A new study has linked poor fitness in middle age to smaller brain volumes almost two decades later.

The results suggest that health and lifestyle choices can have a big effect many years later, in effect shrinking the brains of ‘couch potatoes’.

The new study is online in the journal Neurology. It included 1,094 participants in the US Framingham Offspring Study. The average age was around 40, and participants didn’t have cardiovascular disease, stroke or dementia when the research began and were not taking beta-blockers.

An exercise treadmill test was done between 1979 and 1983. The volunteers exercised with increasing speed and a steeper incline until they were too exhausted to continue, or if they had reached 85% of their estimated maximum heart rate.

The effect of the test on blood pressure was also measured.

The time spent on the treadmill was used to give a score for their estimated fitness level.

Around two decades later the participants did a shorter version of the treadmill test alongside neuropsychological testing and an MRI scan to measure brain volume.

The researchers found that a lower estimated exercise capacity in middle age was linked with smaller brain volumes later in life. Lower exercise capacity in effect is associated with brain ageing.

A smaller group of the original participants who developed cardiovascular disease or began taking beta-blockers after the first treadmill test showed more signs of brain ageing when it was measured later.

Study author Dr Nicole Spartano from the Boston University School of Medicine told Medscape Medical News: "To give some perspective, during the treadmill test, people with about 17 beats per minute higher heart rate, or 14 units higher diastolic blood pressure, had smaller brains later in life, at a rate that we would say was equal to 1 year accelerated ageing.”

However, authors stress that their findings shouldn’t be interpreted to suggest that exercise is harmful, rather that exercise helped to uncover problems that may result in smaller brain volumes.

Dr Spartano says: "Our results suggest that fitness may be especially important for people with pre-hypertension or hypertension [ high blood pressure], in order to slow the brain ageing process in later life.

"But this hypothesis must be tested with an exercise intervention study.”

The authors say their work can’t yet prove a cause-and-effect between inactivity and brain volume and ageing.

The study only gave a ‘snapshot’ of middle age fitness, and didn’t check brain volumes at the start of the study.It’s not clear at this point how middle age fitness is linked to later brain volume. Evidence from other studies suggests that exercise training programmes that improve fitness may increase blood flow and oxygen delivery to the brain over the short term, says Dr Spartano.