fasting and health

Fasting and Health: How Does Fasting Work?

Fasting is an increasingly popular phenomenon. It is about extending the no-eating, so-called fasting window. This supposedly comes with a myriad of health effects like improving glycemic control, improving metabolic health, and fat loss. Let’s see the interplay between fasting and health.

We’ll focus on the effects of fasting on health, glucose regulation, and weight loss. We’ll also see how different nutrients affect insulin response and whether you’re in a fasted state or not. And, check out the effect of fasting on muscle anabolism, hormone health, and circadian rhythms.

Fasted vs. Fed State

The body can be either in a fasted or fed state. But it isn’t so black and white. There’s a transition between the two. From the time we eat and while we’re digesting that food, we’re in the fed state.

After having a meal, we’re still in the fed state. We don’t go to the fasted state right away. It takes some time for us to digest and break down the food that we still carry in our bellies after a few hours.

How fast we can transition into a fasting state depends on what nutrients we ingested, the quantity of food we’ve eaten, the glycemic index of foods, and whether we had fats or carbohydrates (or mix).

Fun Fact

The feeding window lasts between our first and last meal. If we have our first meal at 12h noon and our last meal at 8 o’clock, we’re doing the 16:8 type of fasting. The 16 hours of fasting helps us regenerate, cleanse, detox, and remove dysfunctional cells.

fasting and health

Benefits of Fasting

There are numerous benefits of passing, especially on metabolic health.

Fasting has been shown to increase HGH hormone, but not necessarily anabolism. By stimulating autophagy, it reduces inflammation, helps us remove dysfunctional cells, and improves regeneration.
(1) (2) (3)

Fasting is also a very practical and effective tool for weight loss since it can conveniently put us in a caloric deficit (10-30%) without counting calories. People practicing fasting have improved glycemic control, utilization of glucose, and insulin sensitivity.
(4) (2) (3)

It can also beneficially impact our cognition by increasing alertness and memory. It can reduce neural inflammation and oxidative stress, thus acting neuroprotective. It may also increase certain neurotropic vectors, like BDNF, which help with neuroplasticity.
(5) (6) (7) (8)

fasting metabolic health

Fasting and Health – Glucose Variability

When we ingest food, especially carbohydrate-rich food, blood glucose increases. This elevates insulin, which works to transfer glucose into muscle tissue. It’s all good until the spikes and drops of both glucose and insulin are too high, frequent, and irregular.

Why? Because increased glycemic variability, or the spikes and drops of glucose, lead to poor metabolic health, increased inflammation, and higher mortality risk. Unstable glucose levels can be a strong predictor for cardiovascular mortality. (9) (10)

Comparing 48,843 Chinese patients with type 2 diabetes – Those with higher glucose variability and mean fasting blood glucose levels showed an increased risk of premature deaths by 23 and 38%. (comparing 95th with 5th percentile) (11)


High variability of glucose and insulin is a strong predictor of mortality, poor metabolic health, and cardiovascular disease. One way to improve metabolic health is to keep our glucose stable

Fasting Offsets Chronic Elevation of Insulin

When we are fasting, we are restricting our feeding window, thus ingesting glucose less frequently. Because we’re not shoving glucose into our bellies all the time, insulin can reset.

When we run off glucose, our body needs to tap into fat stores, and use them for fuel. This happens during a ketogenic or high-fat low-carb diet. Doing so enables our body to improve its metabolic flexibility which leads to better health.

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.

Which fasting method is best for weight loss?

Intermittent fasting is the most practical, as it’s easy to adhere to while maintaining a relatively normal eating schedule – only skipping breakfast. On the other hand, science tips the scale more toward 5:2 fasting, or 5 normal eating days with 2 days of <25% of total calories ingested, which puts you at a significant caloric deficit.

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.

When does autophagy start during fasting?

Autophagy starts within 16-24 hours of fasting, but it greatly depends on the person. More metabolically flexible people can enter ketosis way faster, so they can kick-start autophagy faster as well.

A study in prediabetic men experimented with 6-hour early feeding, from 9 am to 3 pm. Early time-restricted feeding improved insulin sensitivity and beta cell responsiveness, it reduced exceeded stress and blood pressure. (12)

By increasing our fasting window, intermittent fasting can have multiple metabolic effects that go beyond just weight loss. Including decreasing fasting insulin and glucose, improving insulin sensitivity, and decreasing levels of leptin. (13)

Fat vs Carbs for Fasting

Another very important factor that affects fasting is the type of macronutrients we consume. Carbohydrates cause steeper increases in glucose and insulin compared to fats. Carbs also put us out of the fasting state quickly and more effectively.

So if you had a large bowl of pasta, your glucose and insulin spike will be steeper and last longer. On the other hand, if you had half an avocado due to the smaller volume, and only fat content, the glucose, and insulin response will be much lower.

Pro Tip

One simple way to reduce glucose variability is to eat a lower carbohydrate diet. Another powerful tool is to eat low glycemic index, foods like grains, legumes, and lentils. A great option is to switch from cereal to oath from rice to quinoa and from white pasta to whole grain pasta.

How Different nutrients affect insulin

Studies comparing carbohydrate and fat diets and people with obesity or diabetes have shown that three food elements are important. Monounsaturated fatty acids, fiber, and low glycemic index food for better glycemic control and glucose stability. (14)

Research comparing carbs to fats showed fats were more favorable for improving insulin secretion and insulin resistance. MUFA lowered HbA1c and improved HOMA-IR, the insulin resistance assessment model. Polyunsaturated fast showed superior effects compared to saturated fat. (15)


Swapping carbohydrates for fats, as in the ketogenic diet, can mimic some of the effects of fasting. It relies on the primary mechanism of tapping into fats for energy, which can be beneficial for glycemic control and insulin secretion.

Weight Loss: Calories in-out foundation

There have been a ton of trendy diets for weight loss. And while some diets provide extra benefits – It mainly comes to calories in vs calories out. So technically, even if you eat unhealthy, as long as you expend more calories than you ingest, you’re going to be in a fat-loss state.

With exercise or not, negative energy balance – meaning expending more calories than you ingest, leads to weight loss. (16)

Before you order your next pizza, wait a second. With a poor diet that’s high in trans fats and low in nutrients, you’re gonna eventually run into nutritional deficiencies, increase inflammation and throw your hormones out of wack. So, it’s extremely important to keep eating a healthy nutrient-dense diet.

Pro Tip

Fasting has a myriad of different effects, which helps us lose weight. From burning fat for energy, better glucose utilization, to a caloric deficit and control of hunger hormones.

Fasting effect on mTOR, AMPK

The body is usually in a cyclic activity between anabolism and catabolism. mTOR is the anabolic guy, important for growth, proliferation, and glucose metabolism. His friend, AMPK is the catabolic guy we stimulate with fasting. The interplay of the two determines which state (catabolic vs anabolic) will take over. (17)

Anabolism is about creating bigger molecules from smaller ones. Anabolism happens when we build muscle or gain fat. Higher insulin and glucose levels (eating more) correspond with anabolism.

Fasting mimics the catabolic state, the no-eating phase. This is when you shrink a tissue, when you reset and recycle old junk cells, and when you burn fat, for example. By doing so, fasting helps remove dysfunctional cells, clean the brain, detox the liver, etc.

The way fasting stimulates catabolic pathways is by activating the AMPK protein kinase, which stimulates autophagy. In such a situation, there is a decrease in glucose nutrients in amino acids, which correspond to lower mTOR, thus lowering anabolism, protein synthesis, etc. (18)

Anabolism Impaired (muscle growth and fasting)

Eating a high-protein breakfast favors muscle growth, which tells you everything. To support muscle growth, a few factors come into play. Such include resistance training, higher protein (growth) and carbohydrate (energy) intake, and being in a caloric surplus.

Fasting is a better way to get leaner and not the optimal way to get bigger. It’s because pasting stimulates catabolic pathways, which can help you lose fat. If you can fit all four factors mentioned above, you can still grow muscle during fasting. But it’s not the optimal way if it’s your primary focus.

The reason is, with fasting, we reduce insulin and glucose levels, which impair anabolic. This reduces how high the protein synthesis in your body is, being suboptimal for maximal growth. (19)

Pro Tip

One way to optimize for muscle growth or sustain muscle during fasting is adding BCAA’s or branched-chain amino acids in the morning before your first meal. While technically it will break your fast because you ingest calories, the change in metabolism is so minimal, you’d get most of the benefits fasting has to offer.

Irregular Eating Schedule Disrupts Circadian Rhythm

One of the reasons fasting was great for many people is because it forces them to eat on a regular schedule. It’s extremely important where you place your eating window and how frequently you stick to the schedule.

Eating very frequently or irregularly can cause different cardiometabolic changes which result in an increased metabolic syndrome risk. It can elevate glucose levels, blood pressure, and increase fat accumulation in the abdominal region. (20)

Erratic eating patterns or eating at irregular times can disrupt your circadian rhythm. This can result in impaired sleep, impaired glucose tolerance, and increased inflammation – which all lead to obesity. Plus it’s detrimental to metabolic health. (21) (22) (23)

Research shows that fasting can offset this. Time-restricted eating, especially if you can stick to a regular eating schedule can mitigate circadian disruption and improve cardiometabolic health.

Pro Tip

Sticking to a regular eating schedule can ensure you get the maximal benefits from fasting. Placing that eating window differently each day can disrupt circadian rhythm, increase inflammation and be detrimental to metabolic health.

Fasting improves metabolic health markers

Metabolic health has a lot to do with cholesterol levels, glycemic control, and insulin sensitivity. The goal is to produce energy efficiently on the cellular level, while maintaining stable glucose and insulin levels.

Research in humans shows that fasting can improve metabolic markers, and reduce blood glucose and triglycerides in both 16/8 TRF fasting and alternate-day fasting. (24) (25)

Studies show that fasting can improve cholesterol profile by reducing all three total cholesterol by 13% LDL by 15% in triglycerides by 22%.. The alternate date, fasting, as shown, decreased an insulin resistance assessment model and improved the atherosclerotic profile. (26)

I mean, what more could you ask for not eating? It’s quite a cheap and practical solution, nah?

What breaks a fast?

The question that always pops out is, what breaks a fast? Can coffee break a fast? Can water break a fast? Can a handful of sesame seeds break a fast?

It is more about context. So remember when we said that the ketogenic diet uses fat for fuel, and you’re technically still in a semi-fasted state? Well, most of the benefits of fasting come primarily through fasting from sugar. Does that mean that we can eat fat?

No, but in terms of how much will that break a fast, it’s less than that of glucose. See it’s not about whether you’re fasting or not. It’s not a yes or no answer. It’s about how much you’re faster. I’ll show you an example.

The steepness of transition fasted to fed state

The way we measure fasting is through glucose and insulin response. Once your metabolism kick starts, once glucose has increased, insulin is secreted and your fast is broken.

But what if we compare eating a full bowl of pasta versus adding 10 grams of collagen peptides into your coffee. Technically, both will break a fast, but the collagen peptide’s coffee, leaves you fasting at a way higher rate. Since glucose is not increased by as much, the benefits of fasting are still there.

If we use this model, we can see that a marathon runner eating two avocados after a run won’t really break his best. Why? Because certain factors like activity, the type of macronutrient consumed, and the glycemic index of food affect your fasting state or “in how much of a fasted state you are”.

Context of Fasting: The No-Rule Rules

Here are some no-rule rules that can explain to you how much of a fasting state you’re in, depending on the situation.

  • Water, green tea, and black coffee do not break a fast.
  • Fats break your fast less compared to glucose.
  • Eating after an activity breaks your fast less because your muscles need nutrients.
  • Glucose ingested after a workout will increase blood glucose less, as it will be transported to tissues.
  • The deeper you’re in a fast, the harder it is to break it. After 16 hours of fasting, one peanut probably won’t break your fast.
  • Low glycemic foods like lentils, beans, grains, and oats are not increasing blood glucose and insulin as much, thus maintaining more stable glucose variability compared to simpler, white carbohydrates (fruits, bread, pasta, rice).

Instead of asking if is fasting broken or not, a better question would be to which degree am I fasting? And that is determined by the glucose rising in your blood. Hence why low glycemic index foods and fats are better than simpler sugars (white pasta, rice, bread, soft drinks) in this scenario.


  • The body can be in a fasted or fed state. Fasting is about reducing your feeding window so you can fast for longer.
  • Fasting can provide a myriad of metabolic health effects like improving cholesterol profile, glycemic control, weight loss, and insulin sensitivity.
  • The mechanism behind fasting is similar to that of keto. By starving the body of sugar, it can practice metabolic flexibility by tapping into fats for fuel.
  • Fasting is effective because it keeps people on a regular eating schedule. Irregular eating patterns increase inflammation and are detrimental to metabolic health.
  • Prolonged fasting is not the most optimal for muscle anabolism, as it primarily stimulates catabolic pathways by activating AMPK and reducing mTOR activity.
  • Coffee water and green tea do not break a fast. Fats break your fast less than carbohydrates.
  • Determining how much in a fasting state you are depends on the rise of your blood glucose levels in insulin. Anything that doesn’t cause a steep rise in glucose and insulin in the body (like fats, coffee, tea) can offer most of the benefits of fasting.

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