Ready to break away from diet culture, but want some structure to change up your eating habits? Intermittent fasting (IF) isn’t like other fad diets that restrict what you can and can’t eat. Instead, it focuses on when food *should* be eaten to reap certain benefits.
Here’s what you need to know before you start intermittent fasting and how to choose the right plan.
Back in the day, refrigerators and grocery stores were nonexistent. This led our hunter-gatherer ancestors to unintentionally fast until they were able to find their next source of food.
To sum things up, IF is a way of eating where you flip flop between periods of time when you’re chowing down and when you’re not.
Since humans were capable of functioning without food for long periods of time, fasting now and again may be more natural than eating three to four-plus meals a day.
You may even see fasting done for religious or spiritual reasons in Islam, Christianity, Judaism, and Buddhism.
Take your pick from the handful of IF options — some more extreme than others.
This method is good for beginners to give a whirl. Fast for 12 hours each day, which can include when you snooze. Once your evening meal is over and you tuck in at night, your body fasts until breakfast arrives.
The easiest way to do the 12-hour fast is to include your sleeping time into your fasting window. For example, stop eating at 8 p.m. and wait until 8 a.m. to have breakfast.
You might even already be doing the 12-hour fast without even knowing!
Similar to the 12/12 method, the 16/8 method extends your fasting period to 16 hours.
This method could be a bit difficult for breakfast lovers. But people who tend to skip breakfast may already (though unintentionally) fast this long. Depending on how late you eat in the evening, or how early you eat lunch, skipping breakfast alone may give you a 16-hour fasting period.
One perk is that you’re able to drink a cup of coffee (without sugar or cream), water, or other zero-calorie beverages during your fasting period.
If you opt for the 5:2 fasting method, you’ll eat normally 5 days per week and then reduce your calories for the other 2 days.
On reduced days, men usually consume around 600 calories and women eat 500 calories. Most people choose to separate their modified fasting days so there is always a nonfasting day in between. For example, you could choose to “fast” on Monday and Thursday.
There has been a lot more research on this method compared to other forms of IF. A 2020 randomized controlled study found that it’s a beneficial method for weight loss without the addition of exercise.
This large, yearlong study also found that the 5:2 method of IF can be as effective as traditional dieting for weight loss.
This IF method requires a full 24-hour fast one or two times per week. An example is to stop eating at 7 p.m. on Monday and then fast until 7 p.m. on Tuesday, completing the full 24-hour cycle. You’re also allowed zero-calorie beverages during the fasting period.
Fasting for a full 24 hours can be difficult for many people, leading to the inability to stick with this eating style. The best option is to start with a fast that lasts 12 to 16 hours and work your way up.
This option is one step further than eat-stop-eat and includes a 24-hour fast every other day.
There are different versions of this method, with some sticking to strict no solid foods for 36-hours and others allowing around 500 calories on fasting days.
True alternate-day fasting leads to fasts of about 36 hours because you’re not eating for a full calendar day — so you’re fasting overnight, all day, and overnight again.
Again, this method is pretty extreme and can be difficult to continue long term. Any fasting beginners or individuals with certain medical conditions should also skip this method.
Research has also shown it may not even be necessary to fast this strictly to reap benefits. A 2017 study found that weight loss, weight maintenance, cardiovascular protection, and adherence was not better with alternate-day fasting compared to calorie restriction (75 percent of energy needs every day).
Also referred to as The Warrior Diet, this form of fasting isn’t for beginners since it can be quite extreme (but still not as bad as true alternate-day fasting).
Basically, you fast for 20 hours in the day, with the option to eat small amounts of fruits and vegetables, then have one large meal within a 4-hour window in the evening.
Supporters of this method claim humans are natural nocturnal eaters. It’s not for everyone though. People may struggle with eating such a large meal close to bedtime or even sticking to the strict guidelines.
Fasting makes your body go through changes on both a cellular and molecular level. Some specific changes may include:
Human growth hormone (HGH)
Our pituitary gland releases HGH which is beneficial for regulating body fluids, bone and muscle growth, fat and sugar metabolism, and even heart function. This hormone naturally decreases as we age and can be unnaturally low due to certain medical conditions.
A 2012 study found that a 24-hour fast increases total growth hormone levels in young, healthy adults.
The downsides? Continuously high HGH levels can cause bone weakness, fatigue, or decline in muscle mass.
This hormone regulates the amount of glucose (aka sugar) found in our blood by helping it enter our cells for fuel. When our body is resistant to insulin, glucose can’t get into the cells as easily, causing a buildup of glucose in our bloodstream and potentially leading to type 2 diabetes.
A 2019 study discusses the effectiveness of IF on insulin resistance due to its ability to reduce BMI. Individuals taking insulin or medication for type 2 diabetes should be wary of fasting as it can lead to low blood sugar.
If you’re taking these medications or have diabetes, it’s a good idea to chat with your medical provider before fasting.
Sometimes there are old and dysfunctional proteins hanging around in cells that our body digests and removes in a process called autophagy.
A 2018 article discusses the ability of intermittent fasting to activate autophagy in organs and cultured cells, beneficial for enhancing cancer treatments.
Unfortunately, autophagy can be a double-edged sword. The same article discusses that autophagy can be beneficial for suppressing some tumors but can also promote them depending on the stage and type of tumor.
Our genes or DNA hold instructions to create protein or other molecules and are responsible for most of what goes on in our body. Intermittent fasting can alter whether certain genes are stimulated or suppressed.
A 2020 study found that 30 days of a 14+ hour daily fast resulted in changes to genes that could promote health and longevity. For example, the APP gene was reduced, which creates amyloid plaques often seen in Alzheimer’s disease.
Much of this research is short term, with more long-term research needed.
If you’re looking into IF to help you shed a few pounds, you’re likely to see a drop on the scale due to the decreased number of calories eaten over the week. This is only true if you don’t overeat during the eating periods to compensate for the fasting periods.
A 2018 systematic review and meta-analysis examined how effective intermittent fasting was for adults with overweight or obesity trying to lose weight. The various IF methods researched included alternate-day fasting and fasting for 2 to 4 days per week.
The results found that intermittent fasting results were effective, but comparable to groups doing a continuous energy restriction (decrease of about 25 percent of recommended energy intake each day).
Therefore, the overall reduction of calories is what promotes weight loss, not limiting your eating timeframe.
In the end, there is no one weight loss diet that works for everyone. A 2020 review discusses how intermittent fasting is a recommended option to promote weight loss, but only if you’re able to stick with it long-term. Do what works for you!
Beyond the promotion of weight loss, what else does intermittent fasting have to offer? Research has shown other benefits may include:
Inflammation is normal, but too much inflammation can lead to conditions like cancer and heart disease. C-reactive protein (CRP) elevates when inflammation is lingering somewhere in our body.
A 2019 study found that men completing a 16:8 fasting eating pattern for 29 consecutive days led to a significant decrease in CRP levels which is associated with less inflammation.
Cardiovascular disease is the No. 1 cause of death across the world, with most deaths coming from heart attacks and strokes. Intermittent fasting can actually limit a lot of the risk factors that lead to the development of cardiovascular disease.
IF may reduce plaque buildup, prevent high blood pressure, and limit cardiac hypertrophy (enlargement or thickening of the heart muscle) according to a 2019 overview.
The second leading cause of death, cancer, is typically treated by either chemotherapy, radiation, or surgery. A 2016 review concludes that periodic fasting and fasting-mimicking diets without calorie restriction may promote disease prevention and enhance disease treatment when it comes to cancer.
A fair warning from a 2019 article is that intermittent fasting should be taken with extreme caution in the oncology world. That’s because up to 80 percent of people with cancer may be dealing with malnutrition, and the unintentional restriction of calories or other important macronutrients may create further issues.
Our brain is one complicated organ, controlling pretty much our every move. When it comes to fasting, a 2017 study found that a small group of women fasting during Ramadan had increased levels of serotonin, brain-derived neurotrophic factor, and nerve growth factor.
These are all essential in brain health by helping grow and maintain certain neurons and keeping our memory sharp.
Many wonder whether intermittent fasting may add a few more years to your life. A 2014 review found a 30 percent dietary restriction increased the lifespan of rodents mainly due to delaying the onset of chronic diseases.
The problem here? These studies are animal-based which can provide very different results compared to human studies.
Give us more research
Don’t get your hopes up and expect all of these benefits by starting IF. More long-term human studies need to be done to confirm conclusions from current short-term and animal studies.
Intermittent fasting has a good safety record. As long as you’re not severely undereating and choosing nutrient-rich foods, intermittent fasting can be totally safe.
But intermittent fasting can be more harmful than helpful for certain groups of people.
If you’re underweight or have a history of disordered eating, schedule an appointment with a health professional before starting up a fasting diet. The restrictions that come along with IF can trigger harmful eating patterns.
IF may also affect women and men differently. Calorie restriction can disrupt important hormones in female bodies.
A 2014 review found that certain hormones can’t communicate with the ovaries if calorie intake is too low, resulting in irregular periods, infertility, poor bone health, and other negative effects.
Women may want to opt for a modified approach to intermittent fasting, like shorter fasting periods and fewer fasting days.
Play IF safe
Chat with your doc before you start IF, since certain medical conditions can be negatively impacted by the change. This especially true if you:
- have diabetes
- have issues with blood sugar regulation
- have low blood pressure
- take medications
- are underweight
- have a history of disordered eating
- are trying to get pregnant
- are pregnant or breastfeeding
Slow and steady wins the race! If you’re new to the fasting game, start with a beginner approach by fasting for 12 or 16 hours.
If you find this method is easy and you want to advance, start up a 24-hour fast 1 or 2 times per week (eat-stop-eat) or restricting calories 1 to 2 times per week (5:2).
There is no one-size-fits-all approach. Try different methods and see what feels best for you.