Changes in Blood Electrolytes and Plasma Volume in NFL Football Players During Pre-season Training

Abstract

0371 Preseason training for football teams in the National Football League (NFL) consists of several consecutive weeks of two practice sessions per day during the hottest months of the year. Due to the large body surface area (BSA) of these football players (FB) sweat, and therefore electrolyte losses can be high. PURPOSE: To measure blood electrolytes and changes in plasma volume (PV) in NFL players during pre-season training in a warm, humid environment. METHODS: Six NFL veteran players (age = 25.5 ± 2.6 yr., height = 186.3 ± 5.5 cm, weight = 104 ± 17.6 kg and BSA = 2.28 ± 0.19m2) participated in the study. Venous blood samples were drawn from subjects for baseline (BL) measures upon arrival to camp and prior to the morning practice on Days 3, 5, 9 and 11. Blood sodium (Na+), potassium (K+), magnesium (Mg++), calcium (Ca++) and pH were determined by ion-selective electrode and percent change in PV was calculated using hematocrit and hemoglobin. The players underwent physical testing on Day 1, practiced two times per day on Days 2 through 4, and then alternated one or two practice sessions per day for Days 5 through 11. Pre-season training camp for this NFL team was held in Eastern Pennsylvania where it is warm and humid during July and August. Data were analyzed with repeated measures ANOVA. RESULTS: Blood Na+ was lower on Day 3 (136.9 ± 0.55 mmol/l) compared to BL (140.35 ± 0.38 mmol/), Day 9 (140.28 ± 0.35 mmol/l) and Day 11 (140.61 ± 0.82 mmol/l), p < 0.01, but was not different from Day 5 (138.05 ± 0.59 mmol/l). Blood K+ was higher on Days 5, 9 and 11 (4.16 ± 0.13, 4.36 ± 0.08 and 4.42 ± 0.07mmol/l) compared to BL (3.77 ± 0.15mmol/L). A significant linear trend was found for Mg++. It rose from 0.5097 ± 0.032 mmol/l at BL to 0.561 ± 0.012mmol/l on Day 5 and then fell to 0.462 ± 0.043 mmol/l by Day 11. No differences were found for blood Ca++ or pH. PV was lower on Day 3 (4.9 ± 2.39 % below BL) compared to Day 5 (4.5 ± 1.89% above BL). CONCLUSIONS: Blood Na+ levels decline in NFL players by day 3 of preseason training and appear to be maintained at low normal levels (136 mmol/L) at the expense of a contracted (−4.5%) PV. Na+ supplementation may be necessary in professional FB players prior to and during the first few days of pre-season two-a-day training. Blood Mg++ also declines to low normal levels by the second week of pre-season training camp and may also need supplementation. The rise in resting levels of blood K+ in FB players on Days 5, 9 and 11 may indicate muscle cell damage due to the physical stresses of practicing football in the heat. Supported by a faculty development grant from West Chester University.