Hardly a week goes by without some celebrity getting trolled online because of their weight.

Last week it was “The Walking Dead” star Alanna Masterson’s turn. The actress, who plays Tara in the series, was criticized by fans for putting on a few pounds after the birth of her daughter.

In a lengthy Instagram post Masterson struck back at the “trolls, body shamers, and the men and women who think it's OK to comment on my weight.”

In her message, which was posted alongside a picture of her daughter Marlowe, Masterson wrote: “Before you decide to make a comment about my chest being ‘too large’ or how ‘fat’ I've become, just know that this little girl got the best start to life … I would've gladly continued to eat enough calories to produce milk for her little bones to grow.”

Body shaming of celebrities often focuses on “extra” weight — like Amy Schumer for not fitting the Barbie mold in an upcoming movie, Vin Diesel for his "dad bod" pictures, or 99-pound Olympic gymnast Alexa Moreno for being "fat."

But trolls know no bounds. And anyone in the public eye is fair game.

A petition started on change.org was demanding that 22-year-old YouTube performer Eugenia Cooney be banned from the site because viewers said she was “too thin” or possibly anorexic.

The petition reportedly gathered 18,000 signatures before being taken down. But several similar petitions have popped up since then, along with others in support of Cooney.

Body shaming of celebrities is now so common that it has lost some of its shock value.

“Headlines on tabloids in the checkout line of grocery stores regularly target celebrity weight loss or the appearance of cellulite. This behavior has become so normalized that most people probably don’t think of it as trolling anymore,” psychology professor Renee Engeln, Ph.D., author of the forthcoming book “Beauty Sick: How the Cultural Obsession with Appearance Hurts Girls and Women,” told Healthline.

Read more: Body shaming in an age of social media »

Internet fuels body shaming

Social media is often blamed for the rise in trolling, but body shaming is not a new phenomenon.

Dickinson College professor Amy Farrell, Ph.D., author of “Fat Shame: Stigma and the Fat Body,” said in an interview with the International Business Times that fat and body shaming can be traced back to mid-19th century England when the first diet book was published.

Postcards from that time showed images such as “fat women” or a “fat butt, sometimes as a pincushion.” Farrell said that people sent these postcards the way people text each other today, with the images spreading like internet memes.

As with many things, though, the internet has allowed body shaming to reach new levels of vitriol.

“[The internet] has definitely played a significant role in amplifying the voice of people who are perpetuating the stigma and the bias and the shaming,” James Zervios, director of communications for the nonprofit Obesity Action Coalition, told Healthline.

Engeln also said that now when people judge how others look, their words can instantly reach a wide network of friends and even hundreds or thousands of strangers.

The internet also isolates people from their victims — even on social media — allowing them to post comments online that they would never say to a person’s face.

“We start to see other people's bodies as perfectly acceptable targets for ridicule, often forgetting that those bodies house actual human beings with the same thoughts and feelings and sensitivities we have,” said Engeln.

Read more: Does the ‘body positive’ movement promote health? »

Mental health toll of trolls

Like Masterson, other celebrities are calling out trolls for body shaming.

“Their voice is often very strong, and as long as they’re using it appropriately, I think it can help raise some positive awareness,” said Zervios.

But while much media attention is focused on celebrities, anyone can be attacked for their appearance — even if they aren’t online.

In July, Playboy model Dani Mathers took a picture of a naked 70-year-old woman at an LA Fitness in Los Angeles without the woman’s consent. She uploaded the photo to Snapchat with a mocking caption about the woman’s appearance.

The widespread public outcry led to an apology from Mathers. Last month, Los Angeles prosecutors charged Mathers with one misdemeanor count of invasion of privacy.

Body shaming — of celebrities or others — can have severe effects on a person’s mental health. Research by the Center for Advancing Health found that teens who believed they were overweight were more likely to suffer from depression or attempt suicide.

Some think body shaming also distorts how we interact with others.

“Every time someone makes a comment — good or bad — about someone else's appearance, they're sending the message that people’s looks are an important topic of discussion,” said Engeln. “They’re suggesting that looks matter more than other things we could be talking about.”

Read more: The hidden dangers of obesity in childhood »