Post-meal fatigue is caused by a number of factors, primarily relating to volume consumed, sugar and fat content and poor digestion sometimes related to food allergies. Most people are tired after eating a large meal, as it requires significant energy to break down.
Foods high in sugar and refined carbohydrates cause a rise in blood sugar levels followed by a sudden drop, which results in fatigue. Large meals take longer to digest and divert more blood from other parts of the body to help with the process. This creates a feeling of lethargy.
However, those who are tired after every meal, large or small, high in sugars and refined carbohydrates or not, might suffer from an underlying health issue such as diabetes. Those with diabetes tend to suffer post-meal blood sugar spikes that leave them extremely tired in the short-term. These spikes also cause blurry vision and an overall unwell feeling.
In non-diabetics, the body attempts to balance blood sugar levels by secreting excess insulin. This sends tryptophan to the brain, where it is metabolized into melatonin and serotonin, neurotransmitters that help regulate sleep and create a calming effect.
Those with food allergies often become tired after eating. They also experience constipation or diarrhea, bloating and general indigestion.