Growth-Hormone responses to consecutive exercise bouts with ingestion of protein plus carbohydrate
Endocrine responses to repeated exercise have barely been investigated, and no data are available regarding the mediating influence of nutrition. On 3 occasions, participants ran for 90 min at 70% VO2max (R1) before a second exhaustive treadmill run at the same intensity (R2; 91.6 ± 17.9 min). During the intervening 4-hr recovery, participants ingested either 0.8 g sucrose · kg–1 · hr–1 with 0.3 g · kg–1 · hr–1 whey-protein isolate (CHO-PRO), 0.8 g sucrose · kg–1 · hr–1 (CHO), or 1.1 g sucrose · kg–1 · hr–1 (CHO-CHO). The latter 2 solutions therefore matched the former for carbohydrate or for available energy, respectively. Serum growth-hormone concentrations increased from 2 ± 1 μg/L to 17 ± 8 μg/L during R1 considered across all treatments (M ± SD; p ≤ .01). Concentrations were similar immediately after R2 irrespective of whether CHO or CHO-CHO was ingested (19 ± 4 μg/L and 19 ± 5 μg/L, respectively), whereas ingestion of CHO-PRO produced an augmented response (31 ± 4 μg/L; p ≤ .05). Growth-hormone-binding protein concentrations were unaffected by R1 but increased similarly across all treatments during R2 (from 414 ± 202 pmol/L to 577 ± 167 pmol/L; p ≤ .01), as was the case for plasma total testosterone (from 9.3 ± 3.3 nmol/L to 14.7 ± 4.6 nmol/L; p ≤ .01). There was an overall treatment effect for serum cortisol (p ≤ .05), with no specific differences at any given time point but lower concentrations immediately after R2 with CHO-PRO (608 ± 133 nmol/L) than with CHO (796 ± 278 nmol/L) or CHO-CHO (838 ± 134 nmol/L). Ingesting carbohydrate with added whey-protein isolate during short-term recovery from 90 min of treadmill running increases the growth-hormone response to a second exhaustive exercise bout of similar duration.
Hydration: Is your sports drink making you dehydrated?
Women are not Small Men: USA Cycling Coaching Summit presentation 2013
Water and Electrolyte Requirements for Exercise
Authors: Latzka W.A. & Montain S. J.
Source: Clinics in Sports Medicine 18(3), 1999, 513-525
Maintenance of water and electrolyte balance is important for sustaining cognitive and physical performance. Dehydration degrades morale and desire to work. Body water deficits of as little as 2% body weight can impair physical performance. Water deficits of 5% to 7% body weight are associated with dyspnea, headaches, dizziness, and apathy. Water deficit and salt depletion are also considered significant risk factors for the development of heat illness during hot-weather activities. Excessive overhydration can also be detrimental to cognitive and physical performance. People consuming excessive quantities of water during physical activities can produce a relatively rapid dilution of plasma electrolytes if the kidneys are unable to excrete the excess fluid. This behavior can result in intracranial swelling and development of “water intoxication”-a syndrome manifested by altered CNS function, nausea, and impaired physical abilities?
This article summarizes the water and electrolyte losses during exercise and presents strategies to optimize hydration state before, during, and following exercise.
Intestinal fluid absorption during exercise: role of sports drink osmolality and[Na+]
Authors: Gisolfi CV., Lambert G>P>, and Summers R.W.
Source: Med Sci Sports Exerc. 33(6), 2001, 907-915.
This study focused on the combined effects of hypotonicity and high [Na+] in a fluid replacement beverage ingested orally by exercising normal healthy euhydrated adults. Formulation of the ideal fluid replacement beverage during prolonged exercise remains unclear. A single beverage suitable for all environmental and race conditions probably does not exist. To maximize water absorption, consideration should be given to studying beverages formulated with a) CHO in multiple forms (to possibly enhance solute transport) but in reduced concentration (2–4%) from previous formulations to reduce osmolality; and b) Na+ to reduce Na+secretion in the duodenum, which serves to attenuate the osmotic flow of water from the blood into the intestinal lumen.
Effects of carbohydrate type and concentration and solution osmolality on water absorption
Authors: X. Shi, RW Summers, HP Schedl, SW Flanagan, R. Chang, and CV Gisolfi
Source: Med Sci Sports Exerc 27(12), 1607 -1615
We studied intestinal absorption of solutions containing either one (glucose, Glu, or maltodextrin, Mal) or two (fructose, Fru, and Glu or sucrose, Suc) transportable carbohydrate (CHO) substrates using segmental perfusion technique in eight healthy male subjects. These CHO were either free or directly transportable monosaccharides (Glu, Fru), bound as the disaccharide (sucrose, Suc), or as oligomers (maltodextrins, Mal). [CHO] was varied from 6% to 8% (120-444 mmol.1(-1)). All solutions contained low [Na+] (15-19 mEq) and [K+] (3-4 mEq). Solutions osmolalities varied from 165 to 477 mOsm.kg(-1). Osmolalities in the test segment ranged from 268 to 314 mOsm.kg(-1). The regression line of osmolality with water absorption differed for single as compared with multiple substrate solutions. The significantly different intercepts of these two regression lines suggest that solutions with multiple substrates produce greater water absorption at a given osmolality than those with one. Comparing all solutions, test segment solute flux (partial r = 0.69) was more important than mean osmolality (partial r = 0.32). In conclusion, solutions with multiple substrates stimulate several different solute absorption mechanisms yielding greater water absorption than solutions with only one substrate.
Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat
Authors: Stacy T. Sims, L van Vliet, JD Cotter, and NJ Rehrer
Source: Medicine and Sciences in Sports and Exercise, 39 (1), 123-130, 2007.
Purpose: This study was conducted to determine whether preexercise ingestion of a highly concentrated sodium beverage would increase plasma volume (PV) and reduce the physiological strain of moderately trained males running in the heat. Methods: Eight endurance-trained (VO2max: 58 mLIkg-1Imin-1 (SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment. Runners ingested a high-sodium (High Na+: 164 mmol Na+IL-1) or low-sodium (Low Na+: 10 mmol Na+IL-1) beverage (10 mLIkg-1) before running to exhaustion at 70% VO2max in warm conditions (32-C, 50% RH, Va $ 1.5 mIs-1). Beverages (~757 mL) were ingested in seven portions across 60 min beginning 105 min before exercise. Trials were separated by 1–3 wk. Heart rate and core and skin temperatures were measured throughout exercise. Urine and venous blood were sampled before and after drinking and exercise. Results: High Na+ increased PV before exercise (4.5% (SD 3.7)), calculated from Hct and [Hb]), whereas Low Na+did not (0.0% (SD 0.5); P = 0.04), and involved greater time to exercise termination in the six who stopped because of an ethical end point (core temperature 39.5-C: 57.9 min (SD 6) vs 46.4 min (SD 4); P = 0.04) and those who were exhausted (96.1 min (SD 22) vs 75.3 min (SD 21); P = 0.03; High Na+ vs Low Na+, respectively). At equivalent times before exercise termination, High Na+ also resulted in lower core temperature (38.9 vs 39.3-C; P = 0.00) and perceived exertion (P = 0.01) and a tendency for lower heart rate (164 vs 174 bpm; P = 0.08). Conclusions: Preexercise ingestion of a high-sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceived strain during exercise and increased exercise capacity in warm conditions.
Pre-exercise sodium loading aids fluid balance and endurance for women exercising in the heat
Authors: Stacy T. Sims, Nancy J. Rehrer, Melanie L. Bell, and James D. Cotter. School of Physical Education, Department of Human Nutrition, and Preventive and Social Medicine, University of Otago, Dunedin, New Zealand
Source: Journal of Applied Physiology, 103, 534-541, 2007
This study was conducted SD 25.6) vs. 78.7 (SD 24.6) min; 95% confidence interval: 13.3, 26.8; P < 0.0001]. Core temperature rose more quickly with Low Na+ [1.6°C/h (SD 0.2)] than High Na+ [1.2°C/h (SD 0.2); P = 0.04]. Plasma [AVP], [Na+] concentration, and osmolality, and urine volume, [Na+], and osmolality decreased with sodium loading (P < 0.05) independent of pill usage. Thus preexercise ingestion of a concentrated sodium beverage increased PV, reduced thermoregulatory strain, and increased exercise capacity for women in the high-hormone phase of natural and oral contraceptive pill-mediated menstrual cycles, in warm conditions.
Endogenous and Exogenous female sex hormones and renal electrolyte handling: effects of an acute sodium load on plasma volume at rest.
Authors: ST Sims, NJ Rehrer, ML Bell, and JD Cotter.
Source: Journal of Applied Physiology, 105(1), 121-127, 2008
This study was conducted to investigate effects of an acute sodium load on resting plasma volume (PV) and renal mechanisms across the menstrual cycle of endurance-trained women with natural (NAT) or oral contraceptive pill (OCP) controlled cycles. Twelve women were assigned to one of two groups, according to their usage status: 1) OCP [n = 6, 29 yr (SD 6), 59.4 kg (SD 3.2)], or 2) NAT [n = 6, 24 yr (SD 5), 61.3 kg (SD 3.6)]. The sodium load was administered as a concentrated sodium chloride/citrate beverage (164 mmol Na+/l, 253 mosmol/kgH2O, 10 ml/kg body mass) during the last high-hormone week of the OCP cycle (OCPhigh) or late luteal phase of the NAT cycle (NAThigh) and during the low-hormone sugar pill week of OCP (OCPlow) or early follicular phase of the NAT cycle (NATlow). The beverage (∼628 ml) was ingested in seven portions across 60 min. Over the next 4 h, PV expanded more in the low-hormone phase for both groups (time-averaged change): OCPlow 6.1% (SD 1.1) and NATlow 5.4% (SD 1.2) vs. OCPhigh 3.9% (SD 0.9) and NAThigh 3.5% (SD 0.8) (P = 0.02). The arginine vasopressin increased less in the low-hormone phase [1.63 (SD 0.2) and 1.30 pg/ml (SD 0.2) vs. 1.82 (SD 0.3) and 1.57 pg/ml (SD 0.5), P = 0.0001], as did plasma aldosterone concentration (∼64% lower, P = 0.0001). Thus PV increased more and renal hormone sensitivity was decreased in the low-hormone menstrual phase following sodium/fluid ingestion, irrespective of OCP usage.
The Effect of Adding Caffeine to Postexercise Carbohydrate Feeding on Subsequent High-Intensity Interval-Running Capacity Compared With Carbohydrate Alone
Authors: Conor Taylor, Daniel Higham, Graeme L. Close, and James P. Morton
Source: International Journal of Sport Nutrition and Exercise Metabolism, 2011, 21, 410 -416.
The aim of this study was to test the hypothesis that adding caffeine to postexercise carbohydrate (CHO) feedings improves subsequent high-intensity interval-running capacity compared with CHO alone. In a repeated-measures design, 6 men performed a glycogen-depleting exercise protocol until volitional exhaustion in the morning. Immediately after and at 1, 2, and 3 hr postexercise, participants consumed 1.2 g/kg body mass CHO of a 15% CHO solution, a similar CHO solution but with addition of 8 mg/kg body mass of caffeine (CHO+CAFF), or an equivalent volume of flavored water only (WAT). After the 4-hr recovery period, participants performed the Loughborough Intermittent Shuttle Test (LIST) to volitional exhaustion as a measure of high-intensity interval-running capacity. Average blood glucose values during the 4-hr recovery period were higher in the CHO conditions (p < .005) than in the WAT trial (4.6 ± 0.3 mmol/L), although there was no difference (p = .46) between CHO (6.2 ± 0.8 mmol/L) and CHO+CAFF (6.7 ± 1.0 mmol/L). Exercise capacity during the LIST was significantly longer in the CHO+CAFF trial (48 ± 15 min) than in the CHO (32 ± 15 min, p = .04) and WAT conditions (19 ± 6 min, p = .001). All 6 participants improved performance in CHO+CAFF compared with CHO (95% CI for mean difference = 1–32 min). The study provides novel data by demonstrating that adding caffeine to postexercise CHO feeding improves subsequent high-intensity interval-running capacity, a finding that may be related to higher rates of postexercise muscle glycogen resynthesis previously observed under similar feeding conditions.