This post is not about dirtbagging per se, but the question of how to eat cheaply and effectively during exercise may be of interest to the same group of people.
While running or hiking, the average person consumes about 1 kcal/kg/km (0.73 kcal/lb/mi), or about 100 kcal per mile, in addition to his or her base metabolic rate (generally ~2000 kcal/day). In addition to energy, exercise also requires small amounts of electrolytes like sodium, calcium, and magnesium, but these are easy to get from salt tablets or salty energy drinks — energy is the key.
This energy can come from one of three sources: (1) liver and muscle glycogen, (2) fat metabolism, and (3) carbohydrate metabolism. First, the typical athlete can store 2000 to 2500 kcal of glycogen, enough for about 20 miles, and metabolize it as quickly as necessary. Glycogen stores are replenished most effectively by eating a carbohydrate-rich meal within an hour exercising. Second, the average athlete can metabolize body fat at roughly 300 kcal/hr, and has an essentially unlimited supply — at 3500 kcal/lb, even an elite runner with 5 percent body fat has enough fat to last well over 100 miles. Finally, one can metabolize about 300 kcal/hr of sugars when ingesting a suitable mixture of simple carbohydrates. Protein and fat metabolism are not a significant factor during exercise.
Given these requirements, what can we say about effective eating at various distances and intensities?
While running at marathon pace or faster, glycogen will be the main energy source — you can’t get energy fast enough from sugar and body fat. Some sugar and fat metabolism is necessary in a marathon, but it’s tough to digest food while running at race pace. So while they are expensive (100 kcal/$), energy drinks and gels are the way to go. They should be consumed in small, frequent doses to avoid overwhelming the digestive system.
At ultra-marathon pace (6 MPH), it becomes both possible and necessary to keep up with one’s energy needs without significant glycogen metabolism. The key is to eat roughly 300 kcal/hr of sugars, again in small doses to avoid creating a big lump of undigested food in your stomach. At this intensity it is possible to digest more normal food, which saves a lot of money. My favorite food for this purpose is pop-tarts (1000 kcal/$, 3.8 kcal/g), because they are cheap, light, and contain a variety of sugars. They’re much better than fancy energy bars (250 kcal/$, 3.4 kcal/g). Eating one tart every 40-45 minutes provides enough energy without being overwhelming. Pop-tarts are low on electrolytes, though, so you should supplement them with something.
Granola bars are somewhat higher fat, but also fairly cheap (600 kcal/$) and sugary enough. Trail mix usually has higher energy density, but most trail mix has nearly 50% of its calories from fat, so it’s useless at this pace.
I don’t have much personal experience here, but have read about and talked with through-hikers. On a longer, multi-day hike (3 MPH) one has more options. A typical Pacific Crest Trail (PCT) through-hiker may plan to pack 4000 kcal/day. Since minimizing pack weight is key and resupplies can waste a whole day, energy density is crucial. Since fats have the highest energy density (9 kcal/g), PCT hikers often carry olive oil (8.9 kcal/g), peanut butter (5.9 kcal/g), and other high-fat foods, along with a dried carbohydrate like couscous or oats (3.9 kcal/g). This makes it possible to survive on less than 2 lbs/day.