Does Your Body Replace Itself Every 7 Years?
MYTH!
Some parts of your body may replace themselves every seven years but not all. Different cells, based on their function, may replace themselves sooner or later (see below). And some, like the cells in half of your heart, will never get the chance to be replaced1 leading you to have some cells from birth until death.
Your Body Replaces About 330 Billion Cells a Day!
The stomach lining is replaced every few days. As a result of the constant acid the stomach is exposed to, it must replace itself quite often.1 The same occurs to the lining of the intestines; they replace themselves every 5-7 days.2
Some are replaced less often such as those found in the bone. These are replaced about every decade.3
And of course, some body parts are either replaced so slowly that parts of it will never be replaced, like the heart, and others will not regenerate at all, such as your tooth enamel.4
Resources: https://www.npr.org/sections/health-shots/2016/06/28/483732115/how-old-is-your-body-really, https://www.nih.gov/news-events/nih-research-matters/intestinal-mending-depends-key-protein, https://news.harvard.edu/gazette/story/2017/07/understanding-how-the-intestine-replaces-and-repairs-itself, https://orthoinfo.aaos.org/en/staying-healthy/bone-health-basics, https://www.scientificamerican.com/article/our-bodies-replace-billions-of-cells-every-day/, https://www.ncbi.nlm.nih.gov/pmc/articles.