Beth Blackwell was told that her medical condition meant she could not lose weight.
But the Keighley woman proved her doctor wrong by losing 2st 9lbs in just six months, despite being unable to exercise.
Beth, who has the auto-immune disease myasthenia gravis, puts her success down to joining WeightWatchers.
Since last August she has attended weekly sessions run by Long Lee woman Denise Walker, in Bingley.
Beth, 30, who lives at Kirkstall Edge Farm in Hainworth, was diagnosed with myasthenia gravis in 2010 after suffering symptoms of tiredness for about six months.
The rare condition, which is similar to muscular dystrophy, causes muscle weakness and excessive muscle fatigue.
It strikes about 15 out of 100,000 people in the UK, with varying degrees of severity.
Beth, a professional care worker, said: “I was told myasthenia gravis would change my life, but about a year ago I decided I needed to get back in control.
“When I told my neurologist I would lose weight, come off medication and lead a ‘normal’ life, he near enough laughed at me.
“He said that because of the problems I have with my muscles I wouldn’t be able to exercise.
“Basically, he told me I would not be able to be a healthy weight and have an active life.”
Beth was about 16 stones when she started Weight Watchers, and is now nearer 13 stones.
She said: “I couldn’t walk very well and I was always tired. After joining I was able to start walking the dog, and now I’m working full time.”
Beth said she had also dropped her medication, which was used to control her immune system.
She paid tribute to the encouragement and support she received from both Denise and her fellow group members.
Denise runs several classes in the Keighley and Bingley area. She can be contacted on (01535) 610657.