"Young people in West Dunbartonshire have a life expectancy seven years shorter than those a couple of miles away in the leafy suburbs. So, although you could say it's an act of faith, I regard what we're doing here as a long-term investment in our pupils' future," he says.
What they are doing at Braidfield High - besides installing all-weather pitches for football, basketball, hockey and tennis - includes equipping a classroom as a fitness centre that would be the envy of many a private gym, refurbishing the changing rooms to provide privacy and individual showers and devising a range of innovative Standard grade physical education courses that appeal to children.
As a keen cyclist and hill-walker, Mr Macdonald needed no convincing of the benefits of exercise when he was invited, as one of only two schools representatives, to be a member of the national Physical Activity Task Force. But even he was surprised by what he has learned.
"At one session we had an academic over from Texas - an expert on the relationship between good health and activity - and what he showed us was quite startling.
"Essentially, physical activity is the single most important factor. Healthy eating and staying slim are beneficial, of course, but as long as you get plenty of exercise you can be overweight and still healthier than someone who stays slim but isn't active."
A particularly successful initiative at Braidfield High has been the efforts to enhance the appeal of physical activities to girls, who are much less inclined than boys to be active when they get to secondary school.
According to the 1998 Scottish Health Survey, the differences appear when children are only four years old and grow steadily more unfavourable to the girls. By the age of 16 two out of three of them, compared to one in three boys, fail to achieve minimum recommended activity levels.
There are a number of reasons for this, explains Mr Macdonald. Basic sports skills such as running, jumping and catching do not necessarily come naturally to children. "If girls aren't taught to throw at primary school they might not learn and they'll later find it very difficult to play sports like tennis," he says.
"Girls also tend to enjoy team games less. Hockey, for example, isn't popular here at all. And girls are very conscious of their bodies, so they hate communal changing-rooms, which is why we've installed individual cubicles with showers."
Another initiative, resulting in girls' participation in Standard grade physical education soaring from just one or two a year to 19 currently, is the provision of two distinct programmes of study, one that includes football and basketball while the other comprises netball and dance.
"We don't tell them one programme is for girls and the other for boys," says Mr Macdonald, "and if some of the boys want to take dancing they can. They just don't though - not around here."
Along in the gym a group of girls are going through their contemporary dance paces to the accompaniment of some extremely loud and funky music. Their PE teacher, Grace Shaw, takes a few minutes to draw breath and talk about the importance of providing something for everyone: dance, team-games, individual sports.
"The girls love dancing, so in March we organised the West Dunbartonshire Festival of Dance. We had youngsters here from 14 schools competing in eight dance categories. Three hundred children took part and it was a fabulous day with a buzzing atmosphere. A fortnight later all the winners and runners-up put on a show that sold out, so we're going to make it an annual event."
One very important but sometimes neglected influence on outdoor physical activity in the west of Scotland, says Mr Macdonald, is the frequently dreadful weather. At one time this discouraged large numbers of pupils from taking part in games and playing fields were often waterlogged for long periods. The new artificial surfaces, on the other hand, provide good conditions even in the depths of winter. PE teachers are now kept busy laundering spare kit for pupils who genuinely forget theirs but are still keen to play.
"I sometimes wonder just how many potential world champions we have had through our schools," says Mr Macdonald. "But they weren't discovered, simply because they never got the chance to play the game. Maybe we'll now produce a Wimbledon champion at Braidfield. You never know."