Let's get this out of the way: the phrase "heat training" is a little misleading. It'd be way more accurate to call it "carefully controlled, purposeful heat damage for increased manageability", but that's not very catchy, now is it?
Heat training involves intentionally weakening the internal proteins of your hair through repeated heat exposure. The goal of this process is to make afro-textured hair less kinky/coily/curly so it can be more easily brushed and styled.
Heat training isn't the same thing as relaxing, since it won't leave hair straightened forever/doesn't involve chemical applications, but it's still quite damaging. The particularly tricky thing about heat training is that you can't really pick and choose between the types of heat damage your hair will incur.
There are three overarching types of heat damage:
While the goal of heat training is to achieve a controlled version of the third type of damage, it's pretty hard to completely avoid 1 and 2 (or the breakage that comes along with 3). And this leads us to another, extremely common question about heat training:
No, but also possibly yes. Unless you're severely burning your scalp (please don't do that), heat training shouldn't affect the rate that hair grows out of your head. However, heat training hair may cause your ends to snap off very easily, leaving you stuck at a certain length.
It's difficult to predict if heat training will affect your hair growth or not. Some people have hair that can withstand a heck of a lot of damage without snapping. Others experience breakage from a single hot-combing session. Hair types are way more diverse than we give them credit for, and there's really no one-size-fits-all approach for safely straightening hair.
Generally, the best way to predict if you'll experience breakage from heat training is to critically consider how your hair has responded to heat in the past. For example, if you know that you've lost hair due to heat in the past (and it wasn't because you did something like leave the curling iron on one section of your hair for five minutes) you probably shouldn't heat train. On the other hand, if you used to straighten your hair once a week and never experienced a bit of breakage, you'll probably be OK to start heat training. And this might seem obvious, but if you decide to start heat training, be sure to pay attention to how your hair responds. If you start to feel like the heat damage is noticeable or if you feel like you're stuck at a certain length, it may be best to discontinue the heat (or at least switch to a different heat protectant -- silicones are your friends).
If you really want to experiment with heat training but don't want to risk length-loss, try straightening some of your shed hair. This won't be a perfect approximation, but you'll be able to see if a certain temperature causes the hair to become especially brittle.
On the flip side, it's possible that some individuals may experience a decrease in breakage when they begin heat training. Some people have hair that can withstand a lot of heat, but not a lot of mechanical stress from brushing and detangling. Since heat training can promote manageability, these individuals may find that they're able to grow their hair longer while heat training then when their hair is curly.
The answer to this question depends really on what you want from your hair, how much damage you're willing to potentially endure, and how much time you want to invest in working on your hair.
If you want perfectly, permanently straightened hair, you may want to go with a relaxer. Hair that has been relaxed won't re-curl from anything, not even water, so relaxers are especially appealing to people who don't want to deal with the hassle of having to straighten all the time. Hair will only need to be re-relaxed once enough growth has occurred that curly hairs at the scalp are noticeable -- for most people, this is about 8 weeks.
Still, as you probably know, relaxers aren't all sunshine and daisies. Relaxing, quite literally, breaks and reforms the bonds in your hair -- once a hair strand has been relaxed, it stays relaxed. A relaxer may be extremely effective for permanently straightening hair, but it will also extremely effective in permanently damaging hair (with how chemical relaxants work, you can't really have one without the other).
As we've covered above, heat training is damaging to hair, but it's not nearly as damaging as a relaxer. If you want to grow your hair to be quite long, you'll probably experience more success with heat than with a relaxant -- just be sure to protect, protect, protect with silicones when straightening.
Heat training also won't permanently change your hair from kinky to straight, which is great for if you ever decide to spend a few days heat-free. This makes heat training preferable for people who don't want to commit to permanently straight hair. Heat training may alter your curl pattern to be a little looser than before, but it most certainly won't prevent your hair from curling up in water the way a relaxer does.
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