For actors, suffering for one's art usually means weight gain, rather than weight loss – think Robert De Niro in Raging Bull, or Renée Zellweger in Bridget Jones's Diary. Michael Fassbender, who shed two stone for his role in Hunger, was a rarity.

But recently, the reverse has been true. First there was Anne Hathaway, who lost 7kg (16lb) to play a tuberculosis victim in Les Miserables. Then came Matthew McConaughey. Normally chunky, he was seen on Tuesday looking practically emaciated – cheekbones the size of craters. He lost 14kg to play an Aids patient in a new film called The Dallas Buyers' Club. Finally, there's Antonia Campbell-Hughes, who underwent a rapid, drastic diet to play starving kidnap victim Natascha Kampusch in 3096.

"There was an understanding from the beginning," Campbell-Hughes told the Evening Standard, "that I would suffer as much as she did."

As ever, such news has prompted speculation about which particular crash diets these actors have endured – speculation that is usually woefully wide of the mark. "When [journalists] write things about the weight loss I did for Les Mis and they publish a diet that had nothing to do with what I did and people believe it to be true," said Anne Hathaway earlier this year, "I think that's really dangerous."

How dangerous? Very, says dietitian Sian Porter, spokeswoman for the British Dietetic Association. "If you're on less than about 1,300 calories a day, there's a risk that you'll start to break down your muscle tissue. When you're losing weight, your body wants calories more than anything, so it will get those calories from where it can."

By muscles, Porter doesn't just mean biceps and triceps. She's also talking about heart muscle: there's a risk that too rapid a diet will cause organ failure. Matt Damon, who lost 18kg for his part in the 1996 film Courage Under Fire, was once told that the process almost caused permanent shrinkage of his heart.

In the short term, it is likely to cause headaches, tiredness, irritability – and sometimes constipation. "Your skin can be affected, hair loss – and women can get a disrupted menstrual cycle."

Some rapid dieters drink large amounts of water to avoid dehydration. But this can have the opposite effect: "You can get dehydrated by drinking too much water, which brings problems itself. Your body can become waterlogged and that can include your brain, and that can lead to brain damage."

Crash dieting could also cause psychological issues for actors, even if their diet has been initiated for professional reasons. "There's always a danger with some people that disordered eating can turn into an eating disorder," says Porter. "They can become obsessed with keeping it up, or losing even more weight. You forget about normal eating patterns."

So the long and short of it, says Porter, is not to try it at home. "Part of a film star's contract will include expert advice on how to lose weight and then how to gain it afterwards. They would be doing it in a controlled environment." The best way to lose weight, Porter advises, is simply to watch your portion size and be more physically active.