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Curious how it's possible to Fast with Food? Find frequently asked questions here.

What is it and how is it used?

Fast Bar® is an intermittent fasting bar that can help curb your hunger without breaking your fast, so you can fast for longer. It has been clinically shown to keep your body in a fasting state even after eating. You can eat it in the morning after your overnight fast to help curb your hunger and extend your fast until your next meal.  A clinical trial showed that eating a Fast Bar after an overnight fast has a similar impact on ketones and a nearly identical impact on blood glucose as drinking water*.

* Benefits seen in a randomized, controlled study in 105 adults after a 15-hour overnight fast comparing the glucose and ketone results every hour for 4 hours in the Fast Bar group, breakfast group and water-fast group. Click here to read the published scientific article.

You should eat Fast Bar once a day, as we want people to follow a healthy, balanced diet including regular meals during the day. 

You can eat it in the morning after your overnight fast or as a healthy snack anytime during the day, in addition to meals, to help curb your hunger.

The impact and safety of consuming more than one Fast Bar per day hasn’t been studied, and we recommend that you eat only one Fast Bar in the morning or evening, along with a healthy, balanced diet for your other meals. 

Fast Bar is great for supporting Time Restricted Eating and other short fasts.  If you seek the benefits of Prolonged Periodic Fasting, our 5-Day ProLon Fasting Mimicking Diet (FMD) has been extensively tested, has been safely used by hundreds of thousands of generally healthy consumers, and has been clinically tested to provide many benefits, and support overall metabolic health when used in multiple consecutive cycles. For more info on ProLon, go to www.prolonfmd.com.

Fast Bar isn’t intended as a meal replacement.  It’s designed to provide nutrients and to help you feel fuller without breaking your fast. We recommend that you maintain a healthy, balanced diet and use Fast Bar to support your intermittent fasting goals.* 

* Benefits seen in a randomized, controlled study in 105 adults after a 15-hour overnight fast comparing the glucose and ketone results every hour for 4 hours in the Fast Bar group, breakfast group and water-fast group. Click here to read the published scientific article.

The Fast Bar study tested the effects of eating a Fast Bar at breakfast time (after a 15-hour overnight fast), compared to either skipping breakfast or eating a standardized breakfast bar. The study looked at blood glucose and ketone levels over the 4-hour period after breakfast time and showed that blood glucose levels in patients eating Fast Bar were nearly identical to blood glucose levels in patients who skipped breakfast.

Ketone levels over the 4 hours were also very similar in both groups. These results showed that eating a Fast Bar doesn’t interfere with key physiological markers of fasting.  The study also showed that Fast Bar gave people a feeling of being fuller and curbed the desire to eat for up to 3 hours after consumption. In other words, you can eat a Fast Bar to help make fasting easier and more pleasant, and maintain similar ketone and blood glucose levels as a water-only fast.

Yes! Fast Bar is designed with the same technology as the ProLon Fasting Mimicking Diet®. Our technology is the result of over 20 years of preclinical and clinical testing with over 36 million US dollars in research and development funding from the National Institutes of Health and the European Union.

Fast Bar is designed to support intermittent fasting (overnight fasts, and fasts lasting less than two days) as part of an everyday healthy lifestyle. The safety and efficacy of Fast Bar has not been studied in fasting regimes lasting longer than two days. 

If you’re looking for the benefits of prolonged periodic fasting, including support of weight loss and fat loss, metabolic health, or a jumpstart to lifestyle transformation, we recommend that you try the 5-Day ProLon Fasting Mimicking Diet.

Fast Bar’s ratio of fats to carbohydrates + protein = 1.4:1. That’s 1.4 grams of fat for every 1 combined gram of carbs and protein, which fits nicely into a modified Keto diet. 

In our studies, ketones were measured over the 4-hour period following consumption of a Fast Bar, and the level of ketones were no different than those in participants who continued an overnight fast with an additional 4 hours of water-only fasting and no Fast Bar.

How does it work?

Scientists used to think that calorie balance (i.e.: calories consumed minus calories burned) was the most important factor in weight loss. But scientists now widely accept that not all calories are the same. Factors such as macronutrient ratios, total amounts, how the nutrients are digested and absorbed, and molecular composition all have an impact on how the body processes food.  

The Fasting Mimicking Diet (FMD) and Fast Bar have been carefully engineered, extensively researched, and clinically tested to allow for the consumption of nutrients and calories but still keep cells in a fasting state. Our special patented formula effectively hides from the body’s nutrient sensors, preventing them from sending the hormone signal that typically is sent after eating. The cells respond to the decrease in this hormone signal by entering a fasting state. You can learn more about the science of fasting mimicking here: https://fastbar.com/pages/science-of-fasting

Fasting has transformational power and phenomenal potential health benefits.  However, many people struggle with hunger and cravings when trying to fast. Fast Bar offers an alternative that’s more pleasant and easier to stick with over time. Some studies have also raised concerns that skipping breakfast may not be a healthy practice, so Fast Bar offers a way to consume food at breakfast time, while continuing to keep your cells in a fasting state on the molecular level. And, when compared with water-only fasting, eating Fast Bar after a 15-hour overnight fast had a similar impact on ketones and a nearly identical impact on blood glucose as continuing a water-only fast.* 

* Benefits seen in a randomized, controlled study in 105 adults after a 15-hour overnight fast comparing the glucose and ketone results every hour for 4 hours in the Fast Bar group, breakfast group and water-fast group.  Click here to read the published scientific article.

Though Fast Bar contains about 5 grams of sugar, it’s specifically formulated with plant-based fats and fibers that help to slow the absorption of sugars and reduce the glycemic index. The sugar in Fast Bar comes from honey, a natural and nutritious food. Thus, even though there’s a small amount of sugar in Fast Bar, its design and composition keeps the body in fasting mode because it doesn’t trigger a significant spike in blood glucose. 

According to the colloquial definition, ‘fasting’ means ‘not eating’.  However, it’s important to consider what actually happens inside you during fasting that makes it different than eating. Your body notices food through nutrient-sensing pathways, and when you don’t eat, these same pathways tell your body that you’re in a fasting state.  This is what triggers fasting at a cellular level, and it’s what we mean when we talk about mimicking a fast – it’s possible to trick these sensors into thinking you’re in a fast while still providing the body with nourishment.  This is the science behind the Fasting Mimicking Diet and Fast Bar. This is the nutritional breakthrough which enables consumption of specific foods while still maintaining a fasting state.

Every cell in your body has tools that sense if there are nutrients available. Normally when you eat, the body’s nutrient-sensing pathways are triggered, and they send the cells a signal to grow. Fasting turns off the nutrient-sensing pathways for a while, allowing cells to rest, clean themselves, and rejuvenate. Fasting Mimicking Diets and Fast Bar have been designed and shown to help “trick” these nutrient-sensing pathways, triggering the same fasting state in your cells.  

There are many things that can trigger glucose and ketone changes in the body, but they don’t all mean that your fast has been disrupted on a physiologic level. More importantly, the nutrients in Fast Bar provide some relief to the body from the stress of fasting while maintaining a fasting state. Perhaps most importantly, blood glucose and ketones are only part of the story when it comes to fasting, and the Fast Bar has been designed and tested to follow the Fasting Mimicking Diet’s science, which has demonstrated the ability to trigger a cellular fasting state

Your body fuels itself while fasting by tapping into its internal energy reserves including fat stores and stored energy in your liver and other organs. This one of the reasons why there’s weight loss and fat loss when fasting. Fast Bars are low in sugar and protein, and are designed so that their energy is released slowly in the body. This allows your cells to keep the nutrient sensing pathways in the ‘fasting’ state and draw on fat from the diet and from your body’s fat stores for energy. 

For those of you who are into biohacking, think of Fast Bar as the ultimate bio-hack – fueling your body so that you don’t get hangry and your cells get nourishment, while staying in a fasting state!


One of the main ingredients in Fast Bar are nuts, which satisfy hunger and have many health advantages. They’re highly nutritious, loaded with antioxidants, and have been shown in many studies to promote weight loss. You can find a list of all the healthy nuts and other ingredients used in Fast Bar here: nutrition information.

Fast Bar is vegetarian, not vegan. Our bars contain plant-based protein and are made with honey. 

L-Nutra is working to ensure that all of our products are and remain GMO- free.

Fast Bars are gluten-free. However, they are manufactured on equipment that also processes wheat, so we recommend you review the list of ingredients here: nutrition information. 

L-Nutra is concerned about cross-contamination and we take the quality of our products seriously. As such, we meet or exceed current Good Manufacturing Practices (cGMPs) as well as applicable regulatory standards. Please read our nutrition information for more on potential food allergens.

Fast Bar is not formulated nor designed with soy ingredients.  However, they are manufactured on equipment that also process soy. Importantly, these facilities follow or exceed the current Good Manufacturing Practices (cGMPs) which ensure there are allergen control programs in     place. Please read our nutrition information for more on potential food allergens.

Fast Bars are dairy-free. However, they are manufactured on equipment that also processes milk or products containing milk, so we recommend you review the list of ingredients here: nutrition information.

Each bar contains 200 – 210 calories (equivalent to one serving).

Fast Bar is high in plant-based fat because it contains a whole serving of nuts. Nuts are an important source of healthy fats, which slow the absorption of the sugar to keep the glycemic index of the bar low and not break your overnight fast.*  The low glycemic index of Fast Bar was independently tested and confirmed.

* Benefits seen in a randomized, controlled study in 105 adults after a 15-hour overnight fast comparing the glucose and ketone results every hour for 4 hours in the Fast Bar group, breakfast group and water-fast group. Click here to read the published scientific article.

Fast Bar was designed with the same technology as ProLon’s L-Bars, but there are slight differences in size, calorie count and overall sugar content. Due to the carefully formulated macro and micro-nutrient ratios of the individual bars, they’re not intended to be interchangeable and should only be used as directed.

The information regarding the natural flavors in our recipe is proprietary, which allows us to keep our product delicious and unique. All ingredients in Fast Bar are Generally Recognized as Safe under the FDA.

It’s unlikely that the bar causes constipation. Constipation arises for many reasons, including dehydration, medication use, and lack of physical activity. The small amount of fiber in Fast Bar can actually help alleviate constipation when caused by insufficient dietary fiber. If your symptoms are severe or don’t resolve on their own, please talk to your medical provider. 

Fast Bar can be used as an occasional healthy, convenient snack by anyone, including during pregnancy and breastfeeding. However, to be clear, we don’t recommend any type of fasting during pregnancy or breastfeeding. Pregnancy and breastfeeding have special nutrient requirements and fasting of any kind may take away from the opportunity to consume the required nutrients.

We’re exploring different, potential organic ingredient formulations to ensure that we keep delivering the quality and deliciousness our consumers expect and deserve! We share all consumer feedback with our Marketing, and Research and Development departments, and many of our innovations are the result of consumer feedback. If you feel strongly about organic ingredients, or have any other concerns, please reach out and let us know!

Intermittent Fasting (IF)

Intermittent fasting (IF) can be defined as fasting for less than 2 days. Time-restricted eating (e.g.: overnight fasting for 12-16 hours), the 5:2 diet, and alternate day fasting are all included under the umbrella term of IF (see also the TRE question and answer below). While there are a variety of fasting methods within the category of IF, they are all based on eating patterns that alternate between intervals of eating and fasting.

People practice intermittent fasting for many reasons, including for weight loss and other potential health benefits. 

In terms of weight loss, by shortening the window of time during which food is consumed, generally fewer calories are consumed overall which can result in  weight loss which varies by individual. There are also other potential health benefits associated with synchronizing the fasting window to the sleep cycle (i.e.: not eating after dinner).

Time Restricted Eating is the most popular form of intermittent fasting. With this type of fast, eating is limited to a specific window of time each day. By shortening the window of time during which food is consumed, fewer calories are consumed overall which can result in weight loss. The specific period of the day dedicated to fasting can also influence health, and many studies have shown that avoiding food intake in the hours immediately before bed may have cardiovascular and other benefits. Because TRE focuses on when you eat as opposed to what you eat, it can be paired with other food-specific diets for enhanced results.

One way of classifying TRE is based on the number of hours spent fasting and then the balance of hours during which eating is allowed. 12:12 indicates a 12 hour fasting window and a 12 hour eating window.  16:8 indicates a 16 hour fasting window and an 8 hour eating window. At this point, there’s no conclusive data on the optimal number of hours which should be spent fasting daily, and recommendations should be tailored to the individual based on their health goals and nutrition challenges. 

TRE diets can also follow Circadian Rhythms. The Circadian clock is the body’s imprinted clock that keeps track of day and night through release of different hormones and activation/suppression of different molecular signaling pathways. Its discovery was awarded the Nobel Prize in Medicine in 2017. Circadian TRE times the periods of food intake during the day, and the fasting periods during the night. The rationale is that consuming food during the day, when energy requirements are higher, allows the body to burn the energy it ingests right away. And fasting after dinner and overnight helps keep the energy availability low during the night, when energy needs are lower and excess energy is more likely to be stored in the form of fat. An example of a 12:12 Circadian TRE schedule is to fast every night between 8pm and 8am and allow eating during the 8am to 8pm window. 

Aside from TRE, the common types of IF are OMAD (one meal a day), the 5:2 Diet, and Alternate Day Fasting (ADF). Whereas TRE is easy to fit into a regular lifestyle with minor behavior adaptations, the other forms of IF (OMAD, 5:2, and ADF) typically require drastic changes in eating patterns. What we’re saying is, they’re not for the faint-hearted! The data is inconclusive on the long-term health implications of these other types of IF. 

• OMAD, also known as ‘The Warrior Diet’, typically requires a ramp-up period of TRE where the fasting window is gradually lengthened from 12 or 16 all the way up to 20+ hours.

• The 5:2 diet entails fasting during specific days of the week. For this approach, individuals either eliminate or severely restrict their calories for two (non-consecutive) days of the week and then resume their normal diet for the other five days. 

• ADF entails fasting every other day and eating on the alternate days. 

•Some modified IF approaches allow consumption of up to 500 calories for women, and up to 600 calories for men on the fasting days. 

One potential challenge with IF is meeting nutritional requirements during restricted eating windows, especially with OMAD where the eating window can be as short as 1-2 hours a day. With all forms of IF, and with OMAD in particular, it’s important to remember that not all calories are equal. Choosing healthy, low glycemic index foods for any calories consumed during the fasting window can be helpful in maintaining health while practicing IF. For people who are underweight or have specific nutritional deficiencies, it’s especially important to speak to your health care provider about this particular challenge before contemplating starting an IF, or any other fasting regimen.

Over-eating after completing a fasting window is also a potential challenge, so it’s important to remember to refeed your body with food that will best support your overall health goals, keeping in mind both the amount and quality of the food you use for refeeding. Generally, if you stick to healthy foods and eat mindfully, your body will signal you when you’ve had enough. However, if you struggle with binge eating, it’s especially important to speak to your health care provider about this particular challenge before contemplating IF or fasting of any kind.

In general, you should keep in mind that the refeeding period is a critical aspect of your fasting journey. During the refeeding period, your body is rebuilding its nutrient stores and renewing cellular components that it tapped into during the fast, so giving yourself the right amount of healthy nourishment is key. 

Although fasting for short periods is safe for most generally healthy people, if you have health concerns, you should talk to your healthcare provider before beginning any diet to ensure that it’s safe and right for you.

Don’t fast or use any products for fasting purposes without a doctor or healthcare provider’s direct supervision if you have any pre-existing medical conditions or are taking any prescription medications.

Don’t fast or use any products for fasting purposes if:

• You have a fever, cough, diarrhea, signs of an active infection, or are at risk for recurrent infection,
• You’re underweight, breastfeeding, or pregnant,
• You have dietary restrictions,
• You have a known, suspected, or diagnosed eating disorder, or
• You’re under the age of 18 or over the age of 70, unless you are under the supervision of a healthcare provider.

The difference between intermittent fasting (IF) and periodic or prolonged fasts (PFs) like ProLon is that IF involves fasting intervals anywhere from 12 to 24 hours, while PF generally refers to fasts lasting longer than 48 hours. With both IF and PF, the body typically taps the energy from glucose consumed during the most recent meal in the first 4-8 hours of the fast, and then crosses the threshold into a state in which it uses its own internal energy stores instead. However, with IF, the body doesn’t typically have time to get into the more advanced stages of fasting. In the more advanced stages of fasting reached during PF, the main source of fuel switches to fat, which is part of the reason PF is such an effective way to lose inches off your waistline. And, during the more advanced stages of fasting, cells may increase digestion of older, worn-out components, and become more stress-resistant.