Although carbohydrate is the most important macronutrient for runners training for marathons and half marathons, minimum levels of fat and protein are required for optimal training as well. These standards are more easily met.
Running increases protein needs above the levels needed by sedentary individuals in two ways. First, protein supplies about 5 percent of the energy the muscles use during running. Second, running damages muscle proteins, which must then be replaced during the post-workout recovery period. The sum effect of these processes on protein needs, however, is small. There is broad agreement among experts on protein metabolism that 1 g of protein per kilogram of body weight daily is sufficient for most endurance athletes, whereas 1.6 g/kg is the most needed by any endurance athlete. The average American adult takes in approximately 1.3 g of protein per kilogram of body weight daily. Contrary to popular belief, vegetarians and vegans get about the same amount of protein as omnivores, hence more than enough to support marathon or half-marathon training.
Minimum fat intake requirements are not well established for endurance athletes. The American College of Sports Medicine recommends that athletes get at least 20 percent of their daily calories from fat. Some research suggests that lower fat intake levels are associated with reduced performance and increased injury risk, at least in female runners, but this appears to be the case not because fat intake itself is inadequate but rather because total energy intake is too low.
More important than the total amount of fat in the diet is the amount of omega-3 fats, which are classified as essential nutrients because they are the only fats the body cannot manufacture for itself. Most Americans do not consume enough omega-3s, which are found in high concentrations in only a handful of foods, including salmon, halibut, tuna, almonds, walnuts, kale, and spinach. For this reason I recommend that all runners consume an omega-3 supplement such as fish oil or flaxseed oil daily. Studies have linked such supplements to reduced muscle tissue inflammation and oxidative stress after exercise. Don’t think of these products as an excuse not to eat omega-3-rich real foods, however. No supplement ever truly substitutes for natural whole foods.
Theoretically, you could also meet your daily carbohydrate needs with a diet that’s full of low-quality processed foods such as snack chips and low in natural, whole foods such as vegetables. Should you? Of course not. As I’ve suggested, meeting your carbohydrate needs so you can get the most out of your training is just one of two critical components of the proper everyday diet for marathon and half-marathon training. The second rule of my Two-Rule Diet for runners is to maximize your overall diet quality so you can shed excess body fat and get down to your optimal racing weight.