“If you can’t measure it, you can’t improve it.” This could be the motto for biohackers everywhere, who like to quantify every single aspect of health and performance in order to optimize outcomes and results.

But sometimes, when it comes to health, subjective feelings and assessments might be just as valuable as hard data.

When it comes to ketosis, this is especially true. While it’s easy to measure things like weight loss and ketone levels using numbers, ketosis can also be monitored using ratings of positive side effects, mental energy, and perhaps some of the negative symptoms.

In order to adequately know whether or not you’re in ketosis and how well you’re responding to it, you’ve got to know what to look out for. In this article, we will discuss some common side effects and symptoms of ketosis.

Let’s Review: What is Ketosis

Dietary carbohydrate restriction, fasting, and even prolonged exercise can all lead to a reduction in blood glucose and glycogen in the body. When this happens, the body must find an alternative route to produce energy and fuel cells. One of these “alternative routes” is the utilization of free fatty acids, obtained by breaking down stored body fat, sometimes called “fat burning”.

Free fatty acids in the circulation can be transported to the liver. The liver is able to turn fatty acids into ketone bodies, which can then be used as a fuel source. When blood levels of ketone bodies (mainly acetoacetate and beta-hydroxybutyrate) rise to levels above 0.5mM, one is considered to be in ketosis.

Reducing carbs and eating a high-fat diet isn’t the only way to enter ketosis.

Exogenous ketone supplements—which are essentially ketones that you can eat or drink—can be used to induce a state of ketosis in the absence of carbohydrate restriction. Exogenous ketone supplements can include ketone salts, ketone esters, and MCT oil. The only difference between ketosis achieved endogenously vs. exogenously is that in the latter, the body is producing its own ketones. In addition, ketosis achieved through dietary manipulation is likely much better for fat loss and metabolic health benefits.

How to Tell That You’re in Ketosis

When ketones are produced, they exert a wide variety of actions on the body. Many of these actions will be noticeable, and therefore can serve as an indication that you’ve entered ketosis.

However, the first and most surefire way to tell if you’re in ketosis is to measure.

Measuring Ketosis

If you want a “yes/no” answer to the question “am I in ketosis?”, then testing for blood ketone levels is the way to go. Using one of several devices and methods, you can get a quantitative reading of how many ketones you’re producing.

The first testing method available to keto dieters involves measuring levels of beta-hydroxybutyrate (BHB) in the blood using a blood ketone meter (similar to a blood glucose monitor).

Blood BHB testing is the most accurate way to measure ketones, but also the most invasive.

There are several different commercially-available ketone meters on the market.

Two other testing methods exist. The first measures levels of acetone in the breath using something called a ketone breath analyzer. Acetoacetate (AcAc), another ketone, can also be measured using urine ketone test strips. Urine strips are able to detect the amount of AcAc in the urine.

Urine and breath tests are much easier than blood testing, but are also less accurate, and may go down in accuracy the longer that you’re on a low-carbohydrate diet.

Increased Mental Focus and Reduced Brain Fog

One of the less well studied but anecdotally reported benefits of ketosis is the enhanced mental clarity that people say they fell while in ketosis.

These experiences match up with the science. Ketones have been shown to enhance cognitive performance during exercise and, in mice, improve mental ability.White2011,Murray2016

The idea that ketones enhance brain function makes sense. The ketone BHB has been shown to boost levels of BDNF—a brain growth factor—and stimulate the production of new mitochondria.

The utilization of ketones for energy in the brain also produces less reactive oxygen species than does the utilization of glucose.

Ketones can be thought of as a “cleaner burning fuel”, perhaps leading to less "brain fog."

What will this feel like? Well, you might wake up with a clearer mind, ready to tackle problems or a big workout right away. You’ll also avoid the midday slump in physical or mental energy levels sometimes occurs when blood sugar levels crashed, instead riding on a constant supply of sustainable energy from ketones. All of these benefits should be easy to notice.

Reduced Appetite and Cravings

When we say “reduced appetite”, we mean this in a good way, in that ketosis achieved through a ketogenic diet will promote satiation. This means you’ll be satisfied after eating a meal, and not feeling the urge to overeat or snack mindlessly throughout the day.

This may be due, in part, to the fact that the ketogenic diet contains foods that are high in fat and protein, which are both very filling nutrients. As a result, even though you might be consuming fewer calories, you’ll be more satisfied.

However, some studies report that just being in a state of ketosis can reduce appetite.Giibson2015 The ketone body BHB has been shown to reduce levels of the “hunger hormone” ghrelin.Stubbs2018 This is perhaps one mechanism by which ketosis can quell the urge to (over)eat.

Potential Unwanted Side Effects of the Keto Diet

A necessary criteria of the ketogenic diet is a restriction of carbohydrates—it’s how the diet works, and ketosis can’t be achieved without it.

Carb restriction is also the cause of some of the initial unwanted “side effects” of the ketogenic diet. Sometimes referred to as the “keto flu”, these symptoms include nausea, headache, and fatigue. However, they likely aren’t serious and usually resolve within a few weeks, after you’ve become keto adapted. These symptoms don’t occur in everyone, however. Significant variability will exist in the individual responses to a ketogenic diet.

Along with the minor “keto flu” symptoms, there are just a few frequently reported and often short-term symptoms that people sometimes report after adopting a ketogenic diet.

Digestive Issues

Some of the more commonly reported digestive symptoms of ketosis include dehydration, constipation, and diarrhea. On a carb-restricted diet, the body will initially burn through glycogen stores. Since each gram of glycogen is bound to 3 to 4 grams of water,Olsson1970 this leads to significant loss of water weight and fluid throughout the body. This is one reason why keto diets can cause such rapid weight loss.

However, this water shedding and frequent urination could also lead to dehydration.

Water loss is exacerbated by low insulin levels. Low insulin levels can cause water excretion,Defronzo1975 and this takes excess sodium along with it. Dehydration and low sodium are one of the causes of the keto flu symptoms. They might also lead to constipation. For this reason, be sure to drink plenty of water, especially during the first week of keto, and probably long after.

On the opposite end, diarrhea could result from a rapid shift in diet, which could cause changes to the gut microbiome. The ketogenic diet might also be a major step up in your fat intake, which could also promote loose bowels or diarrhea.

While gut issues are possible, many people report improved gut health after going keto. This is probably due to the fact that they’ve cut out sugar, refined foods, and processed carbohydrates and grains. A whole foods diet is perhaps the best way to promote a healthy digestive system.

“Keto Breath”

One of the more common “side effects” of going on a ketogenic diet is a distinct smell on your breath. While not necessarily harmful, some find it unpleasant. This bad breath is sometimes referred to as “keto breath.”

What is the cause of “keto breath”? Acetone.

Acetone is one of the ketone bodies that is formed as a byproduct in a reaction involving acetoacetate. To get an idea of the smell, acetone is an ingredient found in nail polish remover. However, keto breath isn't nearly as potent a smell.

And while mildly unpleasant, “keto breath” is a good sign that you are in nutritional ketosis and burning fat.

The fact that we have acetone in our breath is also one of the reasons that we can measure ketones in the breath using a breathalyzer.

Interestingly, keto breath might be a short-term side effect and subside within a few weeks of starting a ketogenic diet. If you notice keto breath, you can use breath fresheners to mask the smell. But most people shouldn’t mind.

Get in the (Ketosis) Zone

Now that you’ve been presented with some of the positive feelings and minor negative side effects of ketosis and ketogenic diets, you’re prepared to understand and assess how your body responds to being in this metabolic state.

Whether you’re fasting, eating keto, or doing intense exercise training, knowing the signs of ketosis can help you confirm that you’re in ketosis and that your low-carb diet is effective.

Knowing the side effects of a keto diet can also help you to not freak out when they do happen, knowing that most of them are only temporary. Some people experience no keto-related side effects at all.

Remember, everyone will experience different positive and negative benefits of a ketogenic diet. It’s up to you to find out how you feel, perform, and think while on keto.