When previously sedentary people begin a regular exercise programme, their 'natural killer' (NK) cells go on full alert. Fortunately, NK cells are not brain cells; in fact, they're Iymphocytes (white blood cells) which directly attack virally infected cells and budding cancers. The upbeat response by NK cells to physical activity may be one reason why regular exercise seems to confer resistance against infection.

Research suggests that competitive cyclists also enjoy enhanced NK-cell activity, but the NK profiles of strength-trained athletes haven't looked quite as favourable. In fact, one investigation showed that NK activity was significantly depressed for up to two hours after a strenuous weight-training workout. Is there something about strength training that kills natural killer cells?

To find out how NK cells actually respond to strength training, scientists at Appalachian State University in the United States recently compared 10 young strength trainers who had been training for at least three years with 10 sedentary men of approximately the same age, height, and weight. The 10 weight trainers could all parallel squat at least 150 per cent of their body weight, and they typically trained by completing three to five sets of five to 10 reps of various exercises, three to four days a week for at least one hour per day.

To check out the NK cells, blood samples were taken from all subjects at 8 a.m., at least 12 hours after eating and working out. The good news was that NK activity was not depressed in the strength trainers. The bad news was that NK function wasn't increased, either. In fact, NK cells of strength trainers were indistinguishable from those of the lethargic louts.

In contrast, endurance-trained athletes DO seem to have more robust NK-cell populations, compared to sedentary people. Elite cyclists, for example, have about 50-per cent more NK cells than non-exercising individuals, and elderly female athletes who compete in endurance road-race competitions have 55-per cent more NKs than women sitting at home in rocking chairs. However, it's likely that a few weeks of exercise are not enough to send NK cells soaring; it may be the minimum requirement for an NK-cell tuneup is 45-60 minutes of endurance-type exercise a day, five days per week, for at least three months.

However, pumping iron - even for over an hour each day - doesn't seem to make increased numbers of NK cells come out of hiding. All is not lost, however; the Appalachian researchers suggest that resistance workouts may stimulate other types of white blood cells which are also an important part of the immune system.

('Natural Killer Cell Cytotoxic Activity in Weight Trainers and Sedentary Controls, Journal of Strength and Conditioning Research, vol. 8(4), pp. 251-254, 1994)