Countdown to a Food Coma

You have seconds on turkey and thirds on stuffing. The top button is unfastened to make room for the pumpkin pie. It is time to step away from the table.

Will eating a heavy meal make me sleepy? WSJ’s Christina Tsuei takes a look at the science behind a food coma in the latest installment of “Is It True?”

This post-meal recovery period is being studied by scientists who are increasingly finding that what happens in the body after eating a big meal doesn’t just bring on sleepiness, commonly known as food coma. It can also increase the risk of later health problems.

Everybody absorbs fats, sugars and other nutrients differently. These variations can provide clues about a person’s risk for common medical conditions, including heart disease, stroke and diabetes, research shows. Even in healthy people, cells that line the blood vessels temporarily function less efficiently after a person eats a high-fat meal.

Researchers also are studying strategies for reducing risks in the period immediately after a meal, known as the postprandial phase. While going for a walk after eating might help digestion, for example, recent studies suggest that exercising 12 or more hours before the meal can prevent one of the most damaging effects—a post-meal spike in a type of fat called triglycerides.

One of the biggest tasks for the body after eating is to deal with fats in the blood. Cholesterol, particularly LDL, or “bad” cholesterol, infiltrates the walls of the arteries and forms plaques, which can block blood flow or eventually rupture, leading to heart attack and stroke. The condition is known as atherosclerosis.

Triglycerides, which typically peak after a big meal, are present in food and are also converted by the body from other nutrients, like carbohydrates. Triglycerides are particularly problematic because they are so good at penetrating the arterial wall, says Borge Nordestgaard, chief physician in clinical biochemistry at Denmark’s Copenhagen University Hospital.

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Dr. Nordestgaard and his team tracked about 14,000 Danish adults for almost 30 years. Women who had elevated triglycerides—440 milligrams per deciliter, compared with a normal range of 90mg or below—had a 17-fold increase in the risk of heart attack; men had a five-fold increase. The study was published in 2007 in the Journal of the American Medical Association.

In a follow-up study, also published in JAMA, the research team found that testing for elevated triglycerides after eating was a better predictor of future heart attack than measurements taken while a person was fasting, which is the typical method during a checkup. Triglyceride counts after eating also were a more accurate predictor of stroke in women than were cholesterol measurements, according to a study published this year in Annals of Neurology.

Spurred in part by the research, Denmark has shifted its clinical practice. Lipid tests are now typically given to patients who aren’t fasting to screen for health risks. If any lipids are elevated, patients then are also screened in a fasting state, Dr. Nordestgaard says.

Light exercise like a slow walk, done continuously for 30 minutes or more, appears to reduce the peak in triglycerides that occurs after eating a meal some 12 to 16 hours later, according to research led by Peter Grandjean, director for the Center for Healthy Living at Baylor University in Waco, Texas.

It’s unclear exactly why there is a delay, but exercise induces a number of cellular responses that require different amounts of time before taking effect, Dr. Grandjean says.

The findings were reported in 2008 in the American Journal of Clinical Nutrition. Other research has suggested that the benefits of exercise on fat processing can last as long as 48 hours.

The ideal is to be consistently active. But if people are more sedentary and want to time their exercise, it is best to take that long walk half a day before a big meal, Dr. Grandjean says.

“Those people wishing to start an exercise regimen can see beneficial effects on postprandial lipids with just one session,” Dr. Grandjean says.

Some people take niacin to lower triglyceride levels. This seems to work through a different process than does exercise. People who are taking the medication also should exercise to benefit from both interventions, Dr. Grandjean says.

Statins, which primarily act to reduce cholesterol, not triglycerides, can have side effects that affect muscle. People who are on statins should be careful when exercising, medical experts say.

High levels of triglycerides also could be an early warning of diabetes. People who are insulin resistant, a precursor to diabetes, also typically have elevated triglyceride levels, in the liver and muscle tissues and in the blood after eating.

Gerald Shulman, a professor of medicine and cellular and molecular physiology at Yale University, and his team found that insulin resistance appears to start in muscle. If the sugar isn’t used by the muscle, it goes to the liver where it is converted to fat, which is why triglycerides increase in the blood, according to a report published in 2007 in the Proceedings of the National Academy of Science, or PNAS.

Even in healthy people, eating a fatty meal can impair the proper functioning of cells that line the blood vessels, potentially making them more susceptible to atherosclerosis, the condition leading to heart attack or stroke, according to research by Michio Shimabukuro, a professor in cardio-diabetes medicine at the University of Tokushima in Okinawa, Japan. The changes to the blood vessel appear specifically triggered by fat content; a high-sugar meal doesn’t appear to have the same impact, says Dr. Shimabukuro.

Dr. Shimabukuro says reducing the amount of fat in meals can minimize the negative impact on the blood-vessel cells. Fats from fish and nuts, commonly considered healthier sources of nutrients, don’t appear to cause the same spike in triglycerides as other kinds of fats, and may even help bring down triglyceride levels, he says.

Scientists also expect that eating smaller amounts of food more frequently is better than eating large meals, although research in this area is limited.

Dr. Shulman, also an investigator for the Howard Hughes Medical Institute at Yale, has found that after just six weeks of using a stair-climbing machine, people were essentially able to reverse the effect of insulin resistance.

Even one 45-minute bout of exercise can improve muscles’ ability to use sugar as fuel, according to his 1996 study in the New England Journal of Medicine. In a study published in PNAS this year, Dr. Shulman also showed that the liver’s conversion of sugar to fat slowed after one exercise session.

Nap Time? Mysteries of Food Coma
The urge to nap after a heavy meal like the Thanksgiving feast is commonly referred to as food coma. But what happens in the body to cause this post-feast dip—known as postprandial somnolence in the medical community—isn’t clear, according to nutrition and sleep experts.

Hal Mayforth
One fact that isn’t under debate: Unlike popular belief, sleepiness isn’t caused by a lack of blood supply to the brain as blood rushes from the head to the stomach to aid with digestion.

The digestive tract, essentially one long muscle from end to end, does need additional blood flow when it begins to move and contract to process food, according to Lona Sandon, a clinical nutrition professor at the University of Texas-Southwestern in Dallas. But the blood tends to come from skin and skeletal muscle in our limbs, which tend not to be in use much anyway when we are sitting down and eating. The brain, on the other hand, is the most important organ in the body and therefore is protected and has adequate blood supply unless under duress, like with a head injury, she says.

The after-meal dragging feeling instead could be associated with changes in certain hormones induced by the food, says Ms. Sandon, who is also a spokeswoman for the American Dietetic Association. Or it could be as simple as the feeling of being weighted down by a large amount of food sitting in our stomachs, she says..

“When you just have this heaviness and this sensation of this bulk in your stomach, you just don’t want to move,” she says. “If you have a smaller meal where you feel satisfied but not stuffed, you don’t get sleepiness.”

Another possibility is that food coma isn’t related to food or digestion at all. Rather, it’s because our bodies are wired to feel tired during the afternoons and evenings when there is a natural dip in the circadian rhythm, the internal clock that regulates our sleeping and waking hours, says Robert Basner, director of Columbia University Medical Center’s sleep-studies program.

“I know people seem to think it’s due to a meal and it may have something to do with it, but usually people get sleepy around that time,” Dr. Basner says.

A person with a normal sleep-wake cycle will typically feel sluggish around 1 p.m. to 3 p.m. and again just after midnight, he says. Sitting around after a satisfying meal may make it that much easier to doze off.

Derek Chong, a professor of clinical neurology at Columbia, agrees that a person’s circadian rhythm can play a major role in meal-time lethargy. But he says food can push us over the edge and into a full-on siesta session. Food activates our parasympathetic nervous system, which regulates our resting and relaxation responses, as opposed to the sympathetic nervous system, which regulates our flight or fight responses, Dr. Chong says.

Tryptophan, of which turkey is a source, may predispose us to sleepiness, but we rarely consume enough for it to have a soporific effect, says Dr. Basner. Also, people usually ingest so many carbohydrates during the same meal, which are processed first by the brain for fuel. That makes it unlikely that tryptophan, which helps make proteins, would actually get to the brain, according to UT-Southwestern’s Ms. Sandon.

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