Professor John Birkbeck's academic areas of expertise are, specifically, nutrition and obesity. But right now it's matters more to do with fashion that are eating at him.

He sports a turquoise polar fleece jacket and a mauve long-sleeved shirt. Of the shirt he says: "When I was young, the colour had certain connotations - but not any more." And he will not be captured on camera in the jacket. That's because it belongs to his wife, Adele.

On the day of this interview, Birkbeck's possessions are reduced to little more than what he has on his back - plus a bed and a few antique chairs. The movers have taken everything else away. The Birkbecks are heading for a new home far from here: they are leaving Auckland for fresh premises, in Kuaotunu in the Coromandel Peninsula.

Is the adjunct professor in human nutrition retiring? Well, no. But he won't be putting in as many hours at the Massey University campus in Albany, where he has spent the last 11 years of his lengthy academic life.

Besides, Birkbeck is not a retiring sort of bloke, in any sense of the word. He is happy to make his feelings known about the weighty business of the state of the nation - if he is asked. Which is why we are here in Room 27 at Massey's institute of food, nutrition and human health.

We meet at lunchtime - but not over lunch. A glass of water will do nicely, thank you.

Birkbeck wears iceberg-white hair and a lifetime in medicine and nutrition. He does not, however, display any of the pinch-faced disapproval of the tedious ascetic.

The truth is already out there: we are a nation of fatties and we are just getting fatter. This is Birkbeck's take on it. "My philosophy about obesity is that in all but very rare instances - so rare it can be forgotten about - obesity is a self-inflicted disease. And it is a disease."

Likewise, there is no truth in the claim that some people will, inevitably, get fat, regardless of their dietary intake. "You can't get over-fat without over-eating, to put it crudely."

Does it matter if you're obese? Yes, he says, it does. He refutes the notion that you can be fit and fat. "There's a whole slew of chronic disorders which will not only shorten your life probably, but what is worse, give you years of ill health at vast cost to the taxpayer which could be prevented by somebody not becoming obese - or if they do become obese, getting rid of it."

The disorders he is talking about include coronary heart disease, diabetes and high blood pressure: get fat, get them - and you're signing yourself up to years of chronic ill health. "Which is not only bad for you, but for your family and your employer and society." What is more, he says, all of it is preventable.

Birkbeck prefers to call them the 'over-fat'. These are the members of the ever-growing group who fall between 'overweight' and 'morbidly-obese'. He believes that over-fatness should be accorded the same social stigma that has successfully been applied to smoking.

It is pointed out to him that you cannot ban over-fat people. "No. But you can make it that society does not accept over-fatness as a normal, healthy thing to be - just like society accepts that it's not good to smoke. We've created an environment where smoking is not only accepted as being a major health hazard but also one that is anti-social.

"Until we've achieved that [with obesity], a lot of work people are trying to do to deal with obesity will be less than fruitful."

OF COURSE it's slightly more complex than that - the big difference being that nobody needs to smoke, but everyone needs to eat. Birkbeck (who was once medical director for WeightWatchers) says what he's been saying for decades: we need to eat less and exercise more.

"Personally, I think one of the reasons that New Zealanders are getting fatter is because the days that they went out and built fences with their bare hands are gone and they take less and less exercise."

Birkbeck is 76. He has a body mass index of 23 - which puts him at the midpoint of normal. He eats a healthful diet, he says, which means lots of cereal products, no white bread (he doesn't like the taste), lots of fruit and vegetables and not a lot of 'flesh foods' (meat, chicken and fish). He says you have to be aware of what you're eating, and make good choices - in terms of content and size.

It's all about the laws of thermo-dynamics: you can't get over-fat without eating more calories than you expend.

And this is where the concentration camp analogy creeps in. "What I used to say, and I certainly used it for the students, is you do not see fat people in concentration camps. Why? Because they get hardly anything to eat and they have to do a lot of work."

Let us assume that society does decide to extend its intolerance of nicotine to obesity: what then?

It is a timely question, given that the budget lopped funding to the Obesity Action Coalition this week.

That's one of the problems, says Birkbeck. He believes the reason may go beyond funding; he says it may well be that the methods tried to date have been inappropriate, and a rethink of other approaches is needed. He suggests, among other things, changing the transport environment to increase exercise and coercing food manufacturers to increase the composition of manufactured food. But that goes only so far.

"Of course, the one way to solve the whole problem is, in a dictatorship, you say 'Everybody that comes back in a year's time with a BMI of more than 30 will be shot' - and you'll find hardly anyone has a BMI over 30.

"But you can't do that in society, so what we have to do is find a way to cajole and coerce. And I don't think they've done enough of that."

He speaks with years of experience. Birkbeck graduated with a degree in medicine from Edinburgh University in 1957. He developed an interest in paediatrics and endocrinology, before moving to the University of British Columbia in Canada, where he took up a post as a paediatric endocrinologist. Next he worked at the University of Southampton in the UK (paediatrics again.) By then he had married Adele - a New Zealander - and she was homesick. The Birkbecks moved first to Dunedin (he got a post at Otago University) and then to Auckland, where he took up a job at the Nutrition Foundation. He also taught medicine at the University of Auckland School of Medicine. In 1998 he became associated with Massey.

Birkbeck believes he is one of the few professors to have worked at three of New Zealand's universities. And he also thinks he is the country's first adjunct professor.

He plans to commute on an occasional basis from Kuaotunu to Massey to give a condensed set of sessions on campus. Among the belongings he has taken with him to his new home is what he calls a whole bunch of data on human foetal growth that he wants to finally knock into shape. It's all based on work that he did before he came to New Zealand. Completion of that work is, says Birkbeck, "an attempt to avoid senility".

He's been both inside the tent and outside the tent: working for associations including McDonald's - where he copped some flak, but says that, as a result, they changed the details of the composition of some of their foods. Speaking of which, he supports the introduction of folate acid into bread, and rejects the argument that it's mass medication.

The benefits extend beyond the prevention of neural tube defects to the reduction of risk of certain types of coronary heart disease and congenital heart disease. "My only regret is that we've set the [folate acid] level too low," he says.

With semi-retirement beckoning, is there anything else he regrets? "I suppose I should have done a good deal more table-pounding when I was younger."

Last night he ate pizza. It was a Four Seasons one, and part of the family farewell. Now he is talking about the 'model of outrage' caused when somebody ended up with a broken rib on an airplane "because a humungous person sitting next to them over-oozed on them". This sort of situation, endorsing the call for the over-fat to pay more for their airborne seating, can influence society's approach to his pet subject.

"I think where we can make things uncomfortable for the seriously fat, we should do so with a clear conscience."