Addicted to exercise?

"Exercise can be as addictive as heroin,” warned the Daily Mail. Exercise “junkies” experience the same sort of withdrawal symptoms as heroin addicts when they try to stop exercising, the Daily Mail 's article on exercise says.

The news story is based on research in rats, some of which were given an exercise wheel. The rats were injected with a drug called naloxone, which blocks brain activity that is usually associated with opioid (e.g. morphine) use. Rats that were more active had greater withdrawal symptoms after being injected with naloxone than rats that were inactive.

It is plausible that the improved cardiovascular health, strength, flexibility and general wellbeing associated with exercise can be addictive and may encourage people to continue exercising. Some studies show that opiate-mimicking chemicals are released in the brains of rats and humans during endurance exercise.

However, until research in humans is carried out, the application of these findings to real life is limited. The balance of benefits and harms with exercise compared to heroin is too complex to be summarised by the simple headlines that the newspapers have used.

Where did the story come from?

This research into exercise was carried out by Robin B Kanarek and colleagues from Tufts University, Massachusetts, USA. Funding was provided by the National Institute for Drug Abuse. The study was published in the (peer-reviewed) medical journal Behavioural Neuroscience .

What kind of scientific study was this?

The aim of this animal study was to investigate whether exercise is addictive. It involved 44 female rats, half of which were housed in a standard cage while the other half were housed in cages with activity wheels. The rats’ food and water intake and revolutions on the wheel were measured for a week. In the second week, access to food was restricted to only one hour a day for half of the rats in each group, while the rest of the rats were able to feed continuously.

Previous studies have found that calorie-restricted rats given running wheels increase their time on the wheel and reduce their food intake. The weight loss associated with this behaviour is called activity-based anorexia. Conversely, calorie-restricted rats in standard cages tend to adapt to their new feeding schedules, eating more when they can and eventually gaining weight.

In this study, once the bodyweight of the active rats had reduced to 80% of their weight at the beginning of the study, the researchers tested for withdrawal symptoms. They did this by injecting the rats with naloxone, which blocks activity in the brain usually activated by opioid use (naloxone is used in humans to reverse the symptoms of opioid overdose). The rats were then observed for one hour for symptoms of weight loss, shaking, teeth chattering, escape attempts, abnormal posture, salivation and diarrhoea. The withdrawal symptoms were given an overall score.

Withdrawal scores were compared between the four groups. The experiments were repeated with male rats to see whether there were gender differences in the response to exercise.

What were the results of the study?

Food intake prior to diet restriction did not differ between the active and inactive rats and their body weights were similar. Following food restriction, both active and inactive rats lost a similar amount of weight. However, the food-restricted rats were more active than those who had normal food availability.

Withdrawal scores differed between the groups. Food-restricted active rats had significantly greater withdrawal scores than rats in all other groups. Additionally, the number of wheel turns made by these rats seemed to be related to their withdrawal scores. The group with the second highest withdrawal symptoms were the active rats that were not food-restricted.

Similar results were seen in the experiments on male rats.

What interpretations did the researchers draw from these results?

The authors say that their findings support the theory that exercise induces opioid-like substances in the body that act in a similar way to chronic administration of opiate drugs.

What does the NHS Knowledge Service make of this study?

This animal study assessed the strength of an “induced exercise addiction” in rats. While it is plausible that the improved cardiovascular health, strength, flexibility and general wellbeing associated with exercise may be ‘addictive’ and may encourage people to continue exercising, until human studies have demonstrated this link more convincingly it is difficult to know how these findings can be applied to real life.

Some studies show that opiate-mimicking chemicals are released in the brains of rats and humans during endurance exercise, but whether the ‘addiction’ is anything like an opiate addiction is not yet known. The benefits and harms of exercise are too different to those of heroin to make a comparison. Simple headlines that suggest that exercise is as addictive as heroin over-simplify the issue and aren’t really supported by the findings of this preliminary study.