Energy balance during multi-day backpacking

Doing field research is tough. Those extraneous variables (like the environment or exercise intensity) that scientists love to control often make interpretation of results difficult. And many times, the logistics make certain measurements impossible. Imagine trying to get a blood or urine sample from a long distance paddler during a 300-mile race. It's a catch-22 for field studies. You want to take the athlete out of the lab and place her in the environment in which she is use to training and competing. The results will have more meaning, at least to the athlete. On the other hand, you want your measurements to be scientifically meaningful and this is very difficult when they are not performed under controlled conditions. So when reputable scientists seek to study athletes under real-world situations, I have to hand it to them. A recent study with backpackers fits the bill.

Hill and colleagues studied 3 experienced backpackers during a 5-day hike on the Appalachian Trail in Virginia. The hike totaled 161.5 km (100 miles), ranging from aboutr 18 to 23 miles each day. Elevation ranged from about 660 feet to just over 4400 feet. Temperatures ranged from 46 to 72 degrees fahrenheit, not reaching over 57 degrees on most days. The three hikers were a 32-yr old female, a 34-yr old male and a 52-yr old male.

Prior to the hike, each hikers was tested in the lab to determine VO2max and to determine the relationship between heart rate and VO2 during under various walking speeds and grades and with pack weight added. A graded test was performed beginning at a speed of 3 mph and increased to 4 mph at 20% grade. Heart rate and VO2 were measured during each stage.

During the hike, a heart rate monitor was worn at all times. During rest stops lasting more than 1 minute, the hiker recorded the average heart rate response during the previous hiking interval. Resting energy expenditure was estimated based on body weight and net energy expenditure (what was spent during activity) was estimated from the heart rate response. Energy intake was measured.

On average, the hikers walked at speeds ranging from 2.1 to 2.8 mph, the fastest average speed occurring on days 1 and the lowest on days 2 and 5.

What they found:

• Average heart rate was 105 bpm
• Hikers spent almost 100% of their hiking time in light to moderate level activity
• Total daily energy expenditure approached 5000 calories on days 1 to 4
• Total daily energy intake averaged just over 2100 calories
• Expected weight loss was 1.8 kg (4 lb) and actual average weight loss was 1.7 kg
• Rate of calorie expenditure during hiking was 6.3 cal/min, ranging from 2.1 to 15.6

What can long distance paddlers take home from these data? For obvious reasons, backpackers limit their food intake based on pack weight and will almost always experience negative energy balance during multi-day hikes. This may be comparable to a paddler who limits his or her intake because of constant paddling and the inconvenience of accessing food. On the other hand, these hikers hiked for up to 8 hours a day and no more. They also hiked at relatively light to moderate intensities. No doubt, a paddler engaged in a multi-day race such as the Everglades Challenge will be expending energy at a much higher rate (at least twice as high) and continuously for a much longer period than these backpackers and thus, will experience a more dramatic negative energy balance.

Five days is not very long to determine how well the body adjusts to chronic negative energy balance and how a backpacker adjusts his or her mileage and hiking speed over the long haul. This is a good introductory study into possibly a longer study that would involve hikers during an Appalachain Trail through hike. Likewise, I would like to see studies that test paddlers under similar conditions when they are training or competing. There is much to be learned out there from paddlers who experience the real world day in and day out.

Reference: Hill et all. Energy balance during backpacking. Int. J. Sports Med. 29, 2008.