Na/24h and it frequently reaches the value of 6,000 mg/24h.
The increase in sodium intake up to approx. 16,000 mg/24 h does not result in any significant changes in blood pressure and such amount of Na often in food is recommended to long-distance runners taking exercises in high ambient temperatures . The delivery of 21-27 g/24h per day in food increases the risk of hypertension.
Decreasing the amount of consumed sodium leads to the reduction in blood pressure. The hypotensive effect of low-sodium diet is a result of changes in electrolyte and water content and hormonal changes accompanying the drop in sodium concentration in the body .
The currently available literature provides a lot of information concerning water-electrolyte changes occurring in the body under the influence of thermal stress. Although, relatively little is known about the changes in systemic response to heat load under the conditions of low-sodium diet. The studies of Armstrong et al.  show that low-sodium diet does not exclude the development of thermal acclimation in people and it seems that it should not disable body to efficiently respond to a single heat stress, though presumably different may be the response model to the thermal stress under the shortage of sodium ions.
The aim of this paper was to assess the changes of physiological and metabolic markers after a single thermal stress in the participants being on a low-sodium diet.
Material and method
The study included 11 healthy males non-exercising and non-acclimating to heat. The participants were volunteers (aged 21.44±0.91 years, body height 178.3±2.28 cm, body weight 72.54±4.97 kg) and before the experiment they were informed about the aim and investigation schedule. The investigation schedule was given a permission from the Bioethical Investigation Commission at the Academy of Physical Education, Katowice.
The participants took part in two research tasks.
In the first of them (C-HS) 3 days before the planned impact of thermal stress they consumed mixed food (diet) (C) in which they received 3,200 mgNa/24 h/person and 4,300 mgK/24h/person. In the second task (L-Na-HS) for 3 days they received a low-sodium diet (L-Na) with significantly reduced sodium concentration: down to 539 mgNa/24h/person. The sodium content in this diet amounted to 3,900 mgK/24h/person. The caloric value of both diets was similar and amounted to 3200kcal?75kg-1?24h-1. While receiving both diets participants drank low-electrolyte liquids at the amount of approx. 3 l/24h. The diet type taken by participants was subject to randomization.
On the third day of consuming a particular diet, participants were subject to heat stress. The heat stress (HS) was always applied by staying in dry sauna (sauna temperature amounted to 85°C, whereas its relative humidity did not exceed 25%) for approx. 90 minutes with two several minute brakes for taking a cold shower) . After the stay in sauna and cooling down the body participants recovered for 2h (at that time they did not refill the water loss resulted from staying in sauna). After completing the rest till to the end of 24h recovery they received in particular groups: C-HS – control diet, whereas L-Na-HS a low-sodium diet and they drank low-electrolyte liquids at the amount of at least 3 l/24h.
Before the experiment, after diet, heat stress and the 24 hour long recovery period, the samples of vein blood were collected in order to test the following values: aldosterone concentration (Aldo)-RIA DSL8600 USA, cortisol (Cort)-RIA DSL2100, USA, haemoglobin (Hb) by spectrophotometric method HEMOCUE, Sweden, total protein content (TP) by spectrophotometric method, ANALCO, Poland, and the haematocrite index. In the blood plasma the tests assessed the following concentration levels: sodium (Na+) and potassium (K+) by using a flame spectrophotometer manufactured by EPENDORF-EFOX, Germany. Prior to and after heat stress internal temperature (ELLAB, Denmark) and systolic and diastolic blood pressure were measured. They also assessed the changes in body mass and body composition after receiving the diet and after the heat stress in both tasks (weight, bioimpedance TANITA, Poland) and the changes in water distribution in body water compartments .
The results were subject to standard statistical analysis using the packet of Statistica (ANOVA) software. Checking the difference in significance between obtained values were based on the variance analysis and the Newman-Keuls post-hoc test. Corrected values were subject to assessment, where the correction included the changes in plasma volume resulting from diet and heat stress, affecting the assessed blood plasma parameters.
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