"Anorexia Nervosa is an eating disorder defined as severe, self- inflicted starvation and loss in body weight to at least 15% below that expected for the individual’s sex and height."
There are more symptoms of AN than just "self-starvation". AN sufferers also have severe Body Dysmorphia (a warped mental image of their own body compared to personal goals and ideals, and to the bodies of the people around them and images from the media) and a borderline to severe phobic fear of gaining weight. Plus, AN sufferers do not have to starve themselves to 'qualify' for diagnosis; laxatives, diuretics, binge-purge and excessive exercise are common methods of preventing weight gain employed by ANs. More extreme methods of weight gain have been noted as anywhere from taking up smoking to things as extreme as purposefully becoming hooked on crack cocaine and heroin simply because of the scrawny physique associated with smack'eads.
"Force-feeding is degrading - the patient's right to refuse treatment should be respected."
Whether something is degrading or not is subjective; no, you wouldn't like it if it were happening to you, but a patient's circumstances differ from yours entirely. If a treatment is necessary, works, and is carried out in a private and discreet environment, the argument for it being degrading looks like nothing compared to the chance of curing or improving a patient's condition. Plenty of treatments used to deal with psychological disorders could be considered degrading; insulin coma therapy (though it's not used any more), narcotherapy, electro-convulsive threapy, aversion therapy and interoceptive exposure are all used to cure people of disorders and they work. The (sometimes compulsory) rehabilitation process for certain drugs is horrible; vomiting, convulsions, hysteria, hallucinations, you get the whole package, yet people will say "damn right s/he got bunged in rehab, it's for their own good". Being forced into such a painful process could be considered degrading, yet almost every normal person you ask will be on favour of it because they do not see far enough past the problem to see the crippling process that the cure entails. Saying "waaah it's degrading" is a weak and pointless argument that is easily tossed aside.
"If an anorexia sufferer thinks that they will be force-fed they may be less likely to seek treatment or advice."
If an AN sufferer seriously endangers their own life through their body-warping methods, they may not be given a choice in whether or not they receive treatment. If a patient accepts and acknowledges the fact they have AN, they are in fact incredibly likely to seek treatment and support.
"Compulsory treatment works in the short-term at best, and has associated risks of depression, which may lead to suicide."
Nothing stops at "compulsory treatment" because otherwise there would be no point. A patient suffering any type of mental disorder is treated and continually even after the compulsory actions have been administered. Until someone is given a completely clean bill of mental health, they are helped and encouraged in every possible manner to aid in their recovery.