Even after publishing several books and hundreds of articles that draw upon the science of ketosis and low-carb living, I keep researching, thinking, revisiting, and discussing the underpinnings of ketosis. My writing partner, Brad Kearns, and I maintain a running dialogue on all things keto. The latest conversation revolved around two very common questions or “problems” that keep coming up in the ketogenic community: why am I getting low ketone readings?
It’s a fair question. Why do some people on a keto diet register high ketones while others eating the same way register low numbers?
I won’t offer definitive answers fit to etch into stone. I will offer my exploration of the research, some educated speculation, and actionable advice you can ruminate on. And by all means get back to me with your take on the questions and my explorations, please. Dialogue is essential to understanding.
The generally accepted range that indicates ketosis is 0.5-3 mmol/L. I hesitate to define a prescriptive range, though, because so many factors affect readings – what you ate for your most recent meal, how long ago you exercised, and even the instrument you used to test can affect readings.
Why do some people on ketogenic diets produce low ketone readings when they test?
There are several potential explanations for low ketone readings. Some are hypotheses, some are based on your individual biology. A few possible explanations for low ketone readings include:
You have developed fat-burning efficiency
Keto-adapted people may be able to burn free fatty acids
Genetic predisposition to low ketones
Total caloric intake
Use of exogenous ketones
Let’s dig deeper.
You have developed fat-burning efficiency
One theory is that some keto-adapted people are so efficient at producing and burning ketones that they don’t leave any extra to spill into the urine and breath. They make only as many as they can use and their cells gobble up almost every ketone they produce. Under this argument, low ketone numbers on a ketogenic diet are a reliable sign of full ketone adaptation.
This sounds plausible, but I haven’t seen any empirical evidence that it’s the case.
Keto-adapted People May Be Able to Burn Free Fatty Acids in the Bloodstream
Another theory is that the keto-adapted have built so much fat-burning metabolic machinery in their muscles that they can burn free fatty acids directly and don’t require much additional fuel from ketones. They make enough ketones to fuel the brain, since our brain can’t run on fatty acids directly, but your muscles no longer require as many. Many people who have been in long term ketosis can get by quite nicely on 20-30 net grams of carbs a day and might only show .4 or .7 millimolar ketones on a blood test, but they have plenty of energy from burning free fatty acids and maintain muscle mass on relatively fewer calories than when they were dependent on carbs.
The evidence is inconclusive, but a few indications point to increased ability to extract energy directly from fat as a possibility.
Keto pioneers Stephen Phinney and Jeff Volek found that keto dieters blew higher readings early on in the diet when they were still burning ketones in the muscle. As they grew adapted to free fatty acids as a source of fuel and produced ketones primarily for the brain, ketone levels dropped. It was totally normal. If anything, they were more fat- and keto-adapted at lower ketone readings.
Consider the energy requirements of the brain. Whether it’s running on glucose (most of the population) or mostly on ketones, the brain is a steady state organ that never spikes energy demand. It’s a slow-burn 24/7 at nearly the same output whether you are sleeping, exercising hard, or concentrating hard. While the brain has a substantial caloric requirement (at roughly 2% of our body weight, it uses 20% of our resting metabolic rate), you don’t need a ton of glucose or ketones at any one time to power your brain gracefully all day long. That’s why people can “get away” with lower ketone production and still reap the benefits we expect from eating this way.
Genetic Predisposition to Low Ketones
There’s almost certainly a genetic component to ketone production, too. Take the Inuit, who were rarely in ketosis despite traditionally eating a very low-carb diet.1 It takes several days of deep fasting for them to produce measurable ketones. Yet, they are adept at burning free fatty acids, almost as if they “skip” keto-adaptation and proceed directly to burning fat. Other variants that affect ketone production have yet to be discovered, but they’re out there.
Recent Exercise or Otherwise Increased Energy Demand
If you test your ketones before and after you exercise, you may be surprised to find that your numbers are quite different after you’ve worked out. Remember, ketones are an energy source, not a stamp of accomplishment. Ketones are consumed when there is an energy demand in your body. It doesn’t mean you messed up your diet. Quite the opposite – it simply means your body used the energy source available at the time. Once ketone bodies are in low supply, your body will burn fat to make more, and that’s a win.
Why do Some People on Long-term Keto Diets Still Get High Ketone Readings?
A few of the previous theories assume that you develop an increased ability to burn fat after you have been in ketosis for a while. But there are people who, after being in ketosis for a long period of time, still get high ketone readings when they test. What gives?
Total caloric intake is a factor in ketone levels
A major factor not often mentioned in whether someone on a keto diet registers high or low ketones is overall calorie intake. How much food are you eating?
Ketones are generated when the amount of dietary fat available to be burned exceeds the supply of oxaloacetate (provided by protein or carbohydrates). It’s not that the body thinks, “This woman needs some ketones, stat.” It’s more like, “I’ve got too much acetyl-COA from all this fat, and I can’t find any oxaloacetate. Time to ask for ketones!” If you’re the type to use keto to justify chugging olive oil, you’ll generate lots of ketones simply because your fat intake is outpacing the supply of oxaloacetate. Keto athletes eating tons of calories will probably produce more ketones simply because they’re eating so much fat.
If you’ve attained “caloric efficiency” and are eating fewer calories overall, you’ll generate fewer ketones but still experience all of the benefits of being in ketosis.
Use of exogenous ketones result in high ketone readings
Another factor is the use of exogenous ketones. Realistically, you could take your ketone readings, then take keto salts or keto esters, then take your readings again and see a bump. That is what they’re for.
Don’t Get Caught Up in the Numbers
Above all else, focus on how you feel.
Can you go without a meal and maintain steady, even energy and concentration?
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.