I am in good gen­eral health but I seem to get tired very eas­ily. Every exer­cise class I have tried makes me feel ill. I joined a gym, but each time after about 10 minutes on the tread­mill I felt very strange, sort of off bal­ance, and my heart star­ted racing. So I have can­celled my mem­ber­ship. Any advice?

One of my best “sales pitches” for doing reg­u­lar exer­cise is how it gives you more energy – the whole day through. This always sounds odd, because people assume, per­fectly reas­on­ably, that you’d be using up all your pre­cious energy in the gym, leav­ing very little for all the other things you have to do in an aver­age day.

In fact with exer­cise we can cajole our bod­ies into mak­ing physiolo­gical changes so that we cope bet­ter with doing more: stronger heart, bet­ter use of oxy­gen, more developed muscles add up to less tired­ness.

But this doesn’t hap­pen overnight. And if you’ve spent most of your adult life doing very little exer­cise, you need to start with care, or your body will cer­tainly revolt.

If a workout makes you feel dizzy, sick or gen­er­ally spaced out, it’s nor­mally because you have over­done things: forced your body to work harder than it is yet able to endure. For a body unused to exer­cise, a 45-​minute step class is a massive shock to the sys­tem. As you get fit­ter, your body learns to move into exer­cise mode more effi­ciently and without com­plaint. So to avoid the extremely off-​putting reac­tion you describe, you need to start more gently and build up gradu­ally.

You should always begin your workout with a warm-​up. Then, on the tread­mill, you need to set a com­fort­able walk­ing speed to start with, and increase it gradu­ally to a brisk walk, per­haps with a bit of uphill, for your first few ses­sions, mak­ing sure you use the last 3 to 5 minutes of the workout to slow back down again to your start­ing level. In this way, you should be able to get through a ses­sion without end­ing up feel­ing ill.

Another option might be to switch from the tread­mill to the exer­cise bike, which should pre­vent you feel­ing “off-​balance”.

All this assumes you have no under­ly­ing med­ical prob­lem, and it might be a good idea to double check with your GP that it is OK to be exer­cising. Then, go back to the gym, explain what happened and ask if they will help you to get back into a good routine by giv­ing you a few super­vised ses­sions. A decent instructor should teach you how to warm-​up and what speeds to use on the tread­mill, includ­ing your max­imum speed and the right length of workout time, so you can be con­fid­ent you are mak­ing pro­gress without bad side-​effects.

Finally, although you should never need to feel ill in the way you have been, you may carry on feel­ing tired for the first month to six weeks of reg­u­lar exer­cise, while your body begins to adapt. After which, life is going to feel an awful lot bet­ter.