You have a wealth of options and different fasting styles to choose from, all of which have different effects on our health, metabolism, and body functioning. Before you choose a type of fasting, ensure you do some reading on each type of fasting’s ultimate physiological effects. Each has pros and cons, and depending on what you want to gain from fasting, different approaches will work better for you. Let’s go through some fasting types, what they mean, and how they are carried out.
Intermittent fasting actually refers to fasting at odd times or on odd days. When intermittent fasting, you may fast every other day, or fast for several days and stop fasting for several days or could even restrict your timing. We will discuss further below many of these methods, but they all fall under the major umbrella of “intermittent fasting” (1).
For those who don’t wish to fast every day, alternate-day fasting can provide a happy medium between fasting and not fasting. People doing this type of fasting eat approximately 25% of their typical caloric consumption on fasting days, then on every other day are able to eat however they like (there are no rules on non-fasting days). This is targeted to reduce weight, boost fat burning, and reduce overall calories consumed. Unfortunately, preliminary clinical studies have not supported this approach as a good weight management tool than a daily caloric restriction (2).
Research suggests it may be a better idea to watch your weight every day, rather than fasting on odd days. However, for those facing insulin-resistance (typical of type II diabetes and some metabolic disorders), alternate-day fasting might reduce resistance, which was supported by clinical research with insulin-resistant patients (3). Alternate-day fasting may also compensate for a high-fat diet, yielding similar weight loss and improvements to cardiovascular health as a low-fat diet when paired with alternate-day fasting (4).
As the name suggests, this type of fasting involves eating during a certain time period, but not during others. Most of us fast unintentionally each day for 8-12 hours while we sleep, but time-restricted fasting increases that time without food to 12-24 hours (5).
The definition is flexible and is dependent on the person’s schedule, health, and preferences. The idea behind time-restricted fasting is by limiting the number of hours in a day you are allowed to eat; you can reduce your overall calorie consumption without feeling like you’re on a diet. By allowing yourself dietary freedom for a certain number of hours each day, you are more likely to sustain the habit, unlike dieting, which often leaves you feeling hungry and unsatisfied.
Fasting mimicking diet
Another type of “fasting” (which, as you can tell by the name, isn’t fasting at all) is a diet that mimics a fast by focusing on certain types of foods and calories. Typically, a fast-mimicking diet is very low in calories (particularly carbohydrates and sugar) and high in unsaturated (healthy) fats (6).
The fat is designed to curb appetite and feed your body. In addition, this is thought to convert your body to focus on burning fat, rather than relying on carbohydrates and sugar to run itself.
This type of fasting has been associated with various health benefits, including boosting the effect of chemotherapy by increasing bone marrow immune cells and other members of our immune system, delaying cancer progression (7).
Fasting-mimicking diets have also been associated with improved insulin production and activity through the reprogramming of pancreatic cells, resulting in the restoration of insulin generation (8). This method of fasting has even been linked to the reduction of multiple sclerosis symptoms (9).
Fasting can alter our gut microbiome, which has a variety of long-term effects as well. In a recent study with mice, those participating in a fasting-mimicking diet had microbiome changes that resulted in some measure of protection against Parkinson’s Disease (10).
It seems fasting has many diverse effects on the way our immune system runs and organizes itself, and the downstream effects of this on multiple diseases are numerous, and we likely have even more to discover as research continues to progress in this area.
The jury is in: Eating no food at all for a long period of time (longer than 3-5 days) is fairly bad for your health (11). While boosting your body’s metabolism and immune system by doing a short term fast appears to have some health benefits, starving your body for several days pushes it into “starvation mode,” which is a survival setting built into us.
When our body enters a starvation state, health problems become more prevalent. As soon as you start eating again, you will gain weight faster because your body anticipates another period of no food and feels the need to store fat away for that future time (12). While short-term fasts may work for some, fasting for 3-5 days or longer is not a good idea, and you should always consult your healthcare professional before you do a fast.
Dry fasting is one of the most dangerous forms of fasting. It means going without both food and liquids (even water). This is dangerous for several reasons, and the most important is dehydration. Dehydration is hard on your body because 60% of your body is made of water!
When you become too dehydrated, your tissues and cells are starved for water, blood vessels start to constrict, your heart struggles to pump, muscles tighten, blood pressure drops to a dangerous rate, and cognition decreases. You feel tired, dizzy, lightheaded, dry, and may vomit and eventually lose consciousness (13).
Liquids are vital for our bodies to function properly, and without it, we put our body in immediate danger. Dehydration can also result in urinary and kidney problems because our kidneys need a lot of water to filter our blood (14). For all of these reasons, the majority of healthcare professionals advise against dry fasting. If you are undergoing a dry fast for medical reasons, consultation with a healthcare team is highly recommended.
3-day and 5-day fasting
Fasting for 3-5 days (but ensuring you drink plenty of water) might have some health benefits without the risks of long-term and dry fasting described above. A recent study showed that fasting may force the body to break down stores of glucose and fat and may also trigger stem cell regeneration.
This would protect against immunosuppression (common in several health conditions and during chemotherapy or anti-rejection medication treatment) and potentially revitalize an aged immune system (15).
A 2014 study showed that short-term fasting’s stem-cell benefits were likely due to the downregulation of a specific stem cell pathway. This resulted in stress resistance and self-renewal of stem cells and reduced immunosuppression and mortality from chemotherapy in mouse studies.
These studies’ long-term implications remain unknown, but it shows that short-term fasting could have drastic positive effects on our immune system (16).