Those suffering from anorexia won't eat, even if they're so hungry that they can't think of anything else but food. Usually they can't see how thin they are. They are terrified of putting on weight.
But anorexia is not, as Beat emphasises, a diet-gone-wrong. The illness may start somewhere in our national obsession with the way women's bodies look (about 85% of sufferers are female).
But anorexia is actually a way of coping with emotions or situations that are too painful or too difficult to face – someone dying, or a family that's falling apart, or other people's demands on you.
The trouble is that what starts as a way of trying to take control ends up with the sufferer losing control. Starvation changes the way you think. You don't make rational decisions. As you become physically weaker, you cling on to the one thing that's making you ill.
So what can parents do to protect their children from this terrible disease? Recent research
from Professor Bryan Lask at Great Ormond Street Hospital in London suggests that there may be a genetic pre-disposition to getting anorexia. "This is a much more serious disease than was previously understood," he says.
It's not always easy to see the warning signs, especially with teenage girls whose body shape, moods and behaviour seem to be changing all the time.
As one mother says, "I knew she wasn't happy. I thought it was because her school was piling on the pressure for exams. Then we started having arguments – horrible arguments about how much she hated us.
"I thought it was just teenage rebellion. She hated the meals I made. She refused to eat them. But I didn't think she wasn't eating at all. Then one day I looked at her and I was really frightened. There was nothing of her. She looked like someone from a prison camp."
The good news, according to Beat, is that about half of people with anorexia nervosa go on to make a full recovery.
But it can be a very slow process. Recovering from anorexia takes patience and determination.
What's important is that those with anorexia seek help early. There are trained specialists who understand this complicated and frightening illness, and your GP should be able to direct you to the right person. Or you can contact Beat for advice (see the contact numbers below).
So what are the warning signs of anorexia? Strange behaviour in teenage years is perfectly normal. But as the parent of a teenage girl, you might want to look out for:
1. Sudden weight loss. Your daughter will probably lose and gain weight naturally during adolescence. But Beat suggests that the loss of more than one stone in weight over a three-month period is cause for concern.
2. Wearing layers and layers of baggy clothes. Maybe it's a fashion statement. But she might also be hiding the fact that she's extremely thin.
3. Being cold all the time. If you don't eat enough, you can't get warm.
4. Extreme exercise. She's tired and dizzy, but she's still going for a five-mile run in the rain.
5. Having an obsession with food. Some sufferers of anorexia develop rituals and routines, like using a particular knife and fork, or a special plate. Others take control of the shopping or the cooking, or get anxious about the type of food they're eating, or insist food has to be eaten in a certain way or in a particular order.
6. Lying about eating. She might say she's already had a meal at her friend's house, or she's still full from lunch, or she's going out to eat later.
7. Getting upset about meals. You put food in front of her and she starts crying, or says she feels sick. She prefers to eat alone rather than with friends or family.
8. Talking all the time about being fat. This is a tough one, because girls and women often talk about weight. But if you think she hates her body shape even though it's completely normal (or even way too thin), you might want to think about getting help.
9. Dramatic mood swings. This isn't uncommon during teenage years. But watch out for a refusal to communicate at all, even on the good days. People with anorexia don't like opening up. They keep a tight lid on everything in case they lose control.
10. Wanting everything to be perfect. People with anorexia have very high standards. This isn't always a bad thing. But when they decide that the best demonstration of self-control is starvation, they desperately need help.
Anyone worried about themselves or a friend or a family member can get help and information from Beat www.b-eat.co.uk, by email email@example.com, or by phone 0845 634 1414 (adult helpline); 0845 634 7650 (youth helpline), Monday to Friday, 10.30am-8.30pm, or Saturday, 1pm-4.30pm.