Work up a sweat but do not overdo it. This is probably what a cardiologist routinely tells his patient who asks about exercise. For those who ever wished the doctor could be a tad more specific, a more quantifiable answer may finally be at hand.
The findings from two different sets of two studies, published last month and this month, suggest together that heart patients need and ought to exercise far less than might have been assumed necessary.
Suppose you are the sort who packs on the pounds if you exercise less. Still, you may not need to worry too much: research now shows fatter heart patients may paradoxically have lower heart risks. Let us take the set of two studies published this month first.
One is a report in Mayo Clinic Proceedings which warns heart attack survivors should indeed not over-exercise. This study tracked 2,400 heart attack survivors who had stayed physically active for 10 years. It found heart patients should exercise for no more than 30 minutes a day, running no more than about 7km every day.
Exceeding these limits doubled their risk of dying from heart attacks or strokes.
But running up to 30 minutes and 7km a day still sounds too much for heart patients - or indeed most healthy persons who might be too lazy to exercise.
The good news for such folks is that although most authorities advocate about 30 minutes of moderate exercise five times a week, the second study published this month showed much less is enough. Tracking 55,000 healthy people for up for 15 years, the study published in the Journal of the American College of Cardiology reported running slowly for just five to 10 minutes a day, doing about 1.5km each time, cut the risk of death from heart attacks or strokes by about 60 per cent.
By contrast, running 7.5km every day cut the risks by only 40 per cent. So more is not better.
Thus we may infer that running 7km every day, as the first study suggests, may already be five times too much for healthy persons - and logically for heart patients as well.
Putting all this together, heart patients should perhaps run slowly only for 10 minutes a day, doing at most just 1.5km.
But you may lament you would be so overweight by Christmas if you ran so little. That may be so, but this might not be a bad thing for the heart patient. How so?
This is where the second set of two studies, both published in Mayo Clinic Proceedings last month, comes in.
One of the two is a meta-analysis pooling data from 36 studies. It reported that heart attack rates and death rates in cardiac patients were strangely the lowest among those who were overweight, obese and even grossly obese.
Since obesity is a known risk factor in heart disease, this counter-intuitive finding was dubbed "the obesity paradox". It was postulated that non-overweight heart patients may have genes that make them slim but which also predispose them to the worst forms of heart disease. Perhaps fatter heart patients do not have such genes, which is why their survival rates are better.
But what could these genes predispose slimmer heart patients to that may account for the paradox?
Perhaps these genes give slimmer heart patients more unseen fat inside along with too little lean muscle inside. Why this should matter was suggested by the other study published last month, which looked at the death rates in 48,000 heart patients classified by weight as well as their amounts of body fat and lean mass - or muscles and bones - respectively.
One's total body mass is made up of lean mass plus body fat. To measure body fat, you just have to sit inside the BodPod, a computerised pod that uses Archimedes' Eureka principle: how much air you displace. While body fat is related to heart attacks, it is lean mass that improves survival rates. And lean mass or muscle and bones can be built up with weight training.
This study found that higher lean mass, even with higher body fat, was related to a 29 per cent reduction in heart death rates.
So how does all this explain why fatter heart patients seem to do better than slimmer ones?
Actually, a person who does not look overweight could well have lots of body fat but too little lean mass "inside".
This is now called "normal weight obesity", where any male with over 25 per cent body fat or a female with over 35 per cent body fat is called "obese", according to the World Health Organisation.
A 2008 Mayo study found 55 per cent of adults do have "normal weight obesity". It showed women with "normal weight obesity" were four times more likely to have heart disease.
That is, body fat inside that is not visible outwardly in "normal weight obesity" may suffice to get the heart in trouble.
By contrast, some who look fat could have comparatively lower heart risks if they have lots of lean mass hidden inside all that visible bulk. They are bigger in bulk so they have more body fat but they also have more lean mass.
It might be that many fat-looking cardiac patients naturally have lots of lean mass hidden inside, which protects the heart. Their excess body fat, which is visible outwardly, may have got their hearts into trouble in the first place. But they may have lots of lean mass inside, which can protect their hearts better compared with cardiac patients with "normal weight obesity" - normal body weight but high body fat with less lean mass inside.
Putting all this together, it seems to suffice if the heart patient regularly puts in 10 minutes of light aerobic exercise, such as jogging, swimming or cycling along with, say, 10 minutes of light weight training everyday.
In short, a little bit every day of some gentle exercise probably suffices for cardiac patients - and perhaps for healthy persons too.