Choosing the best time to exercise
We all know that exercising for 30 minutes a day is a prerequisite to a healthy life, but does it matter when we do it? Well, according to recent studies, timing can make all the difference.
Dr Lygeri Dimitriou from Brunel University in London recently made headlines when he suggested that exercising in the morning carried a higher risk of infection. On top of that, Australian PhD student Leanne Redman says that women can increase weight loss and reduce tiredness by exercising at the end of their menstrual cycle.
Both studies were based on careful observation of chemicals in various athletes' bodies, although experts remain sceptical of the findings.
Dr Dimitriou noted that the chemical cortisol, which is produced naturally by our bodies and helps suppress the immune system, is higher in the morning that at any other time. This in itself is not news, but after studying 14 professional swimmers he also discovered that another chemical, IgA, was lowest in the morning. IgA is found in the mouth and nose, and helps destroy infections carried in the air.
By exercising, the level of cortisol is actually increased and, combined with a low level of IgA, he argues, is enough to place someone at an increased risk of picking up an infection, especially if they are already feeling a little under the weather.
Ms Redman's argument is also based on the fluctuation of chemicals in the body. Towards the end of a woman's menstrual cycle, the hormones oestrogen and progesterone are both at a high level, she explained. Both hormones encourage the body to use fat, rather than for example muscle, as a way of storing energy. So, by exercising at this time, a woman will see a comparatively higher loss of fat that at any other time in her cycle. Added to this, when fat is burnt off for energy, it causes fewer waste products, which often cause tiredness and aches and pains.
A sense of rhythm
But, although this sounds plausible, experts are not convinced. It's all to do with circadian rhythms, Dr Brian English, chief medical officer of UK Athletics, explained. These are the natural rhythms of our body that help harmonise us over night and day, and make up our body clocks.
"Athletes exercise when they feel their bodies are ready," he explains. "And these are professional athletes, they know their bodies extremely well. They will know when their mental and physical circadian rhythms are at their highest." As such, any slight increase or decrease in a particular chemical will have a negligible effect.
Dr Richard Budgett, chief medical officer for the British Olympic Association, agrees. "There is a huge step from seeing a change in levels to saying you are vulnerable to infection. The circadian variations are well known and larger in some people than others. There have also been studies that haven't found any change over the menstrual cycle. It's all very variable."
But what about your average Joe, who wouldn't recognise his circadian rhythms if they ran up to him dressed as a banana?
Know your limits
"People tend to find their own times for exercising, when they feel best," Dr English explains, and Dr Budgett points out that most people feel worse in the morning than the evening, anyway. The most important thing, they both say, is for people to squeeze in exercise when they possibly can. Just to get out there and do it.
There are some caveats, however - exercising after work when you're tired or dehydrated will tend to increase the aches and pains the following day. Exercising on a full stomach is not recommended. Warming up, especially in the morning when you are stiff, is a good way to avoid injury, and knowing when to stop or slow down is vital if you want to be able to do it again the next week.
"The most common reason for injury is that people aren't fit enough for the sport," Dr Budgett said. "The classic is the Sunday morning footballer who never does any other exercise and then wonders why he's hurt himself. The message is: Be fit for your sport, don't do your sport to get fit."
Dr English jokes that at least the early-morning study gives people another excuse to avoid exercise. It seems that even the doctors of athletics associations sometimes need a pretext to get out of strenuous activity. "My favourite excuse is after work when I say I can't exercise on a few pints of Stella - even though alcohol is an excellent energy source. But it does tend to rattle around in there."
The British Olympic Association