By Franziska Spritzler, RD, CDE
The ketogenic diet is often considered an extremely strict, low-carb, high-fat way of eating. The truth is that a ketogenic lifestyle is much more flexible than this. It’s not a rigid diet but rather a customizable approach to eating that can be adapted to fit your unique needs, lifestyle, and health goals while ensuring your ketone levels are optimal for you.
Note: When we say that a ketogenic lifestyle is flexible and customizable, we’re speaking about using ketogenic approaches for losing weight, improving metabolic health, and enhancing well-being. Ketogenic interventions for neurological disease, cancer, or other therapeutic purposes may require a stricter approach in order to achieve higher ketone levels, along with close monitoring and guidance from a knowledgeable healthcare provider, dietitian, or nutritionist.
The Metabolic Shift: How Ketosis Begins
The goal of a ketogenic diet is to achieve and maintain ketosis, a metabolic state where your body switches from using glucose to using fat and ketones as its main fuel. Being in ketosis offers several potential benefits, including weight loss, lower blood glucose levels, reduction of cardiovascular disease risk factors, and improvement in neurological disorders like epilepsy. Fortunately, shifting from a glucose-based metabolism to a fat-based one usually takes less than a week.
When your carbohydrate intake is very low, your body’s glucose supply drops, prompting your liver to use its glycogen stores for energy. These glycogen stores are depleted within approximately 24 to 48 hours, depending on physical activity and other factors. When your glycogen stores are significantly reduced, your liver begins converting fatty acids into ketones, which your brain, heart, kidneys, and muscles can use as an energy source.
There are three types of ketones, also known as ketone bodies: beta-hydroxybutyrate, which can be measured in blood; acetoacetate, which can be measured in urine; and acetone, which can be measured in breath. Measuring beta-hydroxybutyrate (BHB) in the blood is considered the most accurate way to assess ketone levels.
Nutritional ketosis, also known as euketonemia, begins when BHB levels reach 0.5 mmol/L and continues up to 5 mmol/L. Your optimal ketone level within that range can vary based on specific health conditions and individual responses.
Different Strategies to Achieve and Maintain Ketosis
Getting into ketosis and staying there isn’t one-size-fits-all. Long-term success comes from finding an approach that suits you based on your preferences, eating frequency, physical activity, and the amount of carbs your body can handle while staying in ketosis.
Let’s explore several methods that you can use to enter nutritional ketosis and remain there.
Fasting
Abstaining from food and calorie-containing beverages for several hours can speed up the depletion of liver glycogen, prompting the body to switch to burning fat for energy and begin producing more ketones. Indeed, fasting is sometimes used in patients with epilepsy to expedite the transition to ketosis prior to beginning a ketogenic diet.
Intermittent fasting includes non-eating periods ranging from 14 hours to 40 hours, whereas extended fasting involves abstaining from food for more than two days in a row.
Research suggests that fasting for 14 hours combined with mild calorie restriction may result in mild ketosis. But although intermittent fasting can raise your ketone levels, it may not enable you to reach or sustain ketosis on its own, depending on what you eat at meals, how active you are, and your level of metabolic flexibility (how easily your body can switch between different fuel sources). Some people can achieve ketosis quickly through intermittent fasting. However, research indicates that those who are not following a low-carb diet may need to fast for up to 72 hours (three days) to reach nutritional ketosis.
Going on an extended fast for several days guarantees that nutritional ketosis will occur. In one study, 1,610 people underwent a fasting program supplemented with vegetable soup for 4 to 21 days under medical supervision. While this regimen reliably induced ketosis, as confirmed by urine ketone strips, individual ketone levels varied significantly depending on factors such as age, gender, overall health, physical activity, and whether participants consumed small amounts of fruit juice and honey in addition to soup.
Of course, extended fasts can’t be sustained indefinitely. However, many people find that combining intermittent fasting with carbohydrate reduction––for instance, eating two low-carb meals within an 8-hour window, or fasting for 24 hours once or twice a week and eating low-carb on other days––makes for an easy, pleasurable ketogenic lifestyle.
Different Levels of Carbohydrate Reduction
Consuming a high-fat, very low-carb ketogenic diet allows you to get into ketosis and stay there consistently. There are several types of ketogenic diets with varying degrees of strictness that can help people achieve specific ketone goals and health outcomes:
The classic ketogenic diet and modified ketogenic diet are typically used to treat epilepsy and certain cancers and neurological disorders. These diets are based on a ratio of fat to protein plus carbohydrates that ranges from 1:1 to 4:1. A person who follows a classic ketogenic diet with a 3:1 or 4:1 ratio consumes 3 or 4 times more fat than the combined amount of protein and carbs. Someone following a modified ketogenic diet consumes 1 to 2.75 times more fat than the combined amount of protein and carbs. The 4:1 ketogenic diet is typically only used in children because this ratio makes it difficult for adults to meet their protein requirements without exceeding their energy needs. The macronutrient distribution range is approximately 75-90% fat, 6-20% protein, and 4-8% carbs as a percentage of calories.
The modified Atkins diet (MAD) is typically used for the same therapeutic purposes as the classic and modified ketogenic diets. But instead of using ratios, net carbs (total carbs minus fiber) are limited to 10-15 grams per day, and protein isn’t restricted. The macronutrient distribution range is approximately 65-75% fat, 25-30% protein, and 5% carbs as a percentage of calories.
A well-formulated ketogenic diet provides less than 20 grams of net carbs (or less than 30-50 grams of total carbs) and 1.2 to 2 grams of protein per kilogram of body weight per day, with the remainder of calories coming from fat. This approach is typically used for weight loss, type 2 diabetes, and polycystic ovary syndrome (PCOS). The macronutrient distribution range is approximately 70-75% fat, 15-25% protein, and 5-10% carbs as a percentage of calories.
While there are standard guidelines for these approaches––and these may be important to adhere to when used for therapeutic purposes––the level of carb reduction needed to reach and stay in the desired level of ketosis can vary from person to person. Many people have experienced this firsthand when comparing their progress with others.
There’s some research that shows this as well. In one trial, all participants who consumed 5% of their calories from carbs consistently achieved BHB levels within the nutritional ketosis range. However, some participants who consumed 15% of their calories from carbs also achieved and maintained ketosis during the 3-week study, and a few who consumed 25% of their calories from carbs were occasionally in light nutritional ketosis.
Exercise
Physical activity, particularly endurance or high-intensity exercise, can enhance fat-burning and promote ketosis. However, high carb intake suppresses ketone production, which can prevent nutritional ketosis even after vigorous exercise. To consistently stay in nutritional ketosis, it may be most effective to combine physical activity with carbohydrate reduction (not necessarily a “strict” ketogenic diet) or intermittent fasting.
In a meta-analysis of randomized trials, overweight individuals who combined a ketogenic diet with vigorous exercise several times a week achieved ketosis (as measured in blood or urine), lost weight, reduced their waist size, and lowered their triglycerides.
One thing to be aware of is that your ketone levels can be influenced by exercise because the body uses BHB as fuel during physical activity. As a result, testing BHB levels immediately after exercise may show lower values. However, this doesn’t indicate a lack of fat burning; it simply means that BHB was consumed as an energy source during the workout. Several hours after intensive exercise, ketone levels typically rise above baseline.
Measuring Ketones to Customize Your Keto Experience
Ketogenic lifestyles are the only approaches that can be accurately measured and monitored with specific biomarkers. Ketone levels are the key indicator of ketosis, providing valuable insight into how your metabolism is working. By using a blood ketone meter once or twice daily during the first week after making dietary changes, you can confirm that you’re in ketosis when your readings reach 0.5 mmol/L. As you continue to monitor your levels, you’ll see how they fluctuate based on your meals, physical activity, sleep, and stress. You’ll also discover at which ketone level you feel your best and what it takes to stay within that range.
Rather than needing to limit your net carbs to less than 20 grams per day, you may find that you can eat 30 or 40 grams of net carbs daily and remain in your ideal ketosis range. Your ability to stay in ketosis may also be impacted by certain low-carb foods you consume, depending on how your body responds to them. Or you might discover that you can handle carbs better earlier in the day than later, or vice versa, based on your bio-individuality. Another possibility is that you learn that staying in ketosis works best for you when you combine low-carb eating with intermittent fasting or high-intensity workouts several times a week.
By testing your ketones, you can adjust your diet or activity levels to align with your specific goals—whether it’s light ketosis for weight loss and type 2 diabetes reversal, or deeper ketosis for therapeutic purposes, such as an adjunct to cancer treatment.
Measuring and monitoring your levels of ketosis can help you customize your entire lifestyle, from your exercise routine to your macronutrient intake to your fasting window to your sleep habits. This process is called “biohacking,” where the insights gained from tracking your ketone levels enable you to create a ketogenic lifestyle that’s tailored to your specific needs and goals.
Ultimately, measuring ketones is about more than hitting target numbers—it’s about gaining self-awareness and individualizing your health journey.
Key Takeaways
A ketogenic lifestyle that can be sustained long term isn’t about following a strict set of rules, such as keeping your daily carb intake below 20 grams. It’s a flexible, adaptable approach to nutrition that can be personalized to fit your life and health goals. Start by measuring your ketones and learning how your body responds to different strategies, and continue to evaluate and fine-tune your approach as often as you need to.
