Eating an apple in the wintertime is a healthy habit. However, contrary to popular belief, eating an apple in the morning is not a good idea because an apple on an empty stomach could upset your tummy. Apples contain a lot of fructose. Some people may have problems digesting and absorbing fructose quickly. When fructose goes down to the large intestine without being absorbed, it can get fermented by intestinal microbes and cause gas or pain in the stomach.

Apples also contain fiber such as pectin and sugar alcohols such as sorbitol. Fiber and sugar alcohol are also not digested and absorbed in the small intestine and go down to the large intestine. Prunes (dried plums), known to be good for relieving constipation, contain a lot of sorbitol. However, when it comes to apples, fructose was found to be the main cause of indigestion. Dutch scientists proved this in 1993. They gave fructose, sorbitol, and both to children who had difficulty digesting apple juice. Then, they found that fructose was not absorbed properly in most cases.

Individuals have differences in sensitivity to fructose. Some have stomachaches even with a small amount of less than 5g, but others do not have any problems even if they take more than 50g. However, if you eat a large amount of fructose on an empty stomach at once, it is likely to cause symptoms such as gas and abdominal pain. In a 2009 clinical trial, participants consumed 50 grams of fructose, and 80 percent of them had problems with digestion and absorption.

As the name suggests, fructose is found in many fruits -- such as apples, dates, pears, grapes, cherries, and bananas. Among vegetables, cabbage, eggplant, and onions contain a lot of fructose. Sweeteners such as honey and soft drinks such as cola and cider also contain fructose in the form of high-fructose corn syrup. However, the number of people with stomach aches after drinking soft drinks is smaller than expected. This is because fructose and glucose are contained in a ratio close to 1:1. Although the mechanism is still unclear, fructose in food is better absorbed when glucose is contained in the same ratio.

People with frequent stomach aches and gas can improve symptoms if they restrain from eating foods high in fructose. However, there is no reason not to eat healthy vegetables and fruits. In most cases, symptoms improve easily just by eating with other foods or after meals. If you eat fructose-containing food after a meal, it stays in the stomach longer and gives you enough time for the digestion and absorption of fructose. If you don't have any problem eating an apple on an empty stomach in the morning, it's fine as it is. But if you have a stomach ache after having an apple in the morning, it's better to eat it after dinner. Eating an apple after dinner is the same as taking laxatives in the evening and going to bed. Eating an apple in the evening will help keep your bowel movements smooth the next morning. The way to a fun and healthy life is to learn about yourself and the food you eat.

Jeong Jae-hoon is a food writer and pharmacist. He covers a variety of subjects, including trends in food, wellness, and medications. This column was originally published in Korean in Joongang Ilbo on Dec. 7. – Ed.