It is difficult to describe hydration status and hydration extremes because fluid intakes and excretion patterns of free-living individuals are poorly documented and regulation of human water balance is complex and dynamic. This investigation provided reference values for euhydration (i.e., body mass, daily fluid intake, serum osmolality; M +/- SD); it also compared urinary indices in initial morning samples and 24-hr collections. Five observations of 59 healthy, active men (age 22 +/- 3 yr, body mass 75.1 +/- 7.9 kg) occurred during a 12-d period. Participants maintained detailed records of daily food and fluid intake and exercise. Results indicated that the mean total fluid intake in beverages, pure water, and solid foods was >2.1 L/24 hr (range 1.382-3.261, 95% confidence interval 0.970-3.778 L/24 hr); mean urine volume was >1.3 L/24 hr (0.875-2.250 and 0.675-3.000 L/24 hr); mean urine specific gravity was >1.018 (1.011-1.027 and 1.009-1.030); and mean urine color was > or = 4 (4-6 and 2-7). However, these men rarely (0-2% of measurements) achieved a urine specific gravity below 1.010 or color of 1. The first morning urine sample was more concentrated than the 24-h urine collection, likely because fluids were not consumed overnight. Furthermore, urine specific gravity and osmolality were strongly correlated (r2 = .81-.91, p < .001) in both morning and 24-hr collections. These findings provide euhydration reference values and hydration extremes for 7 commonly used indices in free-living, healthy, active men who were not exercising in a hot environment or training strenuously.
Context: Authors of most field studies have not observed decrements in physiologic function and performance with increases in dehydration, although authors of well-controlled laboratory studies have consistently reported this relationship. Investigators in these field studies did not control exercise intensity, a known modulator of body core temperature.Objective: To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting.Design: Semirandomized, crossover design. Setting: Field setting. Patients or Other Participants: Seventeen distance runners (9 men, 8 women; age 5 27 6 7 years, height 5 171 6 9 cm, mass 5 64.2 6 9.0 kg, body fat 5 14.6% 6 5.5%).Intervention(s): Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature 5 26.56C): (1) a hydrated, race trial (HYR), (2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal trial (HYS), and (4) a dehydrated, submaximal trial (DYS).Main Outcome Measure(s): For DYR and DYS trials, dehydration was measured by body mass loss. In the submaximal trials, participants ran at a moderate pace that was matched by having them speed up or slow down based on pace feedback provided by researchers. Intestinal temperature was recorded using ingestible thermistors, and participants wore heart rate monitors to measure heart rate.Results: Body mass loss in relation to a 3-day baseline was greater for the DYR (24.30% 6 1.25%) and DYS trials (24.59% 6 1.32%) than for the HYR (22.05% 6 1.09%) and HYS (22.0% 6 1.24%) trials postrun (P , .001). Participants ran faster for the HYR (53.15 6 6.05 minutes) than for the DYR (55.7 6 7.45 minutes; P , .01), but speed was similar for HYS (59.57 6 5.31 minutes) and DYS (59.44 6 5.44 minutes; P . .05). Intestinal temperature immediately postrun was greater for DYR than for HYR (P , .05), the only significant difference. Intestinal temperature was greater for DYS than for HYS postloop 2, postrun, and at 10 and 20 minutes postrun (all: P , .001). Intestinal temperature and heart rate were 0.226C and 6 beats/min higher, respectively, for every additional 1% body mass loss during the DYS trial compared with the HYS trial.Conclusions: A small decrement in hydration status impaired physiologic function and performance while trail running in the heat.Key Words: environmental physiology, dehydration, rehydration, core temperature, heart rate Key Points N The physiologic and performance decrements associated with dehydration that exist in laboratory settings also exist in field settings. N Methodologic challenges in the field setting make isolating these effects difficult.
Compared to a low-fat weight loss diet, a short-term very low-carbohydrate diet did not lower LDL-C but did prevent the decline in HDL-C and resulted in improved insulin sensitivity in overweight and obese, but otherwise healthy women. Small decreases in body mass improved postprandial lipemia, and therefore cardiovascular risk, independent of diet composition.
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