If you want to lose fat, improve metabolism, and experience other health benefits all without giving up your favorite foods, intermittent fasting might be for you!
It’s an emerging area of research and the results are very promising. Similar to calorie reduced diets, intermittent fasting has benefits for weight loss and metabolic improvements, and might even improve brain and mental health.
Intermittent fasting (IF) has a few advantages over regular calorie reduced diets. Not only is it easier for many people to stick with, but it also seems to have a metabolic advantage. These are really good things when it comes to long-term health.
Intermittent fasting is just that - fasting intermittently (periodically). It’s an “eating pattern,” rather than a “diet.” That means regularly reducing your eating and drinking during pre-set times. It’s controlling when you eat and drink, as opposed to what you eat and drink.
There are lots of ways to intermittently fast. It can be done daily, weekly, or monthly. After we go over the health benefits, we’ll look at some of the most popular methods on how to, and who shouldn’t, IF.
Background: History and animal studies
Back in the 1980s and 1990s U.S. studies looked at effects that reducing smoking had on heart disease risk. Interestingly, the risks seemed to reduce more in members of the churches of Latter Day Saints and Mormons than in other people. Researchers wanted to know why, and that’s when they found a possible connection with fasting.
Beyond smoking, researchers started looking specifically at people who fasted. In the early 2000s, they found that people who reported routine fasting (for religious reasons or not) had lower risk of heart disease. People who reported fasting had lower blood sugar levels, body-mass indices (BMIs), and risks of diabetes.
When it comes to animal studies, it’s easy to restrict when an animal eats, so there are a lot of studies on the health effects of IF in animals.
Animal studies show a lot of health benefits of IF including longer lives and reduced risk of atherosclerosis (narrowing of the blood vessels due to buildup of plaque), metabolic dysregulation (includes type 2 diabetes), and cognitive dysfunction (ability to learn, remember, solve problems). They also have lower levels of inflammation and generally live longer.
So, let’s dive into the health benefits of IF.
Intermittent Fasting for Weight and Fat Loss
For people who have excess weight, losing weight and fat reduces the risk of diabetes, improves healthy lifespan, and increases function of both the body and mind.
After about 5-6% of a person’s body weight is lost, even more health benefits are seen - lower blood lipids (LDL cholesterol and triglycerides), better blood sugar management (lower glucose and insulin), lower blood pressure, and lower levels of inflammation (C-reactive protein).
These benefits are seen with both calorie reduced diets and with IF.
When it comes to weight and fat loss, a typical calorie reduced diet works. By consistently reducing the amount ingested by 15-60%, people with overweight and obesity lose weight and fat. This is called “continuous” calorie reduction because one is continuously reducing what is ingested - at every meal and snack, every day. Calorie reduced diets can include eating smaller servings, low calorie substitutions, and/or cutting out some snacks/desserts every day.
Intermittent fasting isn’t a continuous reduction, but rather an intermittent one. It allows you to eat what you want, but only during certain times. It’s an alternative to calorie reduced diets. IF is a way to “diet” without “dieting,” so to speak.
Both continuous calorie reduction and IF have similar weight loss results.
Intermittent fasting has a few key benefits!
Many studies prove what we know already: it’s really difficult to sustain a (continuous) calorie reduced diet for a long time.
This is the reason why many people prefer intermittent fasting - it gets similar weight and fat loss results, plus it’s easier for many people to stick with.
This makes IF a great alternative for anyone who wants to lose weight and fat, but has difficulty sticking with a reduced calorie diet.
Other advantages to IF over calorie reduced diets are that it can help people eat more intentionally (and less mindlessly). Also, some studies show that IF makes our metabolism more flexible so it can preferentially burn fat, while preserving the muscles. This is a great benefit because that can help improve body composition in people with excess weight.
Intermittent Fasting for Metabolic and Heart health
Over and above the weight and fat loss benefits, IF has metabolic benefits and may help not just with overweight and obesity, but with metabolic syndrome, type 2 diabetes, and heart disease as well.
People who IF sometimes have improved insulin sensitivity and blood sugar levels. They also show improve blood lipids and even reduced inflammatory markers. All of these are related to improved metabolism and reduced risks for many chronic diseases.
One study found that people who IFed for 6-24 weeks and lost weight also benefited from reduced blood pressure.
One unique way IF works is by making our metabolism more flexible, which we’ll talk about below. This is really important for blood sugar control and diabetes risk because, according to Harvie (2017):
“Metabolic inflexibility is thought to be the root cause of insulin resistance.”
Another researcher, Anton (2015) says:
“When taken together with animal studies, the medical experience with fasting, glucose regulation and diabetes strongly suggests IF can be effective in preventing type 2 diabetes.”
Most researchers find these results promising, and recommend more high-quality longer-term trials.
Intermittent Fasting for Brain and Mental Health
Many animal studies show that intermittent fasting can help improve their cognition (ability to think). When mice fasted on alternate days for 6-8 months, they performed better in several learning and memory tests, compared to mice that were fed daily. This improvement even happened in mice who started IF later in life.
Studies also show that alternate day fasting protects brain neurons in animal models of Alzheimer’s, Parkinson’s, and stroke, and reduces oxidative stress in the brain.
We know that people with lifestyles that include little exercise and frequent eating (three meals every day plus snacks) lead to sub-optimal brain function and increases the risk of major neurodegenerative and psychiatric disorders.
Researchers are still learning about the brain and mental health benefits of IF in people. Short-term studies show some people report improvement in tension, anger, and confusion from IF; while others report bad temper and lack of concentration as side effects from it.
More longer-term human studies of different calorie reduction diets, including IF, will shed light on effects on cognitive performance and mental health.
How intermittent fasting helps our bodies and brains
How do we explain the health benefits that IF has on our bodies and brains? One way is the “metabolic switch” that is flipped during fasting.
While continuous calorie reduction and IF have many of the same health benefits, IF might have a different biological mechanism at play. Some research suggests that IF might “flip” a metabolic switch.
Here’s how it works.
After we eat our bodies use carbohydrates (e.g. glucose) from our food for fuel. If there is extra left over, then it’s stored as fat for future use.
With fasting, just as during extended exercise, our bodies flip from using glucose (and storing fat), to using that stored fat and ketones (made from fats) for fuel. Sometimes called the “G-to-K switch,” (glucose-to-ketone) the ability to flip what our bodies use as fuel (between glucose and ketones) is called “metabolic flexibility.”
It’s thought that we, and many animals, evolved to have this ability to survive short periods of fasting from when we were hunter-gatherers. There were times when people didn’t have a lot to eat, but they still needed to survive and think clearly enough to successfully hunt and gather food. This can explain why our bodies and brains don’t necessarily become sluggish when we’re fasting. It makes a lot of sense, although it has yet to be tested in current-day hunter-gatherers.
This metabolic switch can explain some of the health benefits of fasting. When our bodies prefer using fats for fuel, the body starts burning our stored fat. This is how IF helps with overweight, obesity, metabolic syndrome, and type 2 diabetes. When the body uses fat for energy this decreases the amount of fat in the body. Reduced fat reduces weight, and health benefits from weight loss (like lower blood pressure and insulin resistance) are felt.
This “flipping” of the metabolic “switch” happens after the available glucose, and the stored glucose are depleted. This is anywhere from 12-36 hours from the last meal, depending on the person. At this point the fats in our cells start getting released into the blood and are metabolized into ketones. These ketones then go to fuel cells with “high metabolic activity” - muscle cells and neurons.
Since the body is burning fat and using ketones to fuel the muscles, IF can preserve muscle mass. Some studies of IF show that it preserves more muscle mass than regular calorie reduced diets do.
The other high metabolic activity cells fueled by ketones are neurons (in the brain and nervous system). IF helps our brains because when our neurons start using ketones for fuel, it preserves brain function and increases brain-derived neurotrophic factor (BDNF) which is very important for learning, memory, and mood. BDNF also helps enhance synaptic plasticity (changes in our brain that help with learning and memory) and allows our neurons to better resist stress. These are all improvements in brain function, and some animal studies also show improvements in the structure of the brain too. For example, new neurons are produced in the hippocampus (the part of the brain important for short- and long-term memory) in animals who IF.
According to Anton, 2018:
“In these ways, events triggered by the metabolic switch may play major roles in the optimization of performance of the brain and body by IF.”
Who shouldn’t try intermittent fasting?
PRO TIP: Before you try any major changes to your diet, check with your healthcare provider.
IF can provide a lot of health benefits, and according to Patterson & Sears (2017):
“Overall, evidence suggests that intermittent fasting regimens are not harmful physically or mentally (i.e. in terms of mood) in healthy, normal weight, overweight, or obese adults.”
There are a few things to keep in mind before considering intermittent fasting, however.
A number of adverse effects have been reported, including: bad temper, low mood, lack of concentration, feeling cold, nausea, vomiting, constipation, swelling, hair loss, muscle weakness, uric acid in the blood and reduced kidney function, menstrual irregularities, abnormal liver function tests, headaches, fainting, weakness, dehydration, mild metabolic acidosis, preoccupation with food, erratic eating patterns, binging, and hunger pangs.
If done too often or for too many days IF can have more serious effects.
Fasting for several weeks (about 5-7 weeks) becomes starvation even in healthy adults. At this point your body starts consuming muscles and vital organs. This can also lead to excessive weight loss, anemia, chronic diarrhea, delirium, lactic acidosis, small bowel obstruction, kidney failure, heart arrhythmias, and eventually death.
Excessive fasting can lead to malnutrition (including vitamin B1 deficiency), decreased bone density, eating disorders, susceptibility to infectious diseases, or moderate damage to organs.
Limit fasting to avoid these effects.
How to Intermittently Fast
There are lots of ways to intermittently fast, and we don’t yet know how these different methods have different health effects for different people with different health goals.
Here are are some different ways to IF:
Alternate-day fasting (ADF) - One day of fasting, one day of “feasting.” Continue fasting on alternate days.
Alternate-day modified fasting (ADMF) - Eat 25-40% of your daily needs one day, then eat normally the next. Continue alternating days.
Periodic fasting (PF) or “Two day” fasting - Each week has 1 or 2 days to eat very few calories per day (e.g. 0-880 cal/day). The other 5 days you eat normally. Example: 5:2 diet, where you eat no more than 500 calories/day for two non-consecutive days each week.
Time-restricted fasting (TRF) - Fast for 12-16 hours every day and eat normally during the other 8-12 hours. (My personal favorite method)
One 24-hour period of fasting each month.
Several researchers suggest that the alternate-day modified fasting is preferable because it is likely the easiest to follow and may cause the least amount of stress on the body and mind.
You may be wondering if fasting intermittently increases what you eat during those times when you do eat. And that’s a great question.
The interesting thing is, it seems not to!
Studies show that alternate-day fasting reduces overall calorie intake. Plus, on the non-restricted days, some people naturally reduce their energy intake by up to 20-30%.
This means another side benefit of IF is that it can help reduce food costs too!
PRO TIP: Keep in mind that reducing your food intake also reduces your nutrient intake. It’s important to ensure you get enough essential nutrients for long-term health.
When it comes to preserving muscle mass, the jury is out on IF, but there are a few tips:
Eat enough protein (1.2 g protein/kg weight), and
Exercise, preferably with resistance training.
Intermittent fasting is a way to get the benefits of a regular calorie reduced diet without restricting what you eat, just when you eat it. Intermittent fasting reduces both weight and fat, and can improve blood sugar and blood lipids. It has been shown to reduce blood pressure and some markers of inflammation. Many animal studies show improvements in brain health too.
While these benefits of IF are similar to those with calorie reduced diets, IF has some key advantages including being easier for some people to stick with and it might help people eat more intentionally. There is also evidence that IF preferentially reduces fat while preserving muscle and may help our bodies become more “metabolically flexible.”
More research is needed to really understand long-term benefits of IF on the body and brain, as well as which IF approach is optimal for different people and different health goals.
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