*It is also worth mentioning that while fasting is considered safe for many, it is not recommended for everyone. Always check with your doctor before embarking on any new lifestyle or dietary change.
What are the most popular approaches to Intermittent Fasting?
Time-restricted eating: Time-restricted eating involves fasting everyday for 12 hours or longer and eating in the remaining hours. The goal is for the fasting window to be equal to or more than your feeding window. A popular approach to time restricted eating is the 16/8 method, where you fast everyday for 16 hours and eat during the remaining 8-hour feeding window. (2)(3). What does this look like in practice? You finish dinner at 7 pm and fast until 11 am the next morning.
The 5:2 Diet: The 5:2 method calls for eating as you normally would 5 days of the week and fasting for 2 non-consecutive days the rest of the week. On fasting days, eaters restrict caloric intake to 25% of their usual caloric intake, which comes out to roughly 500 calories for women and 600 for men. (3)(4)
What does this look like in practice? You eat normally on Tuesday, Wednesday, Friday, Saturday and Sunday and you restrict your calories/food intake on Monday and Thursday.
Alternate-day fasting. Alternate-day fasting (ADF) is one of the more intense forms of IF, calling for fasting every other day and eating whatever you want on the alternate, non-fasting days. But here’s the rub: a fasting cycle equates to 36 hour periods without caloric intake (“fast days”) followed by a 12 hour eating window (“feast days”).(5) Even if eaters consume food around the clock on feast days, they will still be fasting 36 hours for every 48 hour/2 day cycle. The most common version of this diet allows for a more modified approach to ADF, where you consume approximately 20-25% of your normal energy intake on fasting days, which equates to somewhere around 500 calories on fasting days.
What does this look like in practice?
Day 1: Eat as you normally would and finish eating by 8 pm.
Day 2: Fast or If you’re doing a modified version of ADF, you can consume up to 500 calories.
Day 3: Break your fast at 8 am and Repeat day 1.
Day 4: Repeat day 2. And so on
Eat: Stop: Eat: The Eat: Stop: Eat approach to IF involves fasting for 24 hours one to two times a week and eating normally on the remaining 5-6 days of the week. Most people fast from breakfast to breakfast or lunch to lunch.
What does this look like in practice? Fast from 11 am Monday to 11 am Tuesday, then eat as you normally would until your next 24 hour fasting window, which would begin at 11am on Wednesday and end at 11 am on Thursday. And so on.
Wait? Aren’t we supposed to eat three meals a day and have snacks in between to be fueled, happy and healthy?
Proponents of Intermittent fasting point out that humans are hardwired to fast and that fasting is built into our DNA. Before the advent of agriculture 12,000 years ago, humans lived a hunter-gatherer lifestyle, foraging. fishing and hunting for their food. When food was plentiful, especially during the warmer months, they feasted. When it wasn’t, they fasted, tapping into their stored food energy (fat) to fuel them and keep them alive and well until their next meal. (6) Unlike us, they didn’t have access to food on a continual basis, so they depended on their bodies to provide the necessary fuel by tapping into their stored food (fat) energy.
Today, of course, we have the opposite problem our ancestors had. We have an overabundance of food available to us, all day and all night long. This excess of calories in our food system, most of which shows up in the form of processed and convenience foods, is a driver of food-related chronic disease in our country, including obesity, cardiovascular disease, hypertension, stroke, type 2 diabetes, osteoporosis, Alzheimer’s and certain types of cancer (7). With so much food available to us all the time, many of us are not able to access our stored energy. Proponents of fasting call out IF as a useful tool to liberate our fat stores for energy and improve our health.
How exactly does IF address this problem?
Research shows that limiting our feeding windows (fasting) in a controlled, intentional manner triggers a metabolic switch from glucose-based (sugar) energy to ketone-based energy, allowing us to access our stored food calories for fuel, just like our ancestors did. Every time we eat carbohydrates and protein, the extra food energy is stored in our muscles, liver and fat cells in the form of glycogen [sugar]. When you fast, your liver glycogen stores get depleted first for fuel. It typically takes 12 hours of fasting to deplete your liver glycogen stores and start burning fat for fuel. (8) .
How does Intermittent fasting work?
It all comes down to our biology and hormones. When we eat carbohydrates, our digestive system breaks them down into sugar, which then enters our bloodstream. As our blood sugar levels rise, our pancreas produces insulin, a fat storage hormone, that tells our cells to absorb blood sugar for energy and to store the excess in our liver, muscles, and fat tissue. (9) When we fast, our insulin levels fall because we aren’t consuming carbohydrate foods and our body is able to access and use our stored energy (fat) to fuel our body. This metabolic switch has been shown to improve body weight, body composition, insulin resistance, cholesterol, inflammation, blood pressure, brain function, and reduce the risk for chronic disease, including certain cancers. (10)(11)(12) . Fasting has also been shown to trigger a vital physiological process called autophagy, whereby cells literally eat up their damaged and unnecessary parts in order to regenerate new, healthy cells. This cellular clean-up and renewal process, plays a vital role in disease prevention and longevity. (13)(14)
Long story short, therapeutic fasting triggers a cascade of beneficial reactions in the body that have been shown to improve our health in remarkable ways.
Is it really ok to eat anything you want on non-fast days?
Intermittent fasting should never replace a healthy, balanced diet. In fact, it is most powerful when done in combination with a whole foods approach to eating that emphasizes non-starchy, colorful vegetables, fruit, lean protein, legumes, whole grains and healthy fats like avocado, nuts, seeds, and olive oil. It’s important to remember that what you eat impacts every aspect of your physical and mental health. Not only will a diet high in processed foods, saturated fats, simple carbs, and added sugars likely undo many of the benefits of Intermittent fasting, it can also compromise your health in serious ways and set you up for a host of unwanted health problems.
How do I know if Intermittent Fasting is right for me?
Determining if Intermittent fasting is right for you is a personal decision that should be discussed with your trusted health care practitioner, but you should always make sure to get clear on your goals and reasons for fasting before you start. If you are lean, in good shape, and just want to optimize your already healthy habits, a gentle approach to IF, such as a 12-14 hour fasting window may work best for you. If you are someone who needs to lose a significant amount of weight, a more aggressive approach to IF, done under the supervision of your medical doctor, may work better.
Again, it is always best to reach out to your healthcare provider to determine if Intermittent Fasting is right for you and how to add it to your daily routine in a way that is safe and best for you and your unique body.
Are there people who should not fast?
Skipping meals and drastically reducing calories is not safe for everyone. Individuals who are underweight or have an eating disorder, women who are pregnant or breastfeeding, the elderly, and children and teens under the age of 18 should not fast.
In addition, individuals with certain health conditions, such as diabetes or who take medications may also be vulnerable to health problems resulting from fasting. (15) For this reason, it is always recommended to consult your doctor before making any big changes to your health habits and diet.
