How to Know if You are in Ketosis

For the first three to seven days, your body is undergoing glucose withdrawal.  High energy-consuming organs, especially your brain, are used to having glucose as a fast energy source, and have to undergo a transition to primarily fat use when you cut out the carbs.  During this time, you may experience any of a number of common symptoms, including:^2,3

• Fatigue
• Headache
• Irritability
• Poor concentration
• Difficulty focusing (brain fog)
• Low motivation
• Dizziness
• Sugar cravings
• Nausea
• Muscle cramps
• Vomiting
• Constipation
• Diarrhea
• Muscle weakness
• Stomachaches
• Difficulty sleeping

Experiencing symptoms of keto flu is a good indicator that your body has entered ketosis.  However, you will need to use other means to tell if you are staying in ketosis for longer or not.

A byproduct of fat metabolism is acetone - the same chemical that is in fingernail polish remover.  On a very low-carb diet, when your cells are predominantly utilizing fats for energy production, the concentration of acetone rises in your blood and, subsequently, your breath.  Bad-smelling breath is a common complaint of people on the keto diet, but also a good indicator that it is working.

Recently, breath acetone meters have been developed which can measure the level of fat metabolism in the body.  Compared to blood and urine tests, breath acetone testing is even more reliable and less influenced by other metabolic factors.⁴  Since they rely on specialized sensors, breath acetone meters are more expensive than other tests, but they are reusable and easy to take anywhere.

Blood tests have historically been the most accurate means of measuring your blood ketone levels. This can be done using a finger prick and a home test kit.  Though fast and reliable, the finger prick can be uncomfortable.  It also requires a reusable device and disposable strips which can end up being a bit expensive altogether.  Many devices also measure blood glucose.
Ketosis is defined as a blood concentration of ketone bodies of 0.5-3.0 mm/L.⁵  Lower than this range means you need to cut your carb intake more to achieve true ketosis.  If you have a ketone concentration higher than 3.0 mm/L, however, you should see your doctor. 

For cheaper, less-invasive ketosis monitoring than blood testing, urine test strips are inexpensive and easy to use.  However, the ketone level detected in urine can be influenced by other factors, such as fluid dynamics and the presence of other metabolites.  Urine ketone tests are less reliable than blood ketone tests, but they nonetheless can give you a good indication of how your diet is going.⁶