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If you’re like most Zero Members, you love watching your progress on the Zero Timer. It’s inspiring to know your body is doing exactly what it needs to do to maintain your energy levels so that you can feel and be your best self.
However, every body is different, and every Member has different goals for their fasting practice. That’s why we’ve developed Personalized Fasting Zones (a.k.a. PFZ): to account for those differences and help you make faster, surer progress toward your unique goals.
Foundational Research & Fasting Zones
In developing Zero, we invested significant time and effort to synthesize the foundational science of human metabolism. Work by renowned pioneers in fasting research, such as Dr. George Carhill, informed our understanding of the physiology of fasting and gave us our traditional Fasting Zones, which are as follows:
- Anabolic Zone (0–4 hours): Your body is digesting and distributing nutrients you just ate to the rest of your body.
- Catabolic Zone (4–16 hours): After burning recently-eaten food, your body begins to use stored fuels, like glycogen.
- Fat Burning Zone (16–24 hours): Your body transitions to a high reliance on fat for energy.
- Ketosis Zone (24–72 hours): Your fat-burning machinery is humming, and you are gradually increasing ketone production.
- Deep Ketosis Zone (72+ hours): Three days into fasting, your ketone levels plateau and your cells work through cleanup, repair, and recycling processes.
These Fasting Zones form a great foundation, but, as you can see, every zone contains a physiological range based on metabolic shifts that occur during fasting in humans. Suffice to say, not everyone’s physiology follows this exact timeline. In fact, there are many personal habits and unique variables that shift Fasting Zones from day to day and from person to person. This reality inspired us to improve upon traditional Fasting Zones to help you gain greater insight into your own metabolism based upon your individual characteristics, nutrition, and physical activities.
Tip: Can’t find PFZ in the Zero app? Next time you start a fast, tap the current Fasting Zone indicator below the Timer and begin entering information about your lifestyle, diet, and body profile.
Individual Traits Impact Fasting Zone Progression
Did you know that, if you’ve been fasting for at least 6 to 8 hours and not exercising, the first place your body looks for glucose (its preferred source of fuel) is glycogen stored in your liver? This is the simplest example of why “one size fits all” doesn’t apply to fasting zones: People’s livers — and their glycogen-storing capacity — vary in size. If you have a large liver or a liver that is capable of storing more glycogen than someone else (all other factors equal, of course), you will take longer than them to move out of the Catabolic Zone and into the Fat Burning Zone.
Liver size and glycogen storage capacity are just two personal characteristics that influence your Fasting Zone progression — and they’re characteristics most people are unlikely to know about themselves. Here’s an example illustrating some of the other, more knowable factors that influence the progression of your fast.
Two people, person A and person B, are matched for height, weight, age, and all other health metrics, except for body composition. Person B happens to have more lean mass. Because of the differential in lean mass, person B will likely have a higher metabolic rate and has the potential to burn through fuel stores — and progress through Fasting Zones — more quickly during a fast if all other variables are equal. If person A wants to lose some weight by decreasing their fat mass, they will probably need to fast for more hours to reach the same Fat Burning Zone target as person B.
This, of course, is a hypothetical comparison. In reality, most if not all of the variables that were matched between persons A and B would not be identical, so it would be a lot harder to assume either of their Fasting Zone progressions.
Personalized Fasting Zones integrates many of these variables so you don’t have to assume. It’s a tool to help you understand your individual Fasting Zone progression by taking into account characteristics like your age and physical attributes. PFZ also empowers you to affect your progression by integrating elements you can more easily control, like the food you eat and your physical activity.
Nutrition and Personalized Fasting Zones
Many researchers and clinicians believe that nutrition is a powerful tool to prevent disease and promote healing. Similarly, many athletes use nutrition to achieve fitness benefits that maximize their athletic performance. Therefore, it stands to reason that we can use what we eat to optimize our progress toward fasting goals.
Your body’s energy stores are like bank accounts; carbohydrates are the deposits. A high-carbohydrate meal boosts your accounts (i.e., stored glycogen) to a greater extent than a low-carbohydrate meal. And when your stores are greater, you can “draw from” or burn glycogen for a longer period of time before you empty your account.
This knowledge can help you make progress toward your fasting goals. For instance, if your pre-fast meal is high in carbohydrates, then you may extend your fast duration in order to deplete glycogen and successfully transition to the Fat Burning Zone. By logging your meals, you provide the inputs necessary for PFZ to improve its accuracy and give you the information you need to make these adjustments.
Physical Activity and Personalized Fasting Zones
Extending the bank account metaphor a little further: Physical activity can accelerate your progression through a fast by drawing down your energy accounts.
When you activate skeletal muscles during exercise, your body can use glycogen stored in your muscle tissue as well as your liver. The rate at which you use that glycogen — which, as we discussed, impacts your Fasting Zone progression — depends upon how and when you activate those muscles.
When you are engaged in any activity, from scheduled exercise to tasks of daily living, your body burns a mix of glucose and fat. During intense exercise your cells, specifically muscle cells, prioritize glucose because it burns and releases energy more quickly. The faster you burn through your glycogen stores, the faster you’ll progress to the next Fasting Zone. Therefore, if you really want to put the pedal to the floor and reach Fat Burning as quickly as possible, place an intense workout early in your fasting window.
Traditional Fasting Zones give us a firm foundation for understanding the unique cellular and metabolic shifts that take place during a fast. Personalized Fasting Zones takes this knowledge one step further, to help you understand how your unique traits and habits impact your metabolism. With this knowledge, you are empowered to go forth and make tangible changes that help you achieve your goals.