The vegan internet is abuzz over Penn Jillette’s (of Penn and Teller fame) recent weight loss and his current plant-based diet. But when it comes to celebrities, weight and veganism, I’m always more than a little cautious.

Here are three reasons why I think it’s a mistake to view Penn’s experience as good PR for veganism.

He lost weight on a very low calorie diet (which may or may not have been vegan; he hasn’t said). He reports that he lost 100 pounds in 100 days. That’s quite an accomplishment and I don’t mean to dismiss it in any way. But fast weight loss due to extremely low calorie intake is hard to sustain. Only time will tell whether he is able to maintain the weight loss. His current diet doesn’t exactly create a compelling picture of the joys of vegan living. Here is how Penn’s diet is described in one article: “Vegetables—raw, steamed in a salad or stewed—as well as black or brown rice, and fruits for dessert (with plain cocoa powder) is his daily diet.” Aside from the fact that this diet is nutritionally deficient, it doesn’t sound like a very inviting introduction to veganism. In fact, it sounds like a great way to discourage people from ever considering this way of eating. Penn Jillette does not appear to be a friend to animals. He has spoken out against animal rights. And in response to some tweets about his weight loss, he has said that he is not part of “the vegan movement” and has not changed his perspective.

I have to say, I simply do not get this “celebrities and weight loss” brand of vegan activism. It sets vegan diets up to fail, because that’s what happens when vegans (especially those in the public eye) get sick or gain back their weight or start eating meat and eggs again. It presents veganism as the most unattractive eating plan on earth. And it turns its back on the core value of veganism, which is animal rights.

Losing weight on 1,000 calories per day does not show “the power of plants” as some have proclaimed. It shows the power of semi-starvation. And that is probably not the best approach to advocating for animals or for encouraging people to adopt healthy and sustainable vegan diets.