Journal Title (Medline/Pubmed accepted abbreviation): J. Sports Sci.
Page numbers: 537-542
Summary of background and research design:
Background: Disuse of limbs or completely immobilization (such as those in bed rest or in space travel) causes muscle atrophy and weakening of bones. In addition to the lack of weight-bearing activities, it is believed that diet can affect the rate of tissue turnover. Specifically, a high salt diet can exacerbate the loss of strength in muscle and bone.
Hypothesis: When participants are immobilized for 14 days, a high salt diet will lead to higher bone loss and muscle loss than a low salt diet.
Subjects: Eight healthy males, age 26.3 ± 3.5 y
Experimental design: randomized, crossover
Treatments: A high salt diet (7.7 meq Na+/kg body weight per day) or a low salt diet (0.7 meq Na+/kg body weight per day). For an 85 kg person (187 lbs.), this is equivalent to 38 g or 3.5 g of sodium chloride per day, respectively.
Protocol: The participants completed a 4-day adaptation phase, a 14-day intervention phase where they were subjected to all day bed rest, and a 3 day recovery phase. During the bed rest phase, participants lain at a 6° angle with their heads lower than their feet. They were asked to not elevate their heads above 30° at any point throughout the bed rest period. All daily activities, including showering, were performed in this position. Blood and urine samples were collected throughout the 3 phases of the study. After completion of one variable, the participants had a wash out period of at least 6 mo. before repeating the experiment with the other treatment.
Summary of research findings:
- The concentration of sodium in the serum did not differ between diets (p = 0.63) but serum chloride concentrations were higher in the high salt diet (13.9 ± 0.9 mM vs. 100.0 ±1.0 mM, p < 0.001).
- Calcium excretion was 74% higher with the high sodium chloride diet compared to the low salt diet during the immobilization phase.
- The high salt diet significantly increased markers of bone breakdown compared to the low salt diet (p < 0.001).
- The concentration of bicarbonate (acid-base buffer in the blood) was lower during the high salt diet, though capillary blood pH was not significantly different.
- The high salt diet was associated with a more negative nitrogen balance, indicating that protein is being broken down at a faster rate. It is estimated that the high salt diet caused about 3 times the amount of protein loss as the low sodium diet.
Key practice applications:
A high sodium diet can accelerate the rate of bone and muscle loss under immobilization conditions. This study implemented a rather extreme, whole body immobilization phase, but it is likely that salt intake can similar effects on injured and immobilized limbs, although probably to a lesser extent. It should be noted that the high sodium diet in this study provided approximately 15,000 mg sodium, which is 3-5 times the typical dietary intake in the U.S., a country in which dietary sodium intake is high. Recommendations for sodium intake in athletes are variable. The public health benefits of sodium restriction for controlling blood pressure are well recognized and have led to the development of quite conservative Dietary Reference Intakes for sodium. However, athletes exercising in conditions of high heat and humidity may require considerably more sodium than the current adult Adequate Intake (AI) value of 1,500 mg/day or even the Tolerable Upper Intake Level (UL) of 2,300 mg/day (both values based on blood pressure regulation and not specifically on athletes). Given that the current sodium content of the diet is much higher than the AI or UL for sodium, some have questioned how realistic or achievable these values are, especially in the case of athletes who might have higher needs for sodium. Regardless, the high sodium diet far exceeded even what the sodium requirement might be athletes exercising in high heat and humidity.