Physiological Demands of Hiking the Grand Canyon

To the Editor:

Hiking from top to bottom of the Grand Canyon National Park in Arizona has been a popular endeavor for many years. The most common hike starts at approximately 2100 m above sea level and follows the South Kaibab Trail to Phantom Ranch at the bottom of the canyon (732 m above sea level) and back. The length (∼30 km), duration (6–9 hours), and inclination (∼13%) of this journey exerts extraordinary strain on the hikers' physique, which is influenced by dietary intake, backpack weight, and environmental conditions. The ambient temperatures at the canyon vary widely throughout the year, exceeding 38°C (100.4°F) in the summer months and falling below −18°C (−0.4°F) during the winter. ¹ To prevent the risk of dehydration, heat stroke, hyperthermia, and hypoglycemia, especially in unacclimatized and inexperienced hikers, the Park Service does not recommend completing this journey in 1 day, as evidenced by numerous warning signs.

The intent of this letter is to highlight potential areas of further research regarding the physiological responses to desert hiking and severe environmental conditions. To carefully estimate the cardiopulmonary and metabolic demands during hiking in such an environment, 1 male (age: 31 years, weight: 77.2 kg, height: 179.0 cm) volunteered for a trial, following 5 days of acclimatization to the altitude. The hiker′s peak oxygen uptake (Vo_2peak) of 61.2 mL·min⁻¹·kg⁻¹ and maximal heart rate (HR_max) of 191.0 bpm were measured during a treadmill ramp protocol (Woodway, Lörrach, Germany; starting at 12 km·h−1 and increasing 1°·min⁻¹) using a breath-by-breath analyzer (Metamax 3B, Leipzig, Germany) 10 days before the journey.

Accompanied by a research team, the volunteer hiker started from Yaki Point on the South Rim at 7:45 am on March 21, 2009. The descent via the South Kaibab Trail lasted 3:15 hours and led to Phantom Ranch (∼13% mean grade), the reversal point of the hike. The uphill hike followed the Bright Angel trail (∼13% mean grade) and lasted approximately 4 hours. The volunteer was instructed to hike at a self-selected constant pace.

The ambient temperature during the day ranged from 15°C in the morning up to 29°C at 1:48 pm. The volunteer wore shorts and a light wind jacket during the first hour of the hike, thereafter he switched to shorts and T-shirt. Oxygen uptake (Vo₂), respiratory exchange ratio (RER), and heart rate (HR; Polar Belt, Kempele, Finland) were collected telemetrically throughout the entire hike (Metamax 3B). Based on these data, the active energy expenditure (AEE) of the participant was calculated according to previously published data.^2,3 The total weight of the gear was 4.5 kg, including backpack and breath-by-breath analyzer. Samples (20 μL) of capillary blood for the measurement of lactate (Lac) and glucose (Gluc) concentration (Eppendorf EBIO Plus, Hamburg, Germany) were taken during short breaks (see Figure for time points).

Figure

Subject's relative oxygen uptake kinetics (dotted grey), percent of Vo_2peak (dotted black), percent of maximum HR (black), and RER (grey) over time according to the altitude profile. Black arrows above the time scale indicate short breaks for collecting Lac and Gluc.

Downhill Section

According to HR, Vo₂, Lac, Gluc, and AEE, the findings revealed low cardiorespiratory and metabolic demands during the downhill section. HR and Vo₂ remained constant and did not exceed 40% and 60% of the individual's maximum, reflecting light to moderate exercise, according to the American College of Sports Medicine Position Statement. ⁴ Lac values remained below 2.0 mmol·L⁻¹ and RER decreased from 0.95 to 0.97 at the start to approximately 0.80 at the bottom, indicating that fat oxidation was the major source of energy during this part of the hike. ⁵ There was no indication of hypoglycemia during this period, with Gluc in a stable range of 72 to 96 mg·dL⁻¹.

Uphill Section

In general, the uphill part of the hike showed increasing cardiorespiratory values that peaked towards the end. Oxygen uptake and HR reached 75% to 80% of the individual's Vo_2peak and HR_max (see Figure). In comparison, similar peak values have been reported in a 160-km backpacking trip, ⁶ as well as during marathon racing (75%–85%),^7,8 highlighting the exertion of this hike. Also, the Lac concentration of 3.7 mmol·L⁻¹ at the end of the hike confirms an increased amount of anaerobic contribution to energy production. Especially during the last 2 hours of the hike near the top of the rim, RER values increased to 1.05 to 1.10, indicating that carbohydrates (CHO) were the major source of energy.

Close to Indian Gardens (a common resting point to collect water), Gluc levels declined from 138 mg·dL⁻¹ to 58 mg·dL⁻¹, indicating a hypoglycemic state. Food supply (Cliff Bar and 1.0 L of Gatorade) normalized the level of blood Gluc (96 mg·dL⁻¹). The calculation of energy intake and energy expenditure at the end showed an energy deficit of 1226 kcal. This was due to an accumulated active energy expenditure of 3322 kcal, while the accumulated energy intake was 2096 kcal during the hike. This deficit demonstrates the need for proper caloric and fluid intake strategies to prevent harmful effects, such as hypoglycemia and compromised physical function. Nevertheless, despite the physical strain and the negative energy balance, blood Gluc levels were maintained as long as CHO were ingested.

Conclusion

Even though our volunteer could be classified as well trained and experienced in hiking, he developed hypoglycemia, reflecting the necessity for proper nutrition and hydration. From a physiological point of view, the profile of the hike exerts high demands on the cardiopulmonary and metabolic system and should not be recommended for physically untrained persons, especially during hot conditions. Despite the fact that this case only investigated 1 volunteer and 1 hike, this letter may help to highlight further research needs regarding the responses to desert hiking. These include: (1) nutrition; (2) clothing and equipment; and (3) physical fitness preparation for desert hiking.