Dietary nitrate supplementation does not influence thermoregulatory or cardiovascular strain in older individuals during severe ambient heat stress

Abstract

New findings: What is the central question of this study? Does dietary nitrate supplementation with beetroot juice attenuate thermoregulatory and cardiovascular strain in older adults during severe heat stress? What is the main finding and its importance? A 7-day nitrate supplementation regimen lowered resting mean arterial pressure in thermoneutral conditions. During heat stress, core and mean skin temperatures, vasodilatory responses, sweat loss, heart rate and left ventricular function were unchanged, and mean arterial pressure was only transiently reduced, post-supplementation. These data suggest nitrate supplementation with beetroot juice does not mitigate thermoregulatory or cardiovascular strain in heat-stressed older individuals.

Abstract: This study tested the hypothesis that dietary nitrate supplementation with concentrated beetroot juice attenuates thermoregulatory and cardiovascular strain in older individuals during environmental heat stress. Nine healthy older individuals (six females, three males; aged 67 ± 5 years) were exposed to 42.5 ± 0.1°C and 34.0 ± 0.5% relative humidity conditions for 120 min before (CON) and after 7 days of dietary nitrate supplementation with concentrated beetroot juice (BRJ; 280 ml, ∼16.8 mmol of nitrate daily). Core and skin temperatures, body mass changes (indicative of whole-body sweat loss), skin blood flow and cutaneous vascular conductance, forearm blood flow and vascular conductance, heart rate, arterial blood pressures and indices of cardiac function were measured. The 7-day beetroot juice regimen increased plasma nitrate/nitrite levels from 27.4 ± 15.2 to 477.0 ± 102.5 μmol l^-1 (P < 0.01) and lowered resting mean arterial pressure from 90 ± 7 to 83 ± 10 mmHg at baseline under thermoneutral conditions (P = 0.02). However, during subsequent heat stress, no differences in core and skin temperatures, skin blood flow and vascular conductance, forearm blood flow and vascular conductance, whole-body sweat loss, heart rate, and echocardiographic indices of systolic function and diastolic filling were evident following nitrate supplementation (all P > 0.05). Mean arterial pressure was lower in BRJ vs. CON during heat stress (treatment-by-time interaction: P = 0.02). Overall, these findings suggest that dietary nitrate supplementation with concentrated beetroot juice does not attenuate thermoregulatory or cardiovascular strain in older individuals exposed to severe ambient heat stress.

Keywords: beetroot juice; blood pressure; cardiac function; core temperature; skin blood flow.

Figure 1.

Baseline individual plasma nitrate and nitrite concentrations (NOx), systolic and diastolic arterial pressures, and MAP responses before (CON) and after (BRJ) 7 days of dietary nitrate supplementation in older individuals. Error bars indicate standard deviation. * indicates significant difference between CON and BRJ trials (P < 0.05).

Figure 2.

Individual changes in core temperature following 120 min of environmental heat stress before (CON) and after (BRJ) 7 days of dietary nitrate supplementation among older individuals. Symbols and errors bars indicate mean and standard deviations.

Figure 3.

Time-dependent core temperature (top) and mean skin temperature (bottom) responses during 120 min of environmental heat stress before (CON) and after (BRJ) 7 days of dietary nitrate supplementation among older individuals. Data are for means ± standard deviations for nine participants.

Figure 4.

Laser-Doppler flux, expressed as a percentage of maximum (top left); cutaneous vascular conductance, expressed as a percentage of maximum (bottom left); forearm blood flow (top right); and forearm vascular conductance (bottom right) during 120 min of environmental heat stress before (CON) and after (BRJ) 7 days of dietary nitrate supplementation in older individuals. Forearm blood flow and vascular conductance values were collected at baseline (BL) in thermoneutral conditions, and at 45 and 90 min of heat stress. Data are for means ± standard deviations for eight participants.

Figure 5.

Mean arterial pressure (top) and heart rate (bottom) responses during 120 min of environmental heat stress before (CON) and after (BRJ) 7 days of dietary nitrate supplementation in older individuals. Data are for means ± standard deviations for nine participants. * indicates significant treatment-by-time interaction (P = 0.02).