Intermittent fasting claims to leverage the benefits of time-restricted food intake to improve metabolic health, lose weight, and even help you live longer. But does it actually deliver on these promises?
If you’ve been hearing a lot about intermittent fasting and have questions, this article is for you. Let’s look at:
- What intermittent fasting is and how it works
- Potential benefits and drawbacks
- Intermittent fasting for weight loss
- Common myths about intermittent fasting
What is Intermittent Fasting?
Intermittent fasting (IF) is a blanket term encompassing any form of fasting mixed with normal eating. Some common strategies include:
- Time-Restricted Eating (TRE) - involves eating only during a set number of hours each day
- Alternate-Day Fasting - Fasting every other day
- Eat Stop Eat - Fasting for a 24-hour period once a week
- The 5:2 Method - Eat normally for five days and restrict calories to 500-600 on the other two
Time-restricted eating (TRE) is the most common method of intermittent fasting. The 16:8 method is popular, where you fast for 16 hours and eat during an 8-hour window. This could look like fasting from 6 pm to 10 am, and only eating between 10 am and 6 pm, or whatever 8-hour window works for someone's schedule.
Proponents of intermittent fasting claim a host of benefits, including:
- Ease of weight loss
- Improved metabolic health
- Protection from chronic disease
- Brain health
- Longevity and more
However, the research reveals a more nuanced picture of what intermittent fasting does and doesn’t do.
How Does Intermittent Fasting Work?
Fasting results in specific physiological changes in the metabolism of carbohydrates, protein, and fat.[1] How your body fuels itself depends on how long it has been since your last meal.
After you eat: In a fed state, the body metabolizes glucose from carbohydrates as energy to fuel the brain and normal body function. Glucose, also called blood sugar, is the body’s preferred energy source. Any extra glucose you consume is stored in the liver and muscles as glycogen for later use.
3-4 hours after eating: As glucose levels decline, you become hungry, partly thanks to the appetite-inducing hormone ghrelin.
- If you eat (assuming you include carbohydrates), glucose levels rise, and the body keeps using free glucose as its primary fuel.
- If you don’t eat, the body taps into liver glycogen stores to release glucose into the bloodstream for fuel. Glycogen stores can last anywhere from 12 to 24 hours.[2] At the same time as the body taps into glycogen stores, it also begins to convert stored fat into ketones as an alternative energy source.
12-24 hours after eating: The longer you fast, the greater percentage of energy comes from fat in the form of ketones.[3] Once glycogen stores are fully depleted, ketones provide the majority of fuel for the body.
Intermittent fasting on the 16:8 or similar daily method means the body will not fully deplete its glycogen stores. However, it will burn some fat for fuel. Following a 5:2 method, where 2 days are fasting, creates a true fasted state.
Potential Intermittent Fasting Benefits
Weight loss - Intermittent fasting can make it easier for some individuals to eat in a calorie deficit, the primary prerequisite for weight loss. When food intake is restricted to a set number of hours a day, eating less may be a natural result. Many studies have demonstrated intermittent fasting is an effective strategy for weight loss, assuming individuals eat in a calorie deficit outside of fasting hours.[4][5]
Insulin sensitivity - Weight loss is one of the most effective ways to improve insulin sensitivity and reduce the risk of prediabetes and type 2 diabetes.[6] Weight loss through intermittent fasting can improve insulin sensitivity in individuals with existing insulin resistance.[5]
Improved cardiovascular health - As with insulin sensitivity, improvements in markers of cardiovascular health are primarily tied to weight loss.[7] The weight loss associated with intermittent fasting may result in improved blood lipid levels and a reduction in blood pressure.[3]
Reduced inflammation - Systemic inflammation is strongly linked to obesity and increases the risk of chronic diseases such as type 2 diabetes and cardiovascular disease.[5] Body weight reduction through intermittent fasting, decreases the level of circulating pro-inflammatory cytokines and improves inflammatory status.[8]
Research demonstrates that the majority of the documented benefits from intermittent fasting come as a result of weight loss. In other words, losing weight through traditional calorie restriction or fasting doesn't really matter. What matters are the health benefits you get from achieving and maintaining a healthy weight.
Potential Intermittent Fasting Drawbacks
Some people find intermittent fasting to be effortless, while others may experience side effects such as:
- Low blood sugar
- Dizziness
- Fatigue
- Headaches
- Irritability
- Sleep disturbances
- Hunger and food cravings
- Binge-restrict cycle occurrence
As with any calorie-restricted diet, loss of muscle mass is a potential concern. Consuming a high-protein diet may prevent loss of lean mass during weight loss.[9] Individuals practicing intermittent fasting should be extra cautious to ensure they consume enough protein during their eating window. Performing resistance training is also recommended to support muscle mass during weight loss.
Certain individuals should exercise caution if considering intermittent fasting. It is not appropriate for everyone and in some cases, may be unhealthy. This includes:
- Pregnant or breastfeeding women
- Children and teens
- The elderly
- Individuals with a chronic medical condition
- Individuals with a history of low blood sugar
- Individuals using certain medications
Talk to your doctor if you’re considering intermittent fasting to make sure it is safe for you.
Is Intermittent Fasting Better for Weight Loss?
While it is true intermittent fasting can promote weight loss, it is by no means the only way to achieve a calorie deficit.
Proponents of intermittent fasting often talk as though intermittent fasting is superior to other dietary patterns, but the research doesn’t support that claim.
Studies comparing intermittent fasting with standard calorie restriction show similar results.
- A 2024 trial randomized participants into a time-restricted eating (TRE) group who ate within an 8-10 hour window, and a usual eating pattern group who consumed the same diet, but with no time restriction. At the end of 12 weeks, weight loss and glycemic control were similar between the two groups, showing no superiority of TRE over regular calorie restriction.[10]
- A 2018 randomized controlled trial involving 150 overweight or obese adults found no difference in weight loss or metabolic markers between those who followed a 5:2 IF pattern or a standard calorie-deficient diet.
Intermittent fasting can be a useful tool for losing weight. However, it’s not the only one, so if it doesn’t work well for you, there are other options.
Debunking Myths of Intermittent Fasting
Intermittent fasting has taken on an almost mythical status among its proponents. At the same time, critics of IF sometimes push misleading claims about its safety.
Here are a few claims about intermittent fasting that deserve a debunking.
Myth 1: It’s Not Safe for Women
Critics of intermittent fasting argue that cycles of fasting are detrimental to a woman's hormone cycle. However, evidence is lacking to back up this claim.
A 2022 review found no effects of IF on estrogen, gonadotropin, or prolactin in women of reproductive age.[12]
In general, being overweight or obese has an adverse effect on a woman's hormonal health. Women who are overweight are more likely to have irregular menstrual cycles, infertility, PCOS, hormonal imbalances, and poor maternal and fetal outcomes with pregnancy.[13]
Weight loss through appropriate calorie restriction, whether using IF or another method, can improve women’s health.[14][15]
Myth 2: Increases Autophagy
Autophagy is a buzzword in intermittent fasting circles. It is the term for lysosomal degradation that rids the cell of old, damaged cellular components. Essentially, autophagy is the cell's cleansing cycle that maintains cellular health and recycles nutrients.
Proponents of intermittent fasting claim extended periods of fasting “turn on” autophagy mode. However, there are two important notes to make about this claim.
- Autophagy is a constant process. Certain factors can increase or decrease autophagy, but the idea that it is either on or off is misleading.[16]
- Fasting is only one way to increase autophagy. Any form of calorie restriction also increases it, as does exercise.[17] A 2021 randomized controlled trial found no difference in the rate of autophagy between people following IF vs. people following an isocaloric standard diet.[18]
If you’re looking to increase autophagy to promote health and longevity, IF is just one tool to do that.
Myth 3: Improves Longevity
It’s likely fair to suppose intermittent fasting can promote longevity, simply from the perspective of weight loss and its accompanying health benefits.
However, the claim that intermittent fasting uniquely promotes longevity independent of weight loss lacks a basis in research. Many animal studies have examined this claim, but few robust human studies exist.[19] Studies examining whether IF promotes longevity more than a calorie-controlled, healthy diet and exercise are needed to support this claim.
Conclusion
Intermittent fasting can be a useful tool for weight loss and health promotion. However, it isn't magic. The evidence suggests that the "magic" of intermittent fasting is found in its ability to control calories.
For some people, it is very effective at helping them control their calorie intake and achieve a healthy weight. For others, it’s not a sustainable solution.
The important thing is to find a sustainable way of eating that works in the context of your life. The optimal diet is one that:
- Is calorie-controlled to help you achieve and maintain a healthy weight
- Includes plenty of protein
- Is high in fruits, vegetables, and other fiber-rich plant foods
- Brings you pleasure and satisfaction while nourishing your body
If you’re practicing intermittent fasting, consider adding Build Whey Protein to your routine. It’s an easy way to hit your protein goal during your eating window without sacrificing taste and quality.
References
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