I was sitting in clinic recently when a girl studying at a well-known London University was protesting that I should let her go to a spinning class.
The only thing that was spinning was my head looking at her low sodium levels in her recent blood tests, a result of drinking too much water to try and falsify her weight for this appointment.
By loading her body with water she would try and fool the trusty scales. Every litre of water she had imbibed was an extra 1kg on the scales. But that water dilated her sodium in her blood and there it was in black and white on her blood test results.
She smiled, we both knew that this was a fait accompli; I was not going to change my mind, I was not going to let her go to a work out class. She weighed 40 Kg. She was suffering from anorexia nervosa.
"But please Mark, come on tell my parents it's ok?"
The anorexic voice in her head was telling her than an hour of cardio was a good idea.
"I can't do that and you know that - your blood test results are way off and even if they weren't you weigh 40 kilos that gives you a BMI of 15. (Normal BMI is around 18.5 - 25).
I see this every month and when I guest lectured at Kings College London to dietetic and nutrition students teaching them about eating disorders, I often hear how many of their friends have problems with eating.
Christmas is a particularly hard time for some with food being a central part of the holidays.
Dr Jessica Frankl-Weinberger a psychologist at The Blue Tree Clinic on Harley Street says, "Christmas in most households is the time of year for a traditional get-together of the extended family, where everyone sits around a large table that holds the maximum food possible."
"Some experience this event as magical, but for others, Christmas time is daunting, anxiety-provoking, and at times very lonely. Complex emotions around family dynamics may resurface, and result in a setback or relapse of mental health problems."
"Specifically, for a person who is struggling with an eating disorder, the general preoccupation with food in association with social scrutiny inflicted by a relative can be the ultimate feeling of pressure and loss of control, which play a central role in the perpetuation of anorexia and bulimia nervosa."
A lot of my patients find eating in front of other people especially painful to do.
Some need to know exactly what's in their food and drink and how many calories each piece of food has. But these are sensitive issues and the best you can do is to talk calmly and non-judgmentally to your friend or family member. It's not their fault, they are ill - many people think anorexia is a choice. It is not.
It's prevalent - anorexia usually starts in your teenage years (the average age is 15) so by the time people go off to University they might well be struggling with their eating.
Eating disorders can creep up on you - at first you start to try to diet, maybe control your weight, and before you know it you are over exercising, restricting your diet, vomiting and obsessing over food. You might be getting positive feedback from friends and family telling you how great you look after losing some weight.
A woman in her 30s was telling me "I don't know how I got here, one minute my family was telling me I looked great as I lost weight for my holiday and the next minute I was obsessing over what I ate and making myself sick every day."
And don't forget men too, we often think it's just women who suffer - but about 10 % of people with an eating disorder are men.
I constantly get told by people that have never experienced or have no knowledge of eating disorders that people do it to themselves, that they are vain or trying to look better and they should just "snap out of it."
That could not be further from the truth. It's not about vanity.
We know that the causes of eating disorders are often linked to your genetics.
If you find you are having difficulties right now, try and seek help immediately, studies have repeatedly shown the quicker you can get help the better the outcome. Treatment can take months to years but people do come through the other end and put their illness behind them.
A lot of people are embarrassed to ask for help but it's important to remember you are not alone.
Ask to be referred to your GP or a psychiatrist where nice approachable psychologists and doctors are waiting to help you.
Don't suffer in silence at this time of year.