Why can’t I eat after running, but feel very hungry after swimming?

After the running test, I was out of breath and had no appetite for half a day. But after swimming, almost everyone climbed ashore and the only thing in their mind was "I'm starving!" Both are full-body exercises and are very tiring. The two activities have completely opposite effects on appetite. Can exercise still help you lose weight?

Why am I so hungry after swimming?

Compared with running or cycling on land, swimming has many special advantages. Not only does it reduce the pressure of body weight on the muscles and bones, but it also uses the muscles of the whole body during exercise, improving cardiopulmonary function. Swimming is a good way to exercise, but the weight loss effect is obviously not as good as brisk walking or cycling [1]. When comparing the body shapes of swimmers and runners, the former are usually rounder.

Swimming consumes a lot of energy, but the reason why you don't lose weight is because you eat too much. What is so special about this sport that it increases your appetite instead of reducing it?

The most likely suspect is water. When immersed in water, energy consumption is usually higher than on land, and people eat more after exercising in water. However, when the water temperature is further differentiated, people find that only cold water increases consumption, and warm water (32-34°C) does not have a similar effect [2, 3]. As the water temperature rises, the amount of food consumed after exercise gradually decreases, and only when the water temperature is above 28°C will people stop eating more after swimming [4]. The temperature of a general swimming pool is below 28°C, so we are always so hungry after swimming.

In fact, not only water temperature, but also air temperature on land affects metabolism and appetite[5]. After doing the same exercise, the amount of food we eat will gradually decrease as the ambient temperature rises[6-8]. So whether we are in water or air is not the key, the temperature is the reason why we are prone to hunger when swimming.

Exercise to lose weight is not a lie

In addition to swimming, most sports can also have a good weight loss effect if they are performed at the right intensity and duration[9]. This is because exercise not only increases energy consumption, but also temporarily suppresses appetite, preventing us from eating too much. In this way, the output increases while the intake remains the same, and the energy gap is large, so weight loss is fast[10].

If you exercise regularly for a long time, not only will your appetite decrease immediately after exercise, but your ability to control your appetite throughout the day will also improve. After eating the same high-energy food, people who exercise regularly eat less in the next few meals than those who do not exercise. This regulation helps keep your daily dietary energy intake within the standard and prevents you from gaining weight[11].

Exercise can not only reduce food intake, but also improve the quality of diet. The temptation of high-fat and high-sugar foods to people will be reduced after exercise, but low-energy foods will not be affected. Changes in dietary choices can help us eat more nutritious and healthy foods and reduce high-energy snacks that make us fat [12, 13].

Although exercise can help lose weight and improve health by suppressing appetite, what you eat is not entirely determined by appetite[14]. Many people are not hungry after exercise, but think about how hard they worked and feel they must reward themselves with something delicious. If they always think this way, exercise will not be very useful in suppressing appetite.

How to eat less by exercising

"Which exercise is the best for losing weight?" is the first question many people consider when choosing an exercise. Considering the effect of suppressing appetite alone, aerobic exercise may be better than resistance exercise (weight lifting). Among aerobic exercises, except for skipping rope, which has a better effect in suppressing appetite, there is no obvious difference in other types of exercise [15-17].

However, the type of exercise does not have a big impact, the duration and intensity are the most important.

01 Hold on a little longer and you won’t be hungry anymore

Prolonging the exercise time appropriately can have a better effect in suppressing appetite. For example, if you extend the cycling time from 20 minutes to 40 minutes, the energy consumption during exercise will double, but you will not eat more, so it is more effective for weight loss [18, 19]. If you want to use exercise to suppress appetite, it is best to stick to moderate-intensity exercise for more than 40 minutes.

02 It’s OK to feel tired, the intensity of exercise should not be too low

Many people cannot endure such long-term exercise because they are too busy or bored. One solution is to increase the intensity. The effect and duration of exercise in suppressing appetite increase with increasing intensity [20-23]. Under the condition of the same energy consumption, half an hour of high-intensity exercise will make people eat less than one hour of low-intensity exercise[24]. If the intensity is high enough, the appetite reduction can last up to 1 to 2 days[25]. In the long run, high-intensity exercise is more effective in reducing weight than low-intensity exercise[26].

Although the duration is shorter, such a high intensity makes it difficult for some people to persist. You can also change continuous exercise to an intermittent form: previously running at a fixed speed, now turn it into a full-strength sprint, then rest for a while with jogging or walking, and then repeat the sprint and rest. As long as the total energy consumed is similar, the effect of intermittent exercise in suppressing appetite is no worse than that of continuous form, or even better (when the intensity is very high) [26, 27].

People who want to lose weight can use high-intensity exercise to reduce their appetite, and people who want to gain weight can also use low-intensity exercise to improve their appetite [28]. Compared with standing still, the rate of gastric emptying slows down during high-intensity exercise (heart rate > 160 beats/minute), but speeds up during low-intensity exercise (heart rate < 120 beats/minute). Therefore, if you can’t eat after running fast, walking will improve your appetite rather than sitting still[17].

Exercise time to suppress appetite

The debate over the best time to exercise has never stopped. If we only consider the effect of suppressing appetite, some studies have shown that exercising in the morning is worse than exercising in the afternoon[29]. However, the time of morning, noon, and evening is not as important as the relationship between exercise and meals.

Generally, exercising before a meal is more effective in reducing hunger, and eating less energy and fat, which makes it easier to lose weight and improves blood lipid levels [14, 30]. The advantage of exercising after a meal is that it maintains a stable blood sugar level, which is more beneficial for diabetic patients. In addition, everyone's gastrointestinal response to exercise is different. Some people feel uncomfortable lifting weights before a meal, while others feel stomach pain when running after a meal. Therefore, whether to exercise before or after a meal should be determined based on your exercise goals and your physical condition.

If you schedule exercise before a meal to suppress your appetite, the interval should not be too long. Food intake will be significantly suppressed within 30 minutes after exercise, the effect will weaken after 60 minutes, and it will basically disappear after 180 minutes. Not only appetite, but also our love for fat will recover over time [10, 31]. So we eat less right after exercise, and the longer the rest time, the more we eat, especially high-fat foods.

Overall, exercising close to lunchtime is most beneficial for suppressing appetite and balancing energy[14]. However, for individuals, the best time is when they can ensure regular exercise without causing discomfort, so there is no need to worry too much about the details.

With the right intensity and duration, most sports except swimming can be used to suppress appetite. With a sense of fullness, weight loss also requires controlling other lifestyle habits. In addition, the above suggestions are all aimed at reducing appetite. When the purpose of exercise is to improve physical health, mood, sleep or physical fitness, different arrangements may be required.