How Does Ketosis Work?



Whether or not you’re following the ketogenic diet, you’ve probably heard the word “ketosis” floating around recently.


You might have a vague idea of what it means, but let’s find out how ketosis works.  


What is ketosis?


First, in order to understand how it works, we need to know what ketosis is. Essentially, nutritional ketosis is a metabolic state that occurs when your body is required to switch from carbohydrates to fat to use for energy.


If you’re following the Standard American Diet (or another eating pattern high in carbohydrates), you shouldn’t experience ketosis (unless you are diabetic and very ill).


A diet high in carbohydrates provides plenty of glucose that can be broken down for energy or stored in the liver and muscles.  


Graphic showing the recommended percentage of protein, fat, and carbs on the ketogenic diet


How does ketosis work?


Meanwhile, when following the ketogenic diet, your carbohydrate intake is severely restricted, and what glucose is available is typically used to provide the brain with energy.


At first, carbohydrates stored in the body (as glycogen) can be broken down to glucose by the liver and provided to the organs and tissues that need energy the most urgently.


Once the body glycogen stores have been exhausted, fatty acids in the diet are broken down to triglycerides, a process that releases ketones (acids) as a byproduct. Ketones can circulate in the blood to allocate energy – which is extremely important since some organs (including the brain) cannot break down fats for energy (but they can use ketones).


As you can see, this is why the ketogenic diet is so high in fat – in order to provide as many fatty acids as possible to be used for energy. Once they have served their purpose, ketones are removed from the body in the urine.


Ketogenic diet friendly foots - such as avocado and raw meat - on a table


Once the body has used up all the circulating fatty acids from the diet for energy, it still needs a constant source of fuel – so body fat is targeted. Stored body fat can be broken down to fatty acids and ketones, just like fat from the diet.


This is a major reason why weight loss on the ketogenic diet tends to be rapid once ketosis is achieved (but this should not be confused with the stored carbohydrate and water weight loss during the first week).


Are there different levels of ketosis?


Yes! Although people tend to discuss ketosis like it’s an “on” or “off” switch, there are actually different levels, and you have two main options for determining the extent of ketosis – urine and blood testing. Breath ketone tests are also available, but much less common.


Blood ketone testing


In order to get a precise measurement of circulating ketones, testing your blood is typically much more reliable. After entering ketosis, most people will test their blood on a weekly basis to ensure that their levels are still appropriate to keep them in ketosis.


Blood ketone testing kits are simple to use, just like blood glucose testing done by diabetics – a small drop of blood and a meter. Blood tests measure one of the most common ketones – beta-hydroxybutyrate (BHB), in order to estimate total circulating ketone levels.


In general, BHB levels of 0.5 to 3.0 mmol/L may indicate ketosis, but this can vary somewhat from person to person. Guidelines for interpreting blood ketone testing results are typically:

  • Normal (not in ketosis) – up to 0.6 mmol/L;
  • “Medium” ketosis – 0.6 to 1.5 mmol/L;
  • “High” or deep ketosis – 1.5 to 3.0 mmol/L; and
  • Diabetic ketoacidosis – above 3.0 mmol/L.


Graph showing the optimal levels of ketosis


BHB levels above 3.0 mmol/L do not provide any additional benefits while on the ketogenic diet, and they have the potential to be dangerous. Most ketogenic dieters are unable to reach ketone levels this high, and if they do, it may indicate a medical problem such as diabetes.


Very high ketone levels (in conjunction with high blood glucose levels) can suggest the presence of a complication of diabetes known as diabetic ketoacidosis (DKA). Diabetics can develop DKA if their blood glucose levels remain extremely high for an extended period, which in turn causes the release of ketones rather than insulin.


DKA should be avoided at all costs, as very high ketone levels can alter the tightly-regulated pH of the blood, making it more acidic.


Urine ketone testing


Many keto dieters test their urine for ketones using testing strips. Although these can be the inexpensive and easy option, their accuracy is sometimes questionable.


Urine testing may be most helpful during the first few weeks after starting the ketogenic diet and you are still working toward achieving ketosis. You can test your urine as frequently as you like, and the test kits are simple to use.


Interpretation of urine testing results may vary across different types of test strips, but many will have color-coded legends that can be matched up to the test strip.


Standard ranges for urine testing strips may include:

  • Negative (not in ketosis) – less than 5 mg/dL (0.5 mmol/L);
  • Trace – 5 mg/dL (0.5 mmol/L);
  • Small – 15 mg/dL (1.5 mmol/L);
  • Moderate – 40 mg/dL (4.0 mmol/L); and
  • High – above 80 mg/dL (8.0 mmol/L).


A similar range to that found in blood ketone testing is favorable for nutritional ketosis, falling somewhere between small and moderate. Likewise, excessively high ketone levels may raise concern about potential DKA.


Now that you understand how ketosis works, it's easy to see there's a big difference between a diet that is simply low carb vs a ketogenic diet.


Therefore, make sure your diet is well planned and consider working with a dietitian or health care professional to avoid nutritional deficiencies.


For a comprehensive guide on what you can eat (and what to avoid), check out our printable keto foods list.