Intermittent fasting (IF) is an eating pattern that consists of periods of food restriction, known as fasting, followed by a period of usual eating.
This dietary habit may help you lose weight, lower your risk of illness, and live longer (1, 2).
Some experts even believe its metabolic benefits make it a better approach to reducing weight than traditional calorie restriction (3).
Intermittent fasting is highly effective for weight loss
Intermittent fasting is a simple, effective fat-loss strategy that is reasonably straightforward to follow (4).
According to research, intermittent fasting can be equally successful as regular calorie restriction in weight reduction, if not more so (5, 6, 7, 8).
A 2014 study discovered that intermittent fasting might help patients lose 3-8 percent of their body weight in 3-24 weeks (9).
According to a 2016 assessment, intermittent fasting may be an ideal way to weight loss to very low calorie diets in persons who are obese or overweight (10).
Surprisingly, this eating style may also enhance your metabolism and metabolic health (1, 11, 12, 13).
There are several methods to experiment with intermittent fasting. Some people adhere to the 5:2 diet, which requires fasting twice weekly. Others use the 16/8 technique or alternate-day fasting.
If you want to attempt intermittent fasting, you may learn more about it in our comprehensive beginner’s guide.
Intermittent fasting increases several fat burning hormones
Hormones are substances that serve as messengers in the body. They circulate throughout your body, coordinating complex tasks, including development and metabolism.
They also play a crucial function in weight control. This is due to their considerable impact on your appetite, calorie intake, and how much fat you retain or burn (14).
Improvements in the balance of various fat-burning hormones have been associated with intermittent fasting. This might make it a valuable tool for weight loss.
Insulin
Insulin is a primary hormone involved in fat metabolism. It signals your body to store fat and prevents it from breaking down fat.
Having chronically elevated insulin levels might make losing weight much more difficult. High insulin levels have also been related to obesity, type 2 diabetes, heart disease, and cancer (9, 15, 16).
Intermittent fasting has been demonstrated to be equally effective in lowering insulin levels as calorie-restricted diets (17, 18, 19).
This eating pattern has cut fasting insulin levels by 20-31 percent (9).
Human growth hormone
Fasting can raise blood levels of human growth hormone, an essential hormone for fat reduction (20, 21).
According to some research, fasting can cause a fivefold rise in human growth hormone levels in men (22, 23).
Increased levels of human growth hormone in the blood boost fat burning but also help to keep muscle mass and have other advantages (24).
However, women do not necessarily get the same advantages from fasting that males do, and it is unclear if women will see the same increase in human growth hormone.
Norepinephrine
The “fight or flight” reaction involves norepinephrine, a stress hormone that promotes alertness and attentiveness (25).
It has some additional effects on your body, one of which is instructing your fat cells to release fatty acids.
Increases in norepinephrine often result in fatter access for your body to burn.
Fasting increases the quantity of norepinephrine in your system (26, 27).
Short-term fasts boost metabolism by up to 14%
Many believe that missing meals causes your body to adjust by slowing its metabolic rate to conserve energy.
It is generally known that fasting for extended periods can decrease metabolism (28, 29).
However, some previous research has indicated that fasting for brief periods might improve your metabolism rather than slow it down (30, 31).
Previous research of 11 healthy men discovered that a 3-day fast improved their metabolism by an astonishing 14%. (32).
This spike is assumed to be attributable to the increased fat-burning hormone norepinephrine.
More high-quality, current research is needed to determine how intermittent fasting affects metabolism.
Intermittent fasting decreases metabolism less than continuous calorie restriction
Your metabolic rate decreases as you lose weight. This is partly because weight reduction promotes muscle loss, and muscle tissue consumes calories.
However, the drop in metabolic rate observed with weight loss cannot necessarily be attributed to muscle mass loss alone (33).
Severe calorie restriction over an extended length of time might cause your metabolic rate to plummet as your body enters what is known as famine mode. “Adaptive thermogenesis” is the scientific word for this.
As a natural defense against starving, your body does this to preserve energy (34, 35).
This was proven in a study of persons who dropped significant weight while participating in “The Biggest Loser,” a TV reality program.
To drop a significant amount of weight, participants followed a calorie-restricted diet and a rigorous exercise plan (36).
According to the study, six years later, the majority of them had regained virtually all of the weight they had lost. However, their metabolic rates had not returned to normal, remaining roughly 500 calories lower than expected given their body size.
Similar findings have been reported in other research looking at the impact of calorie restriction on weight reduction. Weight reduction can cause a decline in metabolism that can equal hundreds of calories per day (37, 38).
This validates starvation mode’s existence and may help explain why many people who lose weight gain it again.
Given the short-term effects of fasting on hormones, intermittent fasting may help decrease the metabolic rate reduction produced by long-term calorie restriction.
However, there is presently no high-quality study on the long-term impact of intermittent fasting diets on metabolic rate.
Intermittent fasting helps you hold on to muscle mass
Muscle is a metabolically active tissue that contributes to a high metabolic rate. This allows you to burn more calories at rest (39, 40).
Unfortunately, when most people lose weight, they lose fat and muscle (41).
Because it influences fat-burning hormones, intermittent fasting has been suggested to retain muscle mass better than calorie restriction (42).
The rise in human growth hormone noticed during fasting, for instance, may assist retain muscle mass even if you lose weight (43).
A 2011 study discovered intermittent fasting was more successful than a standard low calorie diet for retaining muscle during weight loss (44).
However, the outcomes have been varied. A recent study discovered that intermittent fasting and continuous calorie restriction had comparable effects on lean body mass (5, 45).
After eight weeks, there was no change in lean body mass between persons who were fasting and those who were on continuous calorie restriction, according to one recent study. However, individuals in the fasting group had lost less lean body mass after 24 weeks (6).
More extensive research is required to determine if intermittent fasting successfully maintains lean body mass.
The bottom line
Although research has shown some encouraging results, the effects of intermittent fasting on metabolism are currently being studied (3).
Early research reveals that short-term fasts can enhance metabolism by up to 14%, and other studies show intermittent fasting does not significantly reduce muscle mass (6, 32, 44).
If this is accurate, then intermittent fasting offers numerous significant weight reduction advantages over regimens based on constant calorie restriction.
Intermittent fasting may be a highly successful weight loss method for many people.
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