You know what makes you fat, right? Too many takeaways, too little exercise and, er, that's about it.

And in essence, you're right. Men get fat because they consume more calories than they burn. It's that simple.

But those basic facts don't tell the whole story. The difference between getting fat and staying slim can be about small details, rather than large portions. And sometimes, those details aren't what you'd expect.

The fact is, male diet killers can come straight out of nowhere. Because yes, eating extra donuts will certainly make you fat. But often, so will getting married, or sleeping too much, or keeping the wrong company. If you want to know why, read on.

Getting married
Marriage really does make men fat. That's the conclusion of a large 2008 study, published in the Journal of Economics and Human Biology, which found that the body mass index of married men increased by 1.5% after they tied the knot.

The subconscious sigh of relief after getting married can lead to weight gain
Another study has found that married people were much more likely to be obese than co-habiting couples or single people.

In other words, as soon as you say "I do" to her, you also say "I do" to cuddly nights in with a good film and a huge tub of ice-cream.

Psychologists say there is a good reason for it. When we get married, we heave a subconscious sigh of relief. Having bagged 'the one', we're out of the shark-infested waters of the dating pool and can let ourselves go. The theory is backed up by research suggesting that, when men divorce, they go back to their pre-married weight.

Sitting down
It's pretty obvious that the more of a couch potato you are, the fatter you'll be.

But recent research has thrown up something more interesting - and nuanced - than that. Studies have shown that those of us who spend large parts of the day sitting down tend to be fatter than those who don't, regardless of how much exercise we take at other times.

The hours you spend sat at your desk may stop your body from processing fat
One Australian study concluded that, even among people who exercise regularly, those who sit down most have larger waists and higher blood pressure than those who sit least.

These findings are really quite shocking. They mean you can run for an hour every morning, but if you spend the rest of the day slumped in your seat - as most office workers do - some of the benefits are cancelled out.

Experts speculate that sitting down actually makes it difficult for the body to process fat. In response, many now advise us not only to exercise, but also to limit the amount of time we spend sitting down.

Exercising
Exercise makes you thin, right? Well yes, most of the time, and we wouldn't want to discourage anyone from slipping on the trainers and slogging through a few miles in the park.

But you have to be careful about when you exercise. Personal trainers say they are seeing more men who want to train later in the evening, after long hours in the office. This could be a mistake.

"Exercising late at night could have the opposite effect of that intended as the body is not allowed to repair itself sufficiently, adapt and recover," says personal trainer Romey Jacketts (http://www.romeypersonaltrainer.co.uk/) "ideally we should be relaxing and calming down from about 9pm."


Avoid exercising in the evening and subsequently eating late at night - your body may store the fat rather than burning it
And to fuel late night exercise, we tend to eat later in the evenings. "Eating later at night in order for you to workout is bad because the body may store the food as fat rather than using it as fuel," says Jacketts.

Sleeping
It's well known that too little sleep can lead to weight gain. Quite simply, when we're not getting enough sleep we don't have the energy to fuel an active lifestyle. We become couch potatoes from sheer fatigue.

But though sleep helps to fuel exercise, getting too much of it can have the opposite effect. A study by Laval University in Canada discovered that excessive sleeping is just as likely as sleep deprivation to lead to weight gain and even obesity,

Those sleeping for less than seven hours a night gained four pounds in six years, while those sleeping for more gained three pounds over the same period, according to the research.

We all know not getting enough sleep can lead to weight gain, but too much shut-eye can be bad for you too
Nobody is entirely sure why over-sleeping makes you fat, but it could be that the sense of slothfulness that comes after a nine- or 10-hour sleep session has a similar effect to the fatigue brought on by five hours of shut-eye or less.

Friends
Be careful how you choose your friends, and if most of them are overweight, quickly befriend a few thin people to balance it out.

A huge European study found that many men judge their weight by comparing themselves to friends and colleagues. If many of our peers are fat, the study suggests, we see ourselves as normal, even if we're overweight.

And if our friends put on weight, it gives us permission to pile on the pounds too. Psychologically, all we want to be is normal within our peer group.

Choose your friends wisely, they may be inadvertently contributing to your weight gain
Another study confirmed the findings. Fat friends make you fat, even more so than a fat partner or fat siblings. Researchers say our risk of obesity increases 37% if our spouses are overweight, 57% if we have overweight friends and 40% for overweight siblings.