Consider the choice between walking a mile and driving a mile. Walking a mile burns about 200 additional calories, which need to be replaced. McKenzie then looks back into the food supply chain. Only about 13% of the energy used in the production and distribution of food actually ends up as part of the calories that are actually in food. In addition, about one-third to one-half of the calories in the food that is produced are lost somewhere in the chain of production. McKenzie then adds:
Moreover, the human body is also not very efficient at converting the potential energy in the food it consumes into useful work: Only about 15 percent of the potential energy in food eaten goes into activities such as walking, as well as maintaining all bodily functions. This means that the energy that the human body actually converts into work is meager percentage-wise—something on the order of 1.3 percent of the fossil fuel energy that is used along the entire length of the food-supply chain. ...
Derek Dunn-Rankin, a professor of engineering at the University of California, Irvine and an avid environmentalist, computes that a 180-pound person walking one mile to and from work at a pace of two miles per hour will burn 200 calories above the 2,000 calories burned each day to maintain the body's basic metabolism. However, the production of those 200 calories in food takes fifteen to twenty times as much energy in the form of fossil fuels. This means that driving a high fuel economy car (40 miles per gallon) will use, in fossil fuel energy, only about two-thirds to one half the energy that the person uses in replacing the calories expended on walks. (Heavier walkers use even more energy when they walk and when they replace the greater calories they expend in moving their weight.) Energy use and pollution do not have a one-to-one correspondence, which causes Dunn-Rankin to conclude, "My bottom line would be that walking can be 1.5 to 2 times more polluting than driving (if you use a high mileage car). If you use a monster car, you are better off walking always."Of course, this conclusion is a broad average. If you drive an especially huge and polluting car, then walking will be more environmentally friendly than driving. My guess is that if the calories you eat don't involve meat, or are less processed, then refueling from your walk will have a smaller environmental effect. But with those reservations duly noted, by all means walk that mile for your own personal health, instead of driving. But be aware that if you are the sort of person who drives a car with high fuel efficiency and who eats a full range of supermarket and restaurant food, then a decision to walk when running your errands or visiting the neighborhood may be putting your personal health ahead of a cleaner environment.