Understand how your digestive system turns food into fuel and you've taken the first step toward smart eating. The process starts with simply chewing well, which adds saliva (1) and breaks food down for easier digestion. When you swallow, food travels down the esophagus (2) and into the stomach (3). There, acid and enzymes convert food into a paste-like mix, while the pyloric valve (4) prevents this acidic brew from backing up. Digested food leaves your stomach in one to four hours. Carbohydrates go first, followed by protein, then fat. These macronutrients enter the duodenum (5), the first section of your small intestine (6). Enzymes stored in the duodenum, and those supplied by the pancreas (7), complete most of the digestion process.

Food Q&A

What's the difference between whole grains and processed whole grains? Nothing, in terms of nutrition. Here's a primer. Whole grains: Seeds are intact, surrounded by an endosperm and nutrient-rich germ and bran. Processed whole grains: Seeds have been cracked, rolled, crushed, or ground and provide the same nutrition as the original whole grain. Avoid refined grains (nutrient-poor white flour) and enriched grains (same as refined but with some nutrients restored).

Bile manufactured by the liver (8) and secreted by the gallbladder (9) assists with fat digestion and absorption. The smaller molecules produced by this secondary breakdown are absorbed through the walls of the next two sections of your small intestine, the jejunum (10) and ileum (11), which supply more than 20 feet of absorptive tissue. This length is multiplied many times over by numerous folds in the intestinal lining called villi, which are themselves covered by small, fingerlike projections called microvilli and which ultimately provide more than 820 square feet of absorptive capacity—an area larger than a tennis court. From there, nutrients are shipped off to your liver for further processing, then to cells, which create and store glucose, amino acids, and fatty acids.