My husband and I are big Costco fans. We love to nose around to see if they are carrying any new vegan products, argue over the wisdom of buying a 50-pound bag of sweet potatoes, and endlessly discuss whether we should purchase a home surveillance system so we can spy on our cats when we’re traveling.

One fun thing we don’t get to do is try all the freebie samples that the nice ladies in their plastic caps hand out. Once in a while we’ll score some chips and guacamole or—far less exciting—a slice of Anjou pear. But for the most part the offerings aren’t vegan. On the plus side, it saves us from eating a lot of really unhealthy food. While it’s certainly true that vegans can make unhealthy food choices, it’s far easier to do so when your options include cheesy fries, French bread pepperoni pizza, and cream puffs.

And when we get in line with our cart filled with quinoa, peanut butter, extra virgin olive oil, pinto beans, chickpeas, hummus, lemons, and frozen blueberries, it’s pretty obvious that even with a few treats, our basket looks way healthier than the ones piled up next to us.

On one recent trip, my husband looked around at the other shopping carts and said “If diet really matters, we should live to be 150.”

Diet does matter, of course; it matters a lot. (And my husband, who has a PhD in nutrition, knows that.) But it doesn’t make us bullet proof against disease and it’s a mistake to believe that it does. It’s especially a mistake to advocate for animals by promising people that going vegan will somehow make them invincible.

Lately, I’ve been seeing quite a few stories about people with cancer in my facebook feed. Several of my FB friends have breast cancer or have been treated for it. Others are dealing with news of cancer in their loved ones.

Most of the comments on these posts are appropriately supportive and sympathetic. But you can always expect that there will be a few that are downright cringe-inducing. If the person with cancer is vegan, it’s a good bet that someone will suggest that they must have been a “junk food” vegan. That they probably ate “tons of soy” and cooked with “unhealthy olive oil.” (Actually, both of those foods are more likely to be protective against cancer than harmful, but that’s a story for another post.)

Aside from the fact that such comments are incredibly rude (even for facebook), trying to blame someone for their disease is not the least bit helpful. Worse, some vegans with cancer blame themselves. In one instance, a vegan who was diagnosed with cancer before the age of 30 said that it wasn’t surprising since she had been overweight and ate a junky diet. But, cancer in your 20s is unusual even among people who eat the worst diets.

Disease shaming alienates and ostracizes people. Vegans may be afraid to share their diagnosis with others. They may not ask for much-needed support because they worry about feeling judged. Some may be reluctant to admit that they have chosen conventional treatment for their cancer instead of trying to cure it through some raw, low-fat, alkaline, whole foods vegan diet. But, there is no evidence that any particular diet, whole foods plant-based or otherwise, can cure cancer. Believe it or not (sorry, Hippocrates) sometimes you have to let medicine be thy medicine.

And while there are diet and lifestyle choices that lower risk for cancer, there is no surefire way to prevent it through what you eat. The evidence for effects of fiber, vegetables, fruits, meat, eggs, milk, and vegetable oils on breast cancer, for example, is “limited,” at best. That’s the conclusion of the World Cancer Research Fund and the American Institute for Cancer Research, organizations that summarize all of the findings on diet and cancer.

We do have research showing that certain types of diets—mainly those based on plants with lots of fiber and antioxidants—are likely to reduce risk of cancer. And choosing foods accordingly is a good idea. It’s just that we can’t say with any certainty that any type of diet offers a guarantee against cancer.

Nor can we ignore the other things that might affect risk. Not just environment and genes, but also factors like reproductive history. Women who have never had a baby or breastfed a baby are at considerably higher risk for breast cancer. And then there is mental health. Stress and depression are both related to inflammation, a condition that underscores many chronic diseases, maybe including cancer. And for many people, these can be difficult to manage.

The fact is that disease, especially cancer, is complex, and our understanding of the causes and cures is imperfect. Pretending that a vegan diet is an established way to make people disease-proof might seem like a good way to get more people to go vegan. But it can cost us credibility. It can deter people from getting appropriate medical help. And it alienates vegans who have cancer or diabetes or heart disease or who aren’t slender.

Alienating vegans who don’t live up to some particular physical ideal is not the best way to create a vegan world. And it’s especially hurtful when those vegans who are overweight or have cancer or some other disease also happen to be incredible advocates for animals.

Lifestyle choices can raise or lower the likelihood that you’ll have a healthier life and maybe a longer one. But the only real guarantee in this life is that those of us who build our choices around a vegan ethic will leave the world a better place than we found it.