The following clarification was printed in the Observer's For the record column, Sunday 1 March 2009
In the article below we reported on research into exercise and eating patterns by Dr David Stensel of Loughborough University. Dr Stensel has asked us to make it clear that this research is incomplete and unverifiable at this stage. The piece also referred to a book by Dr Stensel entitled Influence of Resistance and Aerobic Exercise of Hunger. This is actually an academic paper, co-authored with three others. Dr Stensel also points out that he would never describe himself - as we did - as "the country's leading expert on sport and exercise science".
The type of exercise you choose to do affects how hungry you feel afterwards, says new research that reveals how some workouts trigger cravings for healthy food such as fruit while others lead to a desire for chocolate.
According to a study by Dr David Stensel, the country's leading expert on sport and exercise science, a long swim in a cold pool can leave you hungry, wanting high-fat foods such as biscuits while running on a warm day has the opposite effect.
"Some people are concerned that exercise will make them want to eat more, but the good news is that there doesn't necessarily have to be that compensation," said Stensel, who has been awarded research funding from the British Heart Foundation. "While some high-intensity exercise, such as running in hot conditions, suppresses the ghrelin hormone, which stimulates appetite, other high-intensity exercise, such as swimming in cold water, actually increases the hormone which, in turn, increases hunger. In addition, medium - to low-intensity exercise, such as walking, has no impact on ghrelin, meaning you are as hungry after a one-hour walk as you are if you sat in a chair for the same period of time."
Stensel is investigating possible causes for these results, which are published in his new book Influence of Resistance and Aerobic Exercise on Hunger, but said one cause could be the effort the body makes to keep its core temperature stable. "When exercising, the priority of the body is to divert as much blood to the muscles as possible, which takes it away from the digestive functions," he said. "An additional cause could be that when you are hot, you need to release that heat by circulating blood to the skin, which also takes it away from the stomach and intestines. The opposite happens when you are cold."
Stensel also suggested the cravings could be psychological: "The body tends to respond to exercise so it can do it more efficiently in future. The lighter you are, the better for long-distance running, so your body will crave watery foods that lower your body temperature by rehydration while not piling on the pounds. But if you are making your body lift weights, then you will crave carb- and protein-rich foods that will bulk up your muscles. Similarly, if you are regularly swimming in cold water, your body benefits if your brain guides you towards foods that will give you a layer of protective fat."
The suppression of appetite lasts until the body's hormonal levels return to normal, said Stensel, usually between one and three hours later, depending on how long and how intense the exercise has been. But there are other, longer-lasting benefits. "What is fascinating is the apparently paradoxical fact that people don't seem to over-compensate for missed meals when their body returns to its rest state," he added. "So if you run for 90 minutes, you will burn around 1,300 calories but will not increase your food intake in the 24 hours after that exercise. In short, you burn all those calories but you don't get hungrier than you would have had you not exercised at all."
The research has been welcomed by Asker Jeukendrup, professor of exercise metabolism at the University of Birmingham. He argued that the appetite-suppressing impact of exercise was one of the biggest challenges he faced in his role as training and nutrition consultant to several top athletes. "The cyclists I train expend between 6,000 and 9,000 calories a day - the equivalent of 27 McDonald's cheeseburgers - and need to start refuelling immediately after exercise," he said. "But eating is very difficult for hours afterwards because their appetite is so severely suppressed. What also doesn't help is that they have a specific craving for high-fat, sweet foods that are hard to digest. It would be much easier for them to digest watery, sugary foods such as fruit, but that's not what they want."
Jeukendrup added "We find that when the temperatures are colder, people get hungrier and vice versa," he said. He found that post-exercise food cravings are very specific: "After a 1,500-metre run, which is quick but of extremely high intensity, they crave sweeter foods. But after lower intensity but more prolonged exercise, such as a marathon, the craving will be for savoury foods."