Digestion 101

It's 5 a.m. and you're getting ready for your favorite Spin class. Wonder what to eat, if anything, before heading to the gym? Or have you ever developed stride-stopping stomach pains during a long run?

Most of us have suffered from some type of digestive problem. And, unfortunately, women are unusually susceptible to regular GI distress. But by learning how your digestive system works, you can solve many of your belly woes and fuel your body for peak athletic performance and overall health.

The ins and outs

Imagine you've just woken up to the aroma of pancakes and bacon. Even before you get out of bed, enzyme-rich saliva begins to form in your mouth, which will soften and start to break down your food.

After you take a bite and swallow, food passes through the esophagus to your stomach. Acting like a mixer, the stomach breaks food into smaller pieces through muscle contractions and adds gastric juices and enzymes to convert food into absorbable nutrients and energy. Once liquefied, food moves, a teaspoon at a time, out of the stomach and into the small intestine. The amount of time it takes for the stomach to empty depends on the type of food being digested and the amount of fluid in it. The more fluid, the faster the stomach empties.

The 22-foot-long small intestine does the bulk of the digestive work. It takes one to four hours for the small intestine to break food down into nutrients -- primarily amino acids, glucose and fatty acids -- and absorb it into the blood.

All the indigestibles left over in the small intestine are passed to the body's waste-processing plant, the five-foot-long large intestine. Here the remaining amount of water is absorbed and waste hardens. Food can stay in the large intestine from hours to days. Finally, stool passes through the colon and is excreted from the body.

It takes anywhere from 18 to 72 hours for food to be processed through the entire digestive system. (The process burns up to 200 calories a day.) Thus, a "normal" frequency of bowel movements can range from three times daily to once every three days or more.

Good foods

So what foods should you eat for optimal digestive health? For starters, women should strive for 21 to 25 grams of fiber a day. Good sources include fresh fruits and vegetables, beans and whole grains. The indigestible fiber slows the digestive system, allowing the body to absorb more nutrients. Eating roughage, as it's commonly called, has many benefits, including lowering the risk of heart disease, diabetes and digestive problems like irritable bowel syndrome and constipation.

It takes about two hours for fiber to leave the stomach and move into the small intestine, so avoid eating large amounts before working out. Also, add more fiber to your diet gradually, over a few days or even weeks, to give your body a chance to adjust, or you may suffer bloating and gas.

Eat yogurt regularly. Cultured yogurt -- look for the "live and active cultures" label -- contains beneficial bacteria that prevent the growth of fungal organisms in the digestive system that can cause bloating and gas. These "friendly" bacteria also help with healthy bowel function.

Choose whole foods instead of heavily processed items like white bread or potato chips. Whole foods generally provide more nutrients and fiber and are more easily digested.

And in general, aim to eat a well-balanced diet with about 45 to 65 percent of your calories coming from complex carbohydrates, 20 to 30 percent from fats (with an emphasis on healthy sources like nuts or olive oil) and 15 to 35 percent from protein.

Carbs provide energy quickly because they are the fastest type of nutrient to be broken down and absorbed by your digestive system. While carbs start to be digested as soon as you put them in your mouth by saliva, the digestion of protein doesn't begin until it reaches the stomach, thus slowing the process. Fats stay in your stomach the longest -- more than four hours -- and are the slowest type of nutrient to move through the system.

Digestion and exercise

Here's another perk of your workouts: better digestion. Regular exercise stimulates intestinal muscles to contract, which causes them to push more food through the system. Athletes generally have faster digestion than their sedentary counterparts partly because they have increased blood flow to the gut.

So, how do you fuel for workouts? In general, you should eat a carbohydrate-rich meal of 400 to 500 calories about three hours before working out. However, for morning workouts, you won't want to down a bowl of spaghetti at 4 a.m., so eat a small snack (100 to 300 calories) of an easily digestible carb, like a slice of bread or a banana about 30 minutes to an hour before exercise.

Fueling during your workout is more of a challenge. For sessions lasting longer than an hour, you'll need to replenish the energy you're burning through. Since solid food is not an option for many people, sports drinks and gels are usually easy-to-process energy providers. But everyone tolerates gels and drinks differently. If your body doesn't do well with them, try a few Cheerios or raisins.

During workouts, consume less than 60 grams of carbohydrate per hour (about 250 calories) to prevent GI problems. Drinking large volumes of fluid (16 ounces or more) helps to speed gastric emptying but can also cause cramping, especially if consumed all at once. Instead, try drinking small amounts of fluid (about six ounces) every 15 to 20 minutes. Bottom line: Experiment to find what works best for you during training and stick to it during competition.

Common digestive problems

Heartburn, constipation, diarrhea -- the list goes on. But many GI problems can be avoided or minimized through exercise and diet changes. Reflux, or heartburn, results from stomach acids being pushed up into the esophagus. Non-medical treatment includes avoiding fatty foods, citrus fruits, caffeine, alcohol and other foods that make heartburn worse, and engaging in exercise. Constipation, which causes infrequent bowel movements, often improves by eating more fiber.

Women are particularly susceptible to irritable bowel syndrome. IBS affects about 10 to 20 percent of Americans, the majority -- women ages 30 to 50. Caused by intestinal muscle spasms, IBS can lead to symptoms including recurrent abdominal pain, bloating, constipation and diarrhea. Symptoms are often worsened by large meals, some medicines, certain foods, caffeine and stress. The treatment for IBS varies; for many people minimizing stress is an important part of a comprehensive plan.

And what's one of the best ways to reduce stress? Exercise.

A registered dietitian with a master's in public health, Natalie Digate Muth is a medical student at the University of North Carolina-Chapel Hill and a master trainer for the American Council on Exercise. Reach her at Natalie_Muth@med.unc.edu.

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