Like many professions who kill people with some regularity, doctors have elaborate systems for seeing what went wrong afterwards, and the answer is rarely "Brian did it". This week the papers have been alive with criticism for quack nutritionism after the case of Dawn Page, a 52-year-old with two children.

She ended up in intensive care, with seizures brought on by sodium deficiency, and left with permanent brain damage, after following the advice of "nutritional therapist" Barbara Nash. Nash denies liability. Her insurers paid out £810,000.

I will now defend the nutritional therapist Barbara Nash.

There is no doubt that people who declare themselves to be healthcare practitioners are a risk, by virtue of sheer uncalibrated self-belief. It takes strong nerves to tell a customer following The Amazing Hydration Diet - dramatically increasing water intake, and reducing salt intake - that their uncontrollable vomiting is simply "part of the detoxification process", as Nash is alleged to have done. In fact, Page's lawyers claimed, at this point she was told by Nash to increase her water intake from four extra pints a day to six.

But I put it to the kangaroo court of the international news media that Nash's confidence in her own judgment cannot be seen outside of its social context, and will doubtless have been bolstered by a number of different factors.

After completing the rigorous training at the "College of Natural Nutrition", anyone would naturally believe themselves to be appropriately qualified, and able to give advice confidently. That is certainly the impression I have from reading their website. Nash's confidence in her own abilities seems entirely congruent with that world view.

Then there are the professional bodies. They have been rather keen to distance themselves from Nash. In the Daily Telegraph, for example: "The British Association for Applied Nutrition and Nutritional Therapy (Bant) which has its own code of conduct, said Mrs Nash was not a member." This is not the entire truth. Nash is advertised on yell.com as a member of Bant. In fact, she was indeed a member of Bant, until 2007.

Membership of Bant carries such privileges as "a listing in the Bant Directory of Practitioners, which is available to the public and entry on the Bant website" and "acknowledgement of professional status by the Nutritional Therapy Council". So endorsed, Nash would once again have perfectly reasonable grounds for a strong faith in her own abilities. The episode with Page on intensive care occurred in 2001. These honours were conferred upon her by Bant in 2005.

Looking at Nash's website, I see she carries testimonials from her own appearances on ITV Central's Shape Up for Summer slot: "When I met Barbara (who was the nutritionist for this Central TV programme), I wasn't really sure how her eating plan would help me ... However, it did involve one aspect that I found very difficult to follow, drinking four pints of water a day. I would be the first person to say that I was sceptical but as I had volunteered to take part, I felt that I at least owed it to everyone to try. Was I surprised by the results!"

Promoted, endorsed, trained and buoyed, Nash had good reason to think that what she was doing was sensible and correct. Dawn Page similarly had every reason to believe that Nash was competent. They were both reinforced in these views by the College of Natural Nutrition, the British Association of Nutritional Therapists, Central TV, and every single journalist, editor, commissioner, and producer who has shepherded this bizarre world of made-up nutritional nonsense into our lives. The specific harm done in this one episode is tragic. It always is.

The real measure of professionalism is how you investigate, and what you change: in this case, everyone is queueing up to hold out Nash as solely responsible, and there is not one single crack of critical self-reflection.

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