Did you just eat lunch a few minutes ago, and now you feel like you're going to pass out? You're not alone. The internet is full of sleepy people (myself included) who report overpowering drowsiness and fatigue after eating. Unfortunately, the internet is also full of misleading information on this topic that may evoke frustration or even anxiety. In this post, I will propose some explanations for this problem and some possible ways to alleviate it.
We're all familiar with the image of Uncle Ted sprawled on the couch after Thanksgiving dinner in a "food coma." Indeed, meals tend to have a sleep-inducing effect, and at least part of it seems to be due to increases in the neurotransmitter serotonin after eating (or "postprandially"). It is a common misconception that the high tryptophan concentration in turkey is what leads to the serotonin spike, since the amino acid tryptophan is the precursor to serotonin. Although it is true that turkey is rich in tryptophan, the rise in serotonin is actually a consequence of the insulin that is released in response to carbohydrate consumption. Insulin secretion decreases the amount of glucose and most amino acids in the blood, but it actually raises plasma tryptophan levels and increases the concentration of serotonin in the brain (Fernstrom and Wurtman 1971). Therefore, the foods that put you to sleep at Thanksgiving are more likely to be the potatoes, stuffing, gravy, and cherry pie rather than the turkey.
Thanksgiving dinner is, of course, a very extreme example of this phenomenon, and the reality is that many people experience this sleepiness after eating normal-sized meals. Nevertheless, brain concentrations of serotonin rise whenever you eat carbohydrates, so this effect could still explain postprandial drowsiness that occurs on a day-to-day basis. It is natural to wonder, though, why can some people eat a peanut butter and jelly sandwich for lunch, wash it down with a half-liter of Coke, and feel fine, whereas others (ahem) fall asleep after eating a side of pasta.
One factor that probably determines your reaction to carbohydrates is your sleep debt. Like it or not, your body needs a certain amount of sleep in order to function properly. Rats that undergo total sleep deprivation experience weight loss, dysregulation of body temperature, hormonal imbalances, and ultimately death (Rechtschaffen and Bergmann 1995), so clearly it is important to get enough sleep to keep yourself healthy. When you don't get enough shut-eye, you accumulate a sleep debt, and your body will make sure that the debt is repaid. When you are sitting through your boring political science class and nodding off, for example, that is a clear sign of sleep debt. Incidentally, falling asleep after eating is also an indication of sleep debt (see Dr. William Dement's Sleepless at Stanford page.) The first step that anyone with postprandial fatigue should take is to ensure adequate sleep over the course of a few weeks. (That is, don't sleep for 12 hours one day, eat a loaf of bread and still feel tired afterwards, and decide that sleep debt isn't the problem.) By "adequate," I mean that you should be able to wake up without your alarm clock. Personally, after I made getting enough sleep a priority over my other obligations, my postprandial fatigue improved tremendously. If sleeping more does not improve the situation, you might want to consider if you have a sleep disorder. Fourteen hours of interrupted sleep (e.g. sleep apnea, RLS) is just as bad as a curtailed period of uninterrupted sleep, so that could certainly contribute to daytime sleepiness, including after eating.
While sleep debt is probably the culprit in many cases of postprandial fatigue, there are other physiological conditions that can cause this problem. For example, patients with metabolic syndrome and type 2 diabetes may experience fatigue after eating because their bodies do not respond properly to insulin secretion. As a result, there are wild fluctuations in blood sugar, which often leads to a feeling of exhaustion. It is interesting to note that sleep deprivation, both short-term and long-term, has also been shown to cause insulin resistance in healthy individuals (Knutson et al. 2007). By the same token, metabolic syndrome and obesity are also associated with sleep apnea, so individuals with metabolic syndrome or diabetes should aim to consistently get adequate sleep. Keeping in mind that sleep deprivation can decrease insulin sensitivity, I have had success with eating a low-carbohydrate diet for alleviating postprandial fatigue. For example, eating a salad with grilled chicken, lettuce, and tomatoes for lunch is much less draining than eating a sandwich and pasta salad.
Other causes of fatigue after eating could be anything that causes fatigue in general, such as anemia or hypothyroidism. Individuals with celiac disease could also conceivably experience fatigue after eating gluten, since doing so triggers an inflammatory response in the small intestine in such patients. It is important to stress that this explanation is only for people that have received a confirmed diagnosis of celiac disease by a physician (e.g. anti-tTg antibodies, genetics testing, biopsy). Unfortunately, the internet is replete with assertions that sleepiness after eating indicates a food allergy (for example, this brochure from the "IBS Treatment Center"). While such an explanation cannot be ruled out completely, it is much more likely that gluten-containing foods cause fatigue because they are typically highly refined carbohydrates. Gluten is found in bread, pasta, pizza, cookies, and so on, all of which have a high glycemic index and cause rapid fluctuations in blood sugar. Since people feel tired after eating these foods, however, it is natural to blame subsequent fatigue on an intolerance to the food, which is very speculative. True food allergies typically produce an anaphylactic response characterized by systemic mast cell degranulation that is mediated by IgE antibodies (i.e. you would probably be well aware if you are having an allergic reaction to food). The food allergy skeptics claim that there are other "hidden" food allergies that do not produce overt symptoms, yet apparently wreak havoc on your intestines.
Again, such a reaction is not impossible, but to blame sleepiness after eating wheat-containing foods on an "allergy" seems to be completely unfounded. Furthermore, it causes undue anxiety because you will most likely start voluntarily avoiding foods and blaming them for non-specific symptoms. Eventually, you might end up conditioning yourself to respond aversively to a food, even if you are completely tolerant of it, which can lead to nutritional deficiencies and a lower quality of life. I know this because I am saying this from experience. It was getting to the point where I was spending all of my free time reading about "food allergies" and worrying about them. I know that blaming nonspecific symptoms like GI problems and fatigue on an "allergy" sounds tempting, especially when there are so many claims that eliminating "offending" foods will improve symptoms. Do yourself a favor and stop Googling your symptoms and being misinformed by off-beat web sites, because life is too short to be wasted on such meaningless activities.
So if you are feeling sluggish after eating, particularly carbohydrates, try getting more high-quality sleep and exercise regularly. Sleep and exercise will both improve insulin sensitivity, and getting enough sleep will make you less sensitive to the serotonin surge that is caused by carbohydrate ingestion. Also cut back on the amount of refined carbohydrates that you consume and replace them with high protein and low glycemic index foods. And finally, get off of your computer and go do something enjoyable!