WHAT are we to make of health advice these days?

After decades of telling us that milk will keep us healthy and fat will kill us, now they tell us that every glass of milk a woman drinks each day increases her chances of dying by 15 per cent and saturated fat has nothing to do with heart disease.

And after 50 years of telling us we need to lose weight, a slew of recent studies have shown that overweight people actually live longer, that losing weight can be more dangerous than remaining fat and that what really matters is not fatness at all but exercise.

I’m starting to think we took a wrong turn a long, long way back, and now we haven’t got a clue where we are.

Anyway, let’s have a crack at getting to the bottom of the fat issue.

Do fat people live longer?

Yes, they do.

We generally use Body Mass Index (BMI) to estimate fatness.

You can calculate your BMI by dividing your weight in kilograms by the square of your height in metres.

The numerically challenged can use this website.

A BMI lower than 18.5 suggests you should be a Calvin Klein model, 18.5-25 is the “healthy” or “normal” range, 25-30 is overweight, 30-35 obese, and over 35 seriously obese.

Dozens of recent studies, including a mega-study involving 2.9 million people, have all found the same pattern: you’re least likely to die at any given age if your BMI is pretty much in the middle of the overweight category, with a BMI around 27.

While being fat means you’re more likely to get diabetes or cardiovascular disease, it also means you’re less likely to die from them.

Researchers, who like a catchy title, call this the Obesity Paradox.

It gets worse: people who lose weight by diet or exercise may be more likely to die, even when you allow for pre-existing conditions like cancer.

You’ll live longer if your weight remains the same. The next best thing, unless you’re very, very fat to start with, is to actually gain weight.

The real mystery is why this happens.

Maybe the unhealthy fat people have already died before the study begins.

Then we’re left with only healthy fat people, and a mix of healthy and unhealthy lean people, so it’s not surprising that the fat people appear to have an advantage. Maybe fat people get better medical care.

The sight of a fat person to a GP is like a bell to Pavlov’s dogs: they automatically reach for their prescription pads.

Or maybe fat really is protective, shielding us from hip fractures when we fall, or providing a nutritional reserve when we reach frail old age.

Interestingly, it hasn’t always been the case that overweight people lived longer.

Back in the 1970s people in the “healthy” weight category did indeed live longer, but since then the best death-defying weight has crept up.

This may be partly explained by changes in the general population.

Since the 1970s, the population has grown older and taller (by about 5cm), and BMI is slightly biased against taller people — at any given BMI, shorter people are a bit fatter.

The news that being fat is not necessarily a slow decline into premature death may come as good news to those for whom the pec-flexing, chest-thrusting atmosphere of the average gym constitutes a fundamentally alien and faintly ridiculous environment.

But it’s false hope, because while fatness may not matter as much as we thought, fitness and physical activity turn out to be even more important than we suspected.

A hot-off-the-presses study on a huge European cohort found that just 20 minutes walking each day does more to reduce on your risk of dying than losing enough weight to move from the obese to the overweight category (a difference of about 3-15 kg).

And here’s an odd thing: in the cheating death department, not all types of exercise are equal. I’m not talking here about core, cardio or crossfit, but about the kinds of exercise we do in our everyday lives.

Physical activity we get at work appears to be particularly ineffective in reducing cardiovascular disease and high blood pressure, even if we work just as hard on the job as we do at play, and hard yakka at work actually makes the risk of depression worse.

Walking to work, much touted by the public health mafia, also has rather modest effects. A walk along the beach with your dog or a session at the gym gives you much more bang for your buck.

And more to look at.

So if you’re a bit overweight and you want my advice — and why wouldn’t you, I can hardly do worse than most health experts — just get out for your daily walk, eat lots of fruit and vegies, and chill: you may be better off fat.

Professor Timothy Olds is a professor of health sciences at the University of South Australia.