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TABLE 12-1 Day 1 Mean (±SE) Characteristics of Subjects Consuming Low Na+ Diets (LNA) (n = 8) and Moderate Na+ Diets (MNA) (n = 9)

 

Diet

Variable (unit)

LNA*

MNA

Statistical Significance

n

8

 

9

 

Age (years)

20

± 1

20

± 1

NS

Height (cm)

180.1

± 2.3

178.7

± 2.3

NS

Mass (kg)

79.80

± 3.18

77.86

± 3.80

NS

Estimated body fat (%)§

14

± 1

14

± 1

NS

peak (ml per kg per minute)

45.73

± 1.69

47.09

± 1.54

NS

HR (heart rate) peak

207

± 9

211

± 7

NS

* 4 g NaCl, 68 mEq Na+.

8 g NaCl, 137 mEq Na+.

NS, not significantly different (p > .05).

§ Calculation from Jackson and Pollock (1978).

were taken: nude body weight (±50 g), first void urine specific gravity (refractometer), and first void urine Na+ and potassium (K +) concentrations (flame photometer). Blood samples also were collected prior to exercise on days 8, 11, 15, and 17. Subjects entered the 41°C environment and stood quietly for 20 minutes before the sample was drawn. Daily HA trials were conducted in ambient conditions of 41°C, 21 percent relative humidity (rh), and 1.2 m per second air speed (8.5 hours per day); during the remainder of each day (15.5 hours per day), subjects lived at an ambient temperature of 21°C in an effort to simulate a 24-hour desert temperature cycle. Exercise involved 8 periods of alternating rest (30 minutes per hour) and moderate (5.6 km per hour, 5 percent grade) treadmill exercise (30 minutes per hour) while wearing shorts, socks, and sneakers. Exercise was terminated (and subjects rested in the heat for the remainder of the trial) if heart rate (HR) exceeded 180 beats per minute, if rectal temperature (Tre) exceeded 39.5°C, or if Tre rose 0.6°C during any 5-minute period. Subjects drank pure or flavored water (<1 mEq Na+ per liter, 10° to 15°C) ad libitum from canteens during treadmill walking and rest periods. Body weight was maintained each hour by requiring that subjects drink a volume of pure water, at the end of each rest period, that matched the amount of body mass not replaced by ad libitum drinking.

Statistical significance was tested by using a repeated measures analysis of variance (ANOVA) with Tukey's post hoc analysis (Zar, 1974). The two factors in this design were diet (LNA and MNA) and days (days 1 to 17; days 8 to 17; days 8, 11, 15, 17). The null hypotheses were rejected at the p = .05 confidence level. All data were expressed as mean ± standard error.



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