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.

Ingesting sport drinks during exercise: does this interfer with training adaptations?

Endurance exercise training has profound effects on the structural and functional characteristics of skeletal muscle. Scientists refer to muscle's remarkable abilities to adapt to chronic exercise as plasticity. Muscle plasticity is evident by two important outcomes from endurance training, improved endurance performance and increased use of fat. The muscle's ability to use more fat is beneficial to prolonged exercise performance because it slows the rate at which muscle glycogen (a finite source or energy) is used, thus allowing the athlete more power output before fatigue. Among the specific adaptations in the muscle that allow greater use of fat are increased fatty acid transporters located in the muscle cell membrane and increased enzymes that promote fat oxidation.

We know what happens to muscle with training, but what isn't yet totally clear is how these changes are initiated. Current evidence suggests that with every bout of exercise, changes in gene expression are taking place. In this particular scenario, this means an increase in protein expression for certain proteins such as fat transporters and oxidative enzymes. Exercise provokes changes in messenger RNA which in turn increases the muscle cell's ability to produce proteins that are needed for exercise. Why an increase in fat transporters? Perhaps it is a survivial mechanism. Exercise heightens the energy state of the muscle cell, which means more energy sources and oxygen are required. From a survival point of view, if you are going to submit your muscles to this demand, the muscle will have to adapt in order to sustain such activity over and over again. It makes sense from a Darwinian perspective.

Consider the question, "What if you keep feeding the muscles glucose during exercise and by doing so suppress the use of fat?" When glucose is ingested, the muscle will prefer glucose over fat. This results in improved performance because the muscle, 1) can produce more power from glucose over fat, and 2) it's being provided another source of glucose and thus, blood glucose levels are more easily maintained. But, what if glucose ingestion during each bout of exercise interfers with gene expression that favors fat oxidation? Will this stunt muscle adaptations that ultimately lead to improved performance?

In steps Thorbjorn and colleagues from the University of Copenhagen, Denmark. They hypothesized that glucose ingestion during exercise would alter the gene expression of fatty acid transporters and enzymes. There results are published in the July 2009 issue of the Journal of Applied Physiology. To test their hypothesis, they asked 9 men to participant in a unique 10-week training program. To make each participant his own control, training consisted of 1-legged cycling. One leg was assigned the glucose trials and one leg was assigned the placebo trials. They trained one leg at a time on alternate days, 5 days per week. This meant each leg received on average 2.5 training sessions per week. They were given a 6% glucose solution (such as Gatorade) at a rate of 0.7 g/kg/hr during the glucose training sessions. Each participant performed a fatiguing test and performance was measured as length of time. The investigators also measured fat metabolism.

What they found:

• Training improved maximal power and time to fatigue, no difference between glucose and placebo
• Training increased fat oxidation during exercise, no difference between glucose and placebo

The investigator's hypothesis that glucose ingestion during training exercise would alter fat adaptations that normally occur with training was not supported by these results. Noteworthy is that training sessions did not differ between glucose and placebo, meaning each leg received the same training stimuli. This is good to know because it is possible that not ingesting glucose under certain circumstances would reduce training power output, thus reducing the training stimulus. Theoretically, because of this, not ingesting glucose could negatively affect training adaptations. But because there was no difference between trials in terms of training stimulus, this confounding effect seems to not be an issue here.

Bottomline: Keep drinking your Gatorade, your muscles will adapt just as well. And for athletes who train several hours daily, not consuming glucose during exercise will likely have a greater negative impact. Despite carbohydrate ingestion, athlete's muscles are trained to use more fat, which is one reason for their awesome endurance.

Reference: Thorbjorn et al. Glucose ingestion during endurance training does not alter adaptation. J. appl. Physiol. 106, 2009.