There are many claims by multiple institutions that hair brushing promotes hair growth, yet there is rarely any hard evidence to support this idea. A study done by StatePearls explain, "These [blood] vessels nourish the hair follicle and support nutrient delivery, waste elimination, and growth. Loss of blood supply to the hair follicles is associated with some forms of hair loss."
https://www.ncbi.nlm.nih.gov/books/NBK513312/
Many people believe the act of brushing one's hair provides stimulation to help the circulation of blood vessels, yet over brushing is rarely addressed as an issue individuals can come across. A clinical trial published by the National Library of Medicine concluded, "A subgroup analysis found strong association (p<0.05) between brushing frequencies and the magnitude of hair loss."
https://pubmed.ncbi.nlm.nih.gov/19016066/
It is important to keep in mind that there are many factors that can attribute to hair loss. The Mayo Clinic explains that hair loss can typically be attributed to one's family history, hormonal changes, medications, stress, radiation, and particular hairstyles or treatments.
https://www.mayoclinic.org/diseases-conditions/hair-loss/symptoms-causes/syc-20372926
Every individual has a different genetic makeup, therefore a solution that works for hair loss prevention may not work for everyone. While hair brushing may appear to aid in slowing hair loss, for one individual, that may not be true for another. Readers should be aware of exaggerated claims within the beauty industry and consult a professional when looking for effective treatments.