I'm sorry for your diagnosed, but glad you found this forum.
Many ALS patients lose weight, but neurologists usually urge patients to keep their weight up. In fact, mine told me that NOT losing weight is one of the most effective ways to slow the progression ... it's even better than Rilutek. So it's something to keep in mind.
I lost 80 pounds in just over 1 year and tried really hard not to. Yes it is common to loose weight when you have ALS. I got a PEG in hopes to stop my weight loss but it wasn't until getting a tracheostomy and vent did this downhill tumble stop. The only explanation I have is that I was expending a lot of energy trying to breathe - I was burning too many calories and could not keep up. Once I got my tracheostomy and vent I was no longer having to expend energy to breathe so I started to gain weight back right away! It is very important to keep your weight up.
Also in the equation is that muscle weighs more than fat. Atrophy is muscle loss due to muscle death. So yes you will lose weight. I've kept my weight pretty well the same for a year and a half. Arms and legs are scrawney but belly is bigger. But then everybody still says I look good.
Yep muscle sure does weigh more than fat.. Always compared it this way.. It only takes a couple of bricks to way 5 pounds, it takes a ton of feathers to weigh a pound..... The muscle and fat explanation.. Atrophy will cause muscle loss, causing weight loss... You got it Granpa...So much for wanting to lose weight huh..lol
My husband was 204 and very muscular. He lost all his muscle in his upper body and now two years after diagnosis of PMA/als, weighs 174. When on Riluek he lost and went down to 161. Now off Rilutek and keeping his 174.