Short sprints and blood sugar

“Six minutes of exercise a week is enough to cut dramatically the risk of heart disease and diabetes”, reported the Daily Mail. It said that a researcher had made the claim after carrying out a study where men did short bursts of intense activity several times a week. The study apparently greatly improved the men’s ability to regulate their blood sugar.

This study involved only 16 healthy young men doing brief, intense exercise over two weeks. Although there were signs of improved metabolism after this time, the study’s small size and short length means it is premature to say this type of exercise boosts metabolism, reduces metabolic risk or improves general health. Claims that intense exercise reduces the risk of heart disease and diabetes are speculation. More research in a larger, more diverse group over a longer time is needed.

Where did the story come from?

John A. Babraj, Niels BJ Vollaard and colleagues from the Heriot-Watt University Edinburgh and Stockholm University, Sweden, carried out this research. No sources of funding are reported. The study was published in the (peer-reviewed) medical journal BMC Endocrine Disorders.

What kind of scientific study was this?

Although it is known that regular physical activity reduces the risk of cardiovascular disease and diabetes, the type, frequency and intensity of exercise needed for optimal benefit is not clear. This experimental study investigated whether low volume, high-intensity interval training improved insulin action and control of blood glucose, in addition to improving aerobic function.

Large amounts of aerobic exercise have been demonstrated to reduce the risk of cardiovascular and metabolic diseases such as diabetes. This study investigated whether smaller amounts of exercise at a higher intensity would have the same benefit.

The researchers speculated that such an exercise regime would be a much more time-efficient way to exercise than regular, more prolonged exercise. To investigate this, the researchers recruited 16 healthy men with an average age 21 to the study.

All the men were of normal body mass and could be classed as either active or sedentary. Over a two-week period, they were asked to perform six 15-minute sessions of exercise. There were typically  one or two rest days between each session. Each high-intensity session involved between four and six repetitions of cycling for 30-seconds against a resistance equivalent to 7.5% of body weight, with four minutes of rest between each repetition.

A glucose tolerance test was carried out before and after the two-week period. This assessed blood glucose and insulin levels after consuming 75mg of glucose. The researchers also looked for changes in levels of non-essential fatty acids (NEFA) in the blood, as increased levels of certain fatty acids are associated with the “pre-diabetes” metabolic syndrome.

The participants’ aerobic performance was also assessed. This involved an intense incremental cycling trial to determine their maximum energy expenditure and oxygen consumption, and a self-paced endurance cycling test in which the men had to burn off 250kJ as quickly as possible. Throughout the study, all men maintained their normal exercise and dietary habits.

An additional nine men took part in a smaller separate part of the study. Instead of exercising, these men had glucose tolerance tests to examine individual differences in glucose response.

What were the results of the study?

After two weeks of high-intensity interval training, there was no change in the men’s fasting glucose level. However, the tests of blood glucose after consuming 75mg of glucose found it to be lower 60 minutes after exercise than it was before (i.e. after training, it took less time for glucose in the blood to reduce to normal blood levels). Similar significant results were seen for both insulin and NEFA. There was also significant improvement in insulin sensitivity. The participants also demonstrated improved aerobic cycling performance after the two-week period.

What interpretations did the researchers draw from these results?

The researchers conclude that this exercise regime substantially improves insulin action in young people. They say that this novel time-efficient training system could be a strategy to reduce metabolic risk factors in young and middle-aged people who would otherwise not stick to more time-consuming exercise regimens.

What does the NHS Knowledge Service make of this study?

This study demonstrated improved metabolic response after two weeks of short interval, high-intensity exercise. However, further research incorporating longer follow-up times is needed.

Until then, it is premature to suggest that brief, intense exercise improves metabolic risk in young and middle-aged people. It is also too soon to claim, as in one news story, that “six minutes of exercise a week is enough to cut dramatically the risk of heart disease and diabetes”.

Points to note when interpreting this study and media coverage:

• The study involved only 16 men, which is a small number of participants. Replication is needed in larger samples.
• The participants were all healthy males, average 21 years, and all of healthy weight, and as such are a very specific population group. At this stage, it is not possible to say if the results would be the same in females, in older age groups, or in people who are overweight or with chronic diseases or risk factors.
• Follow-up in this study did not extend beyond two weeks. It is unclear if metabolic benefit would be sustained over a longer period of exercise or if the participants were followed-up for a longer period after the intervention had finished. In addition, adverse health effects from continued short bouts of intense exercise in the long-term, for example to joints and muscles, would need to be considered.
• Finally, based on this study’s findings, it is only speculation that intense exercise reduces the risk of heart disease or diabetes.

Regular exercise and a balanced diet are the best ways to achieve all-round health. However, each person is different and they should choose a type of exercise and level of intensity that is right for them.