Carrying out high-intensity interval training is one of the best ways to upgrade V02max (maximal aerobic capacity), improve coordination and efficiency at race speeds and boost leg muscle power, but such training is not without its dangers. The popular training maxim, 'Speed kills', reminds us that interval training can increase the risk of injury. In addition, conducting a bit too much interval training can throw athletes into the overtrained state, in which performances sink and previously easy workouts become like death marches.

Now, researchers at the Human Energy Laboratory at the University of Wyoming have identified another peril of interval training: it can drastically impair athletes' iron stores. At Wyoming, iron status was monitored in eight competitive cyclists during three weeks of rugged interval training, followed by two weeks of recovery. The cyclists were fairly m (V02max = 63), young (age = 21 years) and lean (8 per cent body fat).

The interval sessions were tough, carried out at an average heart rate of 94 per cent of maximum (about 90% V02max), and were alternated with easier but longer workouts consisting of 60-minute rides at about 65 per cent max heart rate. Overall, approximately 25 per cent of each week's exercise was carried out at a high intensity (around 90-94 per cent max heart rate).

During the two weeks of recovery which followed the three weeks of strenuous intervals, the total amount of training was cut drastically and short, fast work comprised most of what was left of the training programme. This extremely low-volume, fairly high-intensity kind of tapering or recovery is in line with what experts now think is the best way to maximise physiological improvements following a period of heavy training. Instead of taking it totally easy during such a tapering or recovery period, it's best to do a small amount of intense training.

Indeed, the mix of intervals and recovery worked fairly well for cyclists: performances improved by 6-7 per cent after the intervals and by another 2 per cent after eight days of recovery. However, there was one problem: senum ferritin levels plummeted by about 40 per cent at the end of the interval-training period (ferritin is the key protein in the human body which stores iron).

While ferritin levels didn't drop dangerously low (they were still in the normal range), they might have dipped to sub-normal concentrations if the athletes had been iron-compromised to begin with. It's not clear exactly why iron dropped so precipitously in response to the interval training, although impaired absorption of iron, iron losses in sweat and even gastrointestinal bleeding are all possibilities (all three can accompany strenuous training). The bottom line is that serious athletes should consult carefully with their doctor during periods of especially heavy training to ensure that ferritin levels remain normal. ('Reduced Iron Stores Following Three Weeks of High-lntensity Interval Training and Recovery, Medicine and Science in Sports and Exercise, vol. 27(5), Supplement, no. 1136, p.S203, 1995)