The main appeal of intermittent fasting is that it is not a diet based on deprivation or limiting individual foods. Rather, it’s a timed approach to eating which automatically reduces your food intake and offers many health benefits, including weight loss and blood sugar control.
There is also a variety of fasting diets from short-term fasts to alternate-day fasting that gives you the flexibility to choose one that’s right for your lifestyle. If you’re looking to lose bodyweight, exploring the benefits of intermittent fasting should be a top priority.
What is Intermittent Fasting?
Intermittent fasting is a cyclic way of eating that involves frequent, short-term fasts. Each method has a limited eating window and a defined period of fasting. There are a number of health benefits including weight loss, improved cognitive function, enhanced insulin sensitivity and improvement in health markers such as blood pressure.
When you fast, you consume fewer calories due to reduced overall food intake. You also optimise hormones that are related to weight control and increase your body’s ability to burn fat.
There are several different forms of intermittent fasting, each with its own specific time restrictions. Four popular fasting methods are:
- 16:8 method: Eat until satisfied in the 8-hour feeding window and fast for 16 hours. For example, skipping breakfast every day.
- 5:2 diet: Eat as usual 5 days a week and limit your caloric intake to 500-600 calories on two days either consecutive or spread out over your week.
- 20:4 or Warrior diet: Eat one large or two smaller meals in a 4-hour eating window followed by a fast of 20 hours.
- Alternate day fasting: Fast for a defined extended period such as 24 hours one or two days a week in an eat-stop-eat pattern.
Each of these methods has the potential to lead to weight loss if you don’t compensate by over-eating in your eating window. If you’re new to intermittent fasting, the 16:8 or 5:2 are ideal for beginners. Don’t start out on a 24-hour fast or you’ll be destined to fail.
Once you get used to eating on a schedule and see how your body responds to fasting, you could try longer periods of fasting and find what method works best for you.
How Does Intermittent Fasting Work?
The basic premise behind intermittent fasting is that you limit the eating time-frame and increase the time we spend in a fasted state.
Essentially, you’re increasing the amount of food you consume at one time, trusting your body’s ability to self-regulate as you can only eat so much in the limited eating window. Over the course of the day, you’re reducing your calorie intake without being on a defined calorie-restricted meal plan.
For people who overeat or graze consistently over the course of the day, this method can be beneficial and feel less restrictive than a low-calorie diet.
Intermittent fasting trains your body to utilise fat as fuel instead of relying on energy from the food, in particular carbohydrates, you are constantly consuming.
When you eat, your body breaks down the carbs into glucose that can be used as energy. Insulin levels rise to allow your body to uptake the glucose and what’s not used immediately, is stored away in the liver and muscles in long chains of glycogen.
When these stores are full due to a constant oversupply of energy, the excess glucose is converted to body fat.
Fasting changes the way your body responds and utilises energy. Your body becomes an efficient fat-burning machine rather than a sugar lover. This occurs over several stages:
Firstly, when you begin fasting, your insulin levels start to fall as there is no incoming fuel to respond to. The glycogen stored in your liver breakdowns and releases glucose for energy. These stores can last for about 24 hours.
The liver then begins to make glucose from amino acids in a process known as gluconeogenesis. Your insulin levels continue to fall until they fall to a point that stimulates the body to breakdown fat for energy. This process is known as lipolysis.
At this time your body has become fat adapted and the tissues of the body are being fueled by the fatty acids. Ketone bodies are then produced to provide an energy source for your brain. This also occurs when on a ketogenic diet (also known as keto).
Essentially, your body has switched from burning glucose, which is a short-term fuel, to the long-term fuel source fat in response to having little to no glucose in the bloodstream.
How Does Intermittent Fasting Encourage Weight Loss?
From a weight-loss perspective, intermittent fasting works on a couple of different levels to help you achieve a healthy weight.
- Due to the limited eating windows, intermittent fasting makes it more difficult to overeat during the day and providing your body with more energy than it can burn. While you might be overcome by hunger on the fast days, it is still difficult to overeat during the short feeding window due to your body’s ability to self-regulate. Overall your daily calorie intake should reduce.
- Intermittent fasting trains your body to become efficient at fat burning. As you fast, your insulin levels decrease and fat stored in your liver is released for fuel. Instead of relying on a consistent food supply, your excess fat becomes your body’s primary energy foods. This stimulates ketone production and your body enters a state of ketosis, similar to that experienced on a keto diet. You also have much better blood sugar control and won’t experience the energy dips and sugar cravings as a result.
- Changes in other hormones occur when you fast which increases your body’s ability to burn fat. Levels of human growth hormone rise dramatically in a fasted state which can lead to greater fat loss and encourage muscle gain. Your nervous system will also release more noradrenaline in response to the fast. This can help break down body fat into fatty acids that can be burned for energy.
When coupled with exercise and a low carb or ketogenic diet, you can increase your body’s ketone levels and burn more excess fat. This for some can result in significant weight loss.
Fasting Versus Caloric Reduction for Weight Loss
Intermittent fasting does effectively reduce your overall calorie intake but it is different from a calorie-restricted diet and so are the effects on the body.
When you are on a calorie-restricted diet, your body maintains a set weight that it’s determined as your survival weight. When you restrict your caloric intake, you may initially experience weight loss. However, your body compensates by trying to regain the lost kilos.
Your hunger hormone, ghrelin, increases to encourage you to start eating. Your metabolism slows to decrease your energy expenditure. This decrease in basal metabolism sees the weight loss slow down and you plateau. While you’re reducing the number of calories you consume, your body is also reducing the number of calories you burn as a survival mechanism. You’re lacking in energy, hungry and feel deprived and before you know it, your old eating habits begin to creep back in. It’s no wonder low-calorie diets don’t have a great reputation for being successful long term.
How is intermittent fasting different?
The key difference between a fasting schedule and caloric reduction lies in the changes in your hormones.
Intermittent fasting helps restore and improve your body’s insulin sensitivity whereas these health markers are not reduced when dieting. Insulin resistance and blood sugar imbalances are some of the key factors driving weight gain, obesity and metabolic health conditions such as type 2 diabetes.
When on a calorie-restricted diet there is no change to your insulin resistance. As a result, high insulin levels continue which maintains your high set weight and prevents sustained weight loss long term.
When in a fasted state, your body’s insulin levels naturally fall as there is no energy coming in for your body to process and utilise. It’s your fat cells that release the stored sugar for fuel. When your insulin levels go down far enough, your body will burn fat and you’re in the best position to experience weight loss.
What About Refeeding Syndrome?
Refeeding syndrome is a term given to health complications that may arise when you fast for an extended hour window. A disruption in fluid and electrolyte balance and malnourishment can occur when you resume eating after fasting for longer than 5 days.
Your body relies on fluids, vitamins and minerals from the food you consume. When you fast for an extended period of time, a side effect is the loss of essential minerals. Fasting for prolonged periods while on a ketogenic or low carbohydrate diet can increase the excretion of these minerals.
This is not a scenario you need to worry about on short fasts of less than 24 hours. This is especially the case if you are consuming a well balanced, mineral-rich diet.
If you are considering fasts that are longer than 24 hours, it is recommended you seek advice from your health practitioners such as your GP, nutritionist or dietitian before doing so. This is particularly important if you have any existing medical conditions.
One risk factor that is associated with intermittent fasting is overeating or binge eating during the eating window. This is why fasting is not recommended for people with a current or a history of eating disorders.
Intermittent Fasting and Muscle
A common concern about dieting, in general, is that it will result in a loss of muscle mass in addition to body fat.
There is, however, some evidence to suggest intermittent fasting may help you to retain your muscle while losing excess weight.
There are several theories around why this may occur, although more research is required in this area. Higher metabolic adaptions and improved muscle synthesis may increase your training performance and results. Intermittent fasting is also believed to induce autophagy, a process that cleans up damaged cells. If autophagy isn’t activated and damaged proteins and mitochondria are allowed to remain, muscle cells can begin to die which may result in a loss of muscle and strength.
In one review, intermittent calorie restriction resulted in a similar amount of weight loss as continuous reduced-calorie dieting but with a much lesser effect on muscle mass. Another study found 25% of the weight lost on a calorie-restricted diet was muscle mass compared with only 10% on an intermittent calorie-restricted diet.
Tips to Improve Weight Loss While Fasting
Intermittent fasting is popular largely for its simplicity and potential health benefits, particularly weight loss. For fasting success, take a look at these tips.
- Choose the right method for you
Select a fasting method that meets your individual needs and lifestyle commitments. If you’re new to fasting, we recommend you choose one with a shorter fast such as the 16:8 or 5:2.
- Track your progress
If you aren’t assessing, you’re guessing. For weight loss, in particular, tracking your progress is important to help keep you on track. Start with your base values and then measure frequently.
- Make your calories count
Not all calories you consume are the same. Focus on nutrient-dense foods rather than empty calories found in sugary foods and refined grains.
- Watch your snacking
What and when you eat during your feeding window can impact your ability to lose weight. Avoid grazing and instead set defined times to eat. This allows your body to burn fat between meals.
Explore in more detail
- A Beginner’s Guide To Intermittent Fasting
- A Guide To The 16:8 Fast
- A Guide To The 20:4 Fast
- A Guide To The 5:2 Fast
- Intermittent Fasting And Exercise
- Intermittent Fasting And Hormone Balance
- Intermittent Fasting And Keto
- Intermittent Fasting And Type 2 Diabetes
- Intermittent Fasting And Weight Loss
- Intermittent Fasting: Tips To Starting Your First Fast
- Side Effects Of Intermittent Fasting
- Tips For Intermittent Fasting Success
- Top FAQs Of Intermittent Fasting