Adverts show how the apps can be used to alter body parts, including making waists slimmer and adding muscles.
Eating disorder charities have said technology firms should consider the impact on vulnerable people.
The platforms said the apps did not break advertising guidance, but TikTok added it does reviews its policies.
It said it is “continually looking to enhance” its framework to “support a body-positive environment”.
The social media platform, popular with teenagers, banned adverts for fasting apps and weight loss supplements last year.
‘Fuel this epidemic’
Social media companies need to be held to account and “stop this unhealthy and unhelpful messaging,” said eating disorder campaigner Hope Virgo.
"Over the last year, we have seen a huge increase in the number of people with eating disorders, and while eating disorders aren't necessarily caused by bad body image, we know there are some intrinsic links.
“The fact that Instagram and TikTok are currently advertising body-changing apps will fuel this epidemic of eating disorders further.”
Eating disorder charity Seed said it has seen a 68% rise in children and teenagers aged between 10 and 19 seeking support since the pandemic.