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When you’re consistently following a ketogenic diet—high fat, moderate protein, and severely restricted carbohydrate intake—you can expect to enter the state of ketosis within a matter of a few days. Of course, many factors will influence exactly how long this takes and how "deep" you get into keto. But in general, close adherence to an ideal ketogenic diet is effective and, to be honest, not inhibitively difficult (once you get past the keto flu, at least!).
How can you tell if you’re in ketosis? Some telltale signs will help: early on you’ll have bad breath and may feel nauseous, weak, and tired. Once you’ve fully transitioned, you’ll likely notice increased energy, improved mental clarity, decreased appetite, and accelerated weight loss, among other beneficial effects.
That said, the only way to know for sure if you’re in ketosis—the metabolic state when your body switches from using primarily glucose for fuel to using ketones converted from body fat via the liver—is to have your ketone body levels tested.
But is formal objective testing really worth it? That’s mostly up to you to decide.
The Biggest Reasons Why—and Why Not—to Test Your Ketone Levels
Pro: testing your ketone body level can let you know when you’re actually in ketosis.
If you want to be able to say definitively if you’re in ketosis, you’ll need to measure the concentration of ketone bodies in your system. A ketone body level (as measured in urine or blood, primarily) associated with moderate ketosis is anywhere from 0.6 to 1.5 millimoles per liter (mmol/L). 1.6 to 3.0 mmol/L is considered high ketosis, whereas above 3.0 mmol/L is considered a very deep ketosis state.
Testing your ketone bodies can be particularly beneficial if you decide to buy ketogenic supplements like MCT oil and exogenous ketones, or if you decide to experiment with intermittent fasting. Are those somewhat expensive supplements actually working? Is intermittent fasting actually paying off for you? Being able to answer these questions definitively with the help of a ketone test can make the investment in money or energy feel all that more worthwhile.
Con: testing can feel a bit too science-y and overwhelming.
People who follow strict diets which require weighing and measuring often note that the constant hypervigilence about their food can seem too clinical and time-consuming. Sure, it may simply be a matter of going through an adjustment period (humans can tolerate just about anything if it’s important enough to them), and having more data to support what you’re doing isn’t necessarily a bad thing.
But for a lot of people, the drastic change in diet that comes with ketosis can be enough of a change to get used to. Adding on the complexity of ketone body testing may be simply off-putting.
Pro: cheap testing options exist…
Urine test strips are easy and affordable. And because all it requires is a little careful aiming, it’s completely non-invasive as well.
Con: …but you sort of get what you pay for when it comes to at-home ketone body tests.
Urine tests may not be as reliable, especially after a few weeks of being in ketosis when your body is primarily using a certain type of ketone body (beta-hydroxybutyrate) which isn’t detectable on the strips.
The more reliable and "gold standard" method for at-home testing is measuring ketone bodies in your blood. While much more accurate, this method costs more and can be unappealing to anyone squeamish at the sight of blood or aversive to pricking their own finger.
Pro: knowing your exact ketone body levels can help you learn how to "feel it" better when you’re in ketosis.
If you test your ketone body levels periodically throughout your ketogenic journey, you can begin to learn what different stages of ketosis "feel" like for you, since you can simply link your symptoms to whatever reading pops up during your test. Over time, you may be more accurately able to "tell" when you’re in deep ketosis or, perhaps more importantly, when you’re starting to fall out of it (we all have slip ups in our diet from time to time). This can be a good way to gauge your progress and eventually allows you to become less reliant on objective measurements.
Con: it may be sufficient enough for you to simply intuit when you’re in ketosis without having to get an exact reading.
Is knowing the exact level of your ketone bodies a requirement for learning how to listen to your body more effectively? That is, you may be able to tell intuitively based on how you feel and function without having actual numbers in front of you.
The Final Word: Should You Test Your Ketone Body Levels?
Like most things when it comes to diet and related lifestyle choices, the decision to test your ketone body levels is largely personal. The case for testing becomes much more attractive if you’re adopting a ketogenic diet for a specific medical reason, such as in the case of managing epilepsy, diabetes, or multiple sclerosis. We can assume, in these cases, that you’re consulting with your primary care doctor closely, especially if you’re taking medications to manage your disease. This is important, because with a chronic health problem (especially diabetes), you may have a greater risk of developing ketoacidosis. Having a clear picture of your ketone body levels is instrumental for avoiding this potentially serious complication.
Likewise, if you’re a serious athlete attempting to experiment with ketosis, if you’d like to determine whether your purchase of exogenous ketones is really worth it, or, to put it simply, you love geeking out about the science of stuff, then testing your ketone bodies intermittently can be desirable.
But if you’re following a ketogenic diet for more general health reasons—weight loss, increased energy, appetite control, improved blood lipid profile, improved insulin sensitivity, and so on—then objectively testing your ketone body levels may not be necessary. It may be sufficient to learn how to pay closer attention to your body and the way you feel and function. Then, if you notice the hallmark signs of being in ketosis, it’s probably fair to assume you’re becoming fat adapted.
So, feel free to look into testing your ketone body levels if you’re of the experimental type. But if you don’t want to, that’s fine, too. It’s enough to trust in the food choices you’re making every day.
You might also be interested to read our article on Ketone Strips and how well they work.
About the Author Nate Arnold
I started this website because it was hard to find trustworthy, evidence-based information about the ketogenic diet. Information that was published and peer reviewed by respected scientific journals. After years of research, I'm sure you'll achieve great results in a healthy way following my advice. I do my best to translate scientific research jargon into plain English. Remember, it's always a good idea to consult a doctor before starting a new diet!