Step one to creating a food formula that powers you to new performance peaks is to keep a journal recording how you react to what you eat. Lynn Smith, owner of Boulder, Colorado–based Source Nutrition (source-nutrition.com) and an adviser to dozens of the town's elite endurance runners, says the key to really learning what you need is to go beyond simple observations of food types, quantity, and calorie intake and take stock of four fundamental indicators: mood, mental clarity, energy, and digestion. She doesn't give rules about frequency or format of entries but says a regular routine is key, as it will allow you to recognize the often simple patterns that can hold you back. Of course, identifying a problem—"Huh, I always bonk when I have granola for breakfast"—doesn't give you an immediate remedy, but keep tinkering with your diet and paying attention and you'll soon become an expert on yourself. Meanwhile, here are a few common symptoms, along with Smith's suggestions for potential fixes.

> Bloated Belly: You could be downing too many sugars or other refined carbohydrates, which your body stores by retaining water.

> Losing focus before lunch: Add some oomph to your breakfast with more protein in the form of eggs, peanut butter, cheese, yogurt, or last night's leftover meat dish.

> The 4 p.m. Meltdown: Scarf a small, nutrient-dense snack, like a handful of walnuts and sunflower seeds.

> Hating your post-work workout: Make sure you ate properly yesterday; inadequate meals often won't affect your performance until 24 hours later.