When it comes to tracking calories, your body is like that annoying guy in accounting who catches the tiniest indiscretion in your travel expense reports. Every morsel in the mouth is carefully noted on an internal metabolic balance sheet registering intake versus output. If you’re like the average American, then each and every day you end up at least 20 calories over budget. Which means—come December 31—you’ll weigh two pounds more than you did January 1. That’s a big number if you maintain the same rate year after year, putting on 20 pounds a decade after you stop growing taller.
For a backpacker, those extra pounds are doubly insidious, as they’re both bad for health and bad for hiking (you’ll get more mileage out of that ultralight pack if you’re carrying less around the middle). Fortunately, backpacking itself offers a ledger-busting solution to the accounting problem. Duration and intensity make our sport uniquely efficient when it comes to weight loss.
On average, a 180-pound man carrying a 40-pound pack across steep terrain combusts more than 600 calories an hour while backpacking. (Calorie-burn numbers are approximate, and go up and down with body size, pack weight, and exertion.) By the minute, you can burn nearly twice as many calories running at a 7:50 per mile pace, but the average run lasts slightly more than half an hour. Long after the hares have showered and grabbed a snack, backpackers will still be out there striding, and burning, along.
On the Fatpacking venture in west Texas, we hiked at an easy pace for five to six hours daily. I typically go harder and longer, but even still I was vaporizing more than 3,300 calories. Add this to the 1,800 calories a man my size needs daily to power basic functions like breathing, digesting food, and maintaining a steady body temperature—all part of what’s known as basal metabolism—then factor in another several hundred calories for fetching water, strolling to the latrine, and wandering to an overlook, and the “calories out” side of the ledger was pushing more than 5,300 each day.
Calorie burn on that scale lifts backpackers out of the ranks of recreational duffers and thrusts us into athletically exclusive company, according to Neal Henderson, M.S., director of sports science at the Boulder Center for Sports Medicine. “Daily energy expenditure by a backpacker compares to that of elite cyclists, triathletes, and ultradistance runners,” he says. I’m not sure I’d want to see Dan or Jeff squeezed into spandex, but no one could deny they were Olympians at blasting calories.
The effects might not be obvious on a weekend trek, but play out a scenario of all-day backpacking over a week—or even months in the case of long-distance hikers—and the changes in body composition can be radical. That’s especially true when you maintain a normal diet on the trail. By Silberberg’s estimate, his clients eat about 2,500 calories a day. Which put me 2,800 in deficit—enough to burn 13 ounces of fat. Within days, I could actually detect my body geography shifting. My pants fit loosely in the waist.
Extended calorie burn alone doesn’t account for why backpacking is one of the ultimate pound-shedding, shape-altering activities. Our sport also taps body fat more efficiently than most alternatives. Runners, hikers, and other athletes all burn a mix of carbohydrates and fat. But the fuel mix is different. High-aerobic activities like running can burn an 80/20 blend of carbs to fat; the ratio varies widely for individuals, but in general your body depends on faster-processing carbs to sustain higher speeds. Backpackers, on the other hand, cruise along with about a 45/55 mix, our muscles consuming fuel at a rate more suited to slow-burning fats. In one study, researchers at the University of Birmingham found that maximal fat oxidation—28 grams an hour—is achieved, on average, at 62 percent of max heart rate. For optimum intensity, the university’s Asker Jeukendrup, professor of exercise metabolism, advises, “You should be able to talk without much effort.” Backpackers routinely operate in precisely this zone. Most of us go hiking for the scenery, of course, but when you run the numbers, the weight-loss benefits sure look good.