The bulk of a lettuce head's weight -- almost 95 percent, according to caloriefacts.org -- comes from water, with the remainder primarily from carbohydrates. Lettuce also contains nominal amounts of protein and fat, although these nutrient levels are so small they do not contribute greatly to your nutrition. However, even the smallest quantity of a food's nutrients undergoes digestion, the process of breaking food down into small, absorbable pieces, once you eat the food.
Protein comprises less than 2 percent of the weight of green leaf lettuce. When you consume lettuce, the digestion process for its protein content begins in your stomach. Here, the high acidity of your gastric juices unfolds the protein structure, and an enzyme called pepsin begins to break the protein molecules into smaller fragments known as peptides. As the peptides travel to your small intestine, they meet with digestive enzymes secreted from both your pancreas and the walls of your small intestine. These enzymes continue to break down the peptides to their individual amino acids. At this point, the digestion process stops, and the process of your small intestine absorbing the amino acids begins.
After water, carbohydrate is the most abundant nutrient in green leaf lettuce, although it makes up less than 3 percent of the lettuce's weight. About half of the lettuce's carbohydrate is in the form of dietary fiber. As you chew lettuce, salivary enzymes begin the process of breaking down its digestible carbohydrates into smaller sugar units. This process continues with the help of pancreatic enzymes once the partially digested lettuce carbohydrates reach your small intestine. When the carbohydrates are fully digested into glucose molecules, your small intestine then absorbs the glucose. The indigestible fiber portion of the lettuce continues on to your large intestine, where you eventually excrete it in your feces.
Fat is the least abundant macronutrient in green leaf lettuce, comprising less than 1 percent by weight. The small amount of fat travels to your small intestine, where a specific digestive enzyme, secreted by your pancreas, breaks down the triglyceride, or fat molecule, into its component fatty acids and glycerol. Your small intestine then absorbs the fatty acids and glycerol and moves them into your bloodstream.
Water, Vitamins and Minerals
In addition to small quantities of protein, carbohydrate and fat, green leaf lettuce contains water, vitamins and minerals. However, these nutrients do not undergo digestion, as they are able to be absorbed in their existing state. Water travels to your large intestine for absorption, while vitamins and minerals tag on to other bits of digested food for absorption in your small intestine.