A new study suggests that not every healthy person benefits from regular exercise: for a small 7% minority it may increase heart and diabetes risk factors. The researchers did not suggest this should stop people exercising but point to the importance of using this type of knowledge to personalize exercise programs.

Claude Bouchard, a professor of genetics and nutrition in the Human Genomics Laboratory at Pennington Biomedical Research Center, Baton Rouge, Louisiana, in the US, was lead author of the study, which was published online in PLoS ONE on 30 May.

Bouchard and colleagues write in their background information that public health guidelines suggest adults should do 150 minutes a week of moderate intensity physical activity, or 75 minutes a week of vigorous intensity activity.

However, it is now well established that different people respond differently to exercise in terms of cardiorespiratory fitness and cardiometabolic and diabetes risk factors.

But the question that still remains, is whether there are people for whom the effect of regular exercise on these risk factors could be harmful.

For their study, Bouchard and colleagues analyzed data from six rigorous studies that looked at the effect of exercise in a total of 1,687 adults.

These studies were the HERITAGE Family Study, the DREW Study, the INFLAME Study, and the STRRIDE Study, plus cohorts from two other studies, one from the University of Maryland and another from the University of Jyvaskyla.

They looked to see how many of the participants experienced an adverse response to exercise, which they defined as an “exercise-induced change that worsens a risk factor beyond measurement error and expected day-to-day variation”.

Bouchard and colleagues produced their own parameters for this definition by measuring resting systolic blood pressure (SBP), fasting plasma HDL-cholesterol (HDL-C), triglycerides (TG), and insulin (FI) in 60 people over a period of three weeks.

From these measures they defined an adverse response in these risk factors as: an increase of 10 mm Hg or more for SBP, 0.42 mmol/L or more for TG, or 24 pmol/L or more for FI or a decrease of 0.12 mmol/L or more for HDL-C.

Applying this definition to the results of the six studies, they found that 8.4% of participants had an adverse change in FI. This figure was 12.2% for SBP, 10.4% for TG and 13.3% for HDL-C.

They also found that the proportion of participants that experienced adverse effects in two or more risk factors was 7%.

“This subgroup should receive urgent attention,” they write.

Bouchard and colleagues found no evidence for differences between blacks and whites, or between men and women.

Also, the adverse responses were not explained by the health status of the participants, or their age, amount of exercise they did, or lack of improvement in cardiorespiratory fitness. And there was no evidence to suggest drugs were the cause of adverse responses.

“Thus, some individuals experience [adverse responses] when exposed to regular exercise, but the causes of the phenomenon are unknown at this time,” they write, concluding that:

“Adverse responses to regular exercise in cardiovascular and diabetes risk factors occur. Identifying the predictors of such unwarranted responses and how to prevent them will provide the foundation for personalized exercise prescription.”

“The challenge is now to investigate whether baseline predictors of [adverse responses] can be identified to screen individuals at risk so that they can be offered alternative approaches to modifying cardiometabolic risk factors,” they write.

Some experts are concerned the study will give ready ammunition to those people who don’t need many reasons to excuse themselves from exercise.

For example, William Haskell, emeritus professor of medicine at the Stanford Prevention Research Center told the New York Times he thinks this could be an excuse for some people to say “Oh, I must be one of those [that doesn’t benefit]”.

Written by Catharine Paddock PhD