fasting, metabolic health

Fasting for Fat Loss, Metabolic Health, and Glycemic Control

Fasting can be quite an effective tool for fat loss, metabolic health, and glycemic control. All three of these are crucial for our ability to produce energy on a cellular level. The way we utilize glucose is important for metabolic health and affects our ability to lose weight. But how effective is fasting for fat loss really?

Fasting seems to be beneficial on many different levels such as glucose utilization, energy production, cholesterol transport, and metabolic efficiency. It has been associated with higher fat loss, better brain function, and metabolic health.

Fasting and Obesity

Obesity is a major problem around the world. Having extra weight is related to higher inflammation, higher cardiovascular disease risk, and poor metabolic health. It seems that fasting can be a viable strategy to offset this.

The way fasting works is by reducing glucose and insulin spikes. When you eat a doughnut, glucose spikes in your blood, so insulin is secreted to transfer that glucose into tissues. The problem is that high and frequent secretion of insulin (which happens with overeating sugar) – wastes insulin reserves in the Beta cells, and makes insulin less effective over time, thus developing insulin resistance.

Obesity is a complex problem mainly originating from excessive caloric consumption and leptin imbalance. Research shows that many parameters, like body mass index, body weight, fat mass, LDL, cholesterol, and triglycerides can be improved through fasting. (1)

Calorie restriction is the main strategy that helps us lose weight. But fasting may seem superior in certain aspects as it relates to improving metabolic health factors. (2) Fasting is also effective in reducing weight circumference and central fat distribution. It modulates the apolipoprotein A4 – important for satiety, cholesterol transport, and weight loss. (3)

What is Metabolism and Metabolic Health

In simple terms, metabolism is the way we turn food into cellular energy. We need energy to power physiological processes in our bodies. Metabolic health is about generating energy as efficiently as we can while maintaining stable glucose levels.

The more metabolically efficient we are, the faster we can turn food into energy. Expending more energy equals burning more calories, which results in fat loss.

Fasting can help optimize the machinery in our body that helps us create energy from glucose, thus supporting our metabolism – while keeping healthy metabolic markers.

Why does fasting work?

The main mechanism behind fasting is learning to tap into fat stores to derive energy from. This improves metabolic factors like insulin sensitivity and can lead to weight loss.

Will fasting slow my metabolism?

As with keto, it’s true that fasting decreases energy expenditure. But we primarily want faster metabolism to lose weight, which fasting can effectively do. Conserving energy is a natural response when there’s less food ingested, but weight loss isn’t impaired.

What does fasting do to the body?

Fasting, by removing excess and frequent food teaches the body to clean itself of toxins, tap into fats for energy, and reset insulin levels. This can greatly affect weight loss, lower inflammation, and increase energy levels.

An interconnected complex

It’s important to understand that all of these areas are interconnected. It’s like a pathway, and each step counts. From first putting that sweet potato in your mouth to how it is broken down and digested. Then, how much your blood glucose has increased and the amount of insulin secreted, to how efficiently you metabolized that glucose.

Fun Fact

The faster we can process and utilize glucose and the more stable our glucose and insulin levels are – the better our metabolic health

Why is Fasting Effective?

The main reason time-restricted eating can be effective is that it leads to a caloric deficit. Fasting is also a very practical and effective tool to which people have high adherence.

Data shows that advice on fasting is followed even more than other weight loss methods. It is more sustainable, easier to practice in the long run, and seems to be quite effective in reducing total caloric consumption by 20-30%.

Types of Fasting

  • Time-restrict eating or intermittent fasting is the most popular form of fasting. You’re having a fasting window where you do not ingest any calories at all, and an eating window where you eat all your calories. For example, 16:8 fasting, or 12:12 fasting, are common ones.
  • Alternate day fasting is when every other day you eat only 25% or less of the calories you normally eat. It is more challenging to adhere to ADF, but it seems to have superior metabolic benefits to caloric restriction.
  • The 5:2 fasting model is where you eat normally for five days, and then fast for two days or eat 25% or less of the calories during the weekend. It’s more of a longevity-enhancing and autophagy-stimulating style of fasting.

Disclaimer. Although fasting seems to be an effective tool, it is not for everyone. The 12:12 h fasting is pretty safe, while the extended fasts over 16 hours may cause side effects, hormonal imbalances, or hypoglycemia in some people. It’s very individual. There’s a great podcast at The Human Upgrade, featuring Valter Longo on the 16 or 12 h fasting debate

fasting metabolic health fat loss glycemic control

Fat Loss

The bulletproof formula for fat loss is being in a caloric deficit, simple as that, huh? Fasting seems to make this easier, more effective, and more sustainable. But is fasting for fat loss effective? Well, it seems to provide a myriad of benefits tied to it. From fat distribution to controlling hunger hormones and improving cholesterol profile, it does a lot.

There are numerous studies showing fasting benefits in terms of weight loss, reduced body fat mass, improved body mass index, reduced cholesterol and triglycerides, and lower weight circumference.

Alternative fasting for 3 to 12 weeks showed to reduce body weight from 3 to 7%, body fat from 3 to 5.5 kilograms, and total cholesterol and triglycerides levels. (4) Besides just reducing body mass index, fasting can improve glycemic control and stabilize or reduce hunger. (5)

Research in 139 participants at 12-month follow-up showed fasting’s effectiveness in reducing body weight at – 8 kilograms, on average. However, fasting was not superior to caloric restriction for reducing body weight, fat mass, and metabolic risk factors. (6)

Compared to a non-intervention diet and caloric restriction, fasting was superior in lowering weight and body mass index, reducing waist circumference, and regulating fat mass more effectively. (7)


Fasting is an effective tool for weight loss, easy to adhere to. The magic behind fasting lies in a caloric deficit. It offers additional benefits which accelerate fat loss such as improving glycemic control, reducing inflammation, regulating fat mass, and reducing metabolic risk factors.

Metabolic Health

Metabolic health is about optimizing the process of turning food into energy. It is about more efficient utilization of glucose that leads to higher energy production on a cellular level.

Metabolic flexibility is the ability to switch between energy fuels, primarily between glucose and fats. Being metabolically flexible is greatly associated with greater metabolic markers.

LDL and HDL cholesterol, triglyceride levels, fasting glucose, and insulin say a lot about our metabolic health. The more these metrics are out of control, the more metabolic deficits can arise, thus increasing the risk for metabolic syndrome.

It seems that besides a low glycemic index diet, proper management of stress, optimized sleep, and working out, fasting can be an effective tool for improving metabolic health.

One of the reasons why intermittent fasting works in favor of metabolic health is because we can switch between different fuel sources of energy, from glucose to fatty acids or ketones. Training our body to work on ketones gives our insulin time to reset. (8)

There are three main underlying mechanisms behind fasting’s effect on longevity and metabolic health. One is the activation of AMP-activated protein kinase. The second one is inhibiting mTOR and anabolism. The third one is improving mitochondrial biogenesis. (9)

Research shows that fasting can lead to weight loss and reduction of insulin and glucose, which are important markers for metabolic health. It isn’t superior to caloric restriction, but it’s more practical. (10)

Extra Research: Fasting on Cholesterol and Triglycerides

ADF (alternate) fasting seems to be more effective than TRF (time-restricted) for reducing blood glucose, triglyceride levels, and LDL cholesterol. It reduced insulin by 21% to 42%, and it improved the insulin resistance model. (11) (12)

extra research
Studies on fasting show that it reduces LDL cholesterol and triglyceride levels. And in some studies, it has also impacted HDL and a positive way by increasing it, while in others, there was no difference. (13) (14)

In terms of cardiometabolic health, fasting seems to improve many factors related to CVD risk. It seems that the effectiveness is greater in more metabolically unhealthy people. (15)

Both alternate-day fasting and time-restricted eating seem to be cardio-protective by reducing oxidative stress, reducing blood pressure, and insulin resistance. (16)


Fasting can improve certain cardiometabolic factors, especially in metabolically unhealthy people. These include lowering LDL cholesterol and triglycerides while the effects on HDL are variable. It also may reduce insulin resistance and oxidative stress.

Glycemic Control

Glycemic control is about keeping stable glucose and insulin levels. When there are too many high spikes and drops of insulin and sugar, inflammation in the body arises. Unstable glucose level also poses a risk for cardiometabolic health.

Simple sugars raise blood glucose and insulin faster, compared to complex carbs. The Glycemic Index of foods matters as well. For metabolic health, you’d aim for lower GI foods, complex carbs. That’s why you’d pick the whole wheat pasta instead of the white, simple one. It’s why lentils and oats are better (for stable glucose) than a doughnut or a cake.

There are even companies like Levels Health that are fully devoted to optimizing and tracking your blood sugar levels. It allows you to see when and how blood glucose increases after a certain meal, a different stressor, or a workout.

glycemic variability

A large systematic review and meta-analysis list the myriad of potential benefits fasting has on glycemic control. (17) These include:

  • reduction of fasting blood glucose
  • reduction of glycosylated hemoglobin
  • reduction of insulin plasma levels
  • reduction of BMI, body weight, and waist circumference
  • improving cholesterol profile (lower LDL, TG)

Another powerful mechanism by which fasting may improve weight loss and glycemic control is controlling the levels of leptin and adiponectin. Leptin controls your hunger levels depending on your energy expenditure needs. Higher levels of leptin signalize satiety and fullness.

Fasting helps us shift the levels of leptin and adiponectin in favor of weight loss. It also increases levels of adiponectin, which is important for improving insulin sensitivity and reducing inflammation. (18)

In overweight patients with Type 2 diabetes, fasting for 10 hours – from 08 to 18 h resulted in reduced body weight and hemoglobin A1C, plus reduced insulin resistance and HOMA-IR. (19)


Fasting can provide a myriad of benefits related to glycemic control. Some of these include reduced fasting blood glucose, improved cholesterol profile, and insulin sensitivity. Fasting also helps control hunger hormones like leptin and adiponectin.

Why is fasting effective for weight loss?

The main effect of fasting on fat loss has to do with caloric restriction. Basically, it’s an easy diet to adhere to, and you do not have to count any calories. It conveniently puts a lot of people in a caloric restriction of 20% to 30%. Besides that, it can improve different factors, like insulin sensitivity, regulate blood and glucose levels, and improve cholesterol profile.

How does fasting improve metabolic health?

Fasting can improve metabolic health by allowing your body to switch between fuels. When you don’t have enough glucose, which is when you’re in a fasting state, your body turns to fats or ketones for energy. Doing so gives insulin time to reset, thus improving insulin sensitivity.

Which type of fasting is best?

Data shows alternate date fasting to be the most effective for quicker weight loss in the short term, but adherence to this style of fasting is lower.
16 to 8 fasting has shown effective for weight loss due to its high adherence and practicality. It can also improve cholesterol profile, fasting blood glucose, and insulin sensitivity.
5:2 fasting is most effective for enhancing longevity, removing junk cells, and stimulating autophagy. A more extreme version is the full 5-day fast done with <25% calorie consumption (soup, teas, veggies) also practiced for longevity.


Fasting is a nutritional practice where one abstains from eating food for a certain period of time. Intermittent fasting can be done at intervals of 12:12 h or 16:8 h fasting. Alternate-day fasting is eating <25% of normal calories, every other day.

Fasting seems to be a very effective tool for weight loss, as it’s easy to adhere to, and it leads to a caloric deficit. Not superior to a caloric deficit, but it’s more sustainable.

It can be great for improving metabolic health, as it reduces inflammation, and improves energy creation and glucose utilization. By altering non-eating states you have no glucose, so insulin has time to reset, while you run on fats for energy.

In terms of glycemic control, fasting may improve insulin sensitivity, reduce fasting glucose, and improve cholesterol profile.

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