I think while it is in the Anagen (growing phase), the follicle is still healthy and providing as many nutrients as it can to the follicle. However, if you are eating badly at this time (for example) the strand can get thinner and weaker as there are fewer nutrients available for the follicle to give it, whereas if you eat super well your hair should be much thicker and stronger (providing all your other practices are healthy). The better the practices you have the longer you can stretch the anagen phase too. After the growing phase, it transitions to the Telogen phase. Once it is there it would no longer get nutrients and then fall out in this phase.