Some people have great success with intermittent fasting (IF) because fasting curbs their appetite. The result is that it helps them stick to it and enhance its benefits.

However, people differ greatly in their ability to tolerate fasting so it’s critical to pay attention to your body’s responses and what it is telling you.

During the transition from fed to fasting, a starvation-like response  –  involving extreme hunger, release of stress hormones and a change in metabolism – can kick in.

A side effect is that hormones controlling hunger and satiety can get out of balance, leading us to become unresponsive to cues that we’re full when eating again.

These shifts in hunger hormones may or may not occur depending on the total fraction of time you are fasting, your diet and how you’re implementing IF.

Here, we review the evidence regarding IF’ing and hunger hormones, explore how IF affects appetite and provide you with a few tips to help ease the transition and overcome hunger.

Defining IF and how this article works

This article isn’t a global argument against IF. Rather, it points out that those who are just starting to fast could have trouble. Reportedly, the hardest part of fasting is the first day, and that’s usually only if you eat high-carb and/or not used to go long periods without eating.

There are several kinds of intermittent fasting. The type most frequently used in scientific studies is alternate-day IF (ADF), while the types being used in the public are mostly either 1-2 weekly fasting days or part day fasts, like the 16:8 fast.

As far as the state of the art goes, there are a few studies on ADF, one or two on intermittent fasting reminiscent of 5:2 fasting and a very limited number on part-day fasting (e.g. 16:8) tracking changes in hunger hormones, at the time of writing.

ADF also involves calorie restriction (CR). There are differences between CR and IF where hunger and hormones are concerned and it can be hard to determine which of the fasting or CR is having an effect on hunger hormones.

In this article, the term “fasting” will be used generally to refer to all of the aforementioned IF methods, with specific description of protocols when appropriate.

Fasting and hunger-reducing hormones

Most people don’t realise that there are a plethora of hormones regulating hunger, appetite and satisfaction after a meal.

Their levels and signals change with changing food intake and how much fat you’re carrying. There is also some indication of a gender difference in response to them.

These hormones, released from various organs and tissues, have direct effects on appetite and impact how well or how poorly feeding is regulated in the brain.

Among those directly involved in appetite regulation that play a major role are two of particular interest to today’s discussion: the gut-derived hormone ghrelin and another secreted from fat cells known as leptin.

Ghrelin stimulates appetite, promotes fat storage and lowers energy expenditure while leptin decreases hunger, sends a signal to your brain that you’re full and turns eating ‘off’.

Fasting and ghrelin

Ghrelin becomes entrained to normal eating times and once one starts fasting, its levels and secretion might change, as suggested by research from the University of Birmingham on ghrelin and feeding frequency.

The study compared a fast with a low-frequency meal regimen (two meals) or high-frequency one (12 small meals) across eight hours.

The increased meal frequency tended to maintain stable ghrelin levels during the day and the lower meal frequency caused higher variations in ghrelin. During the fast, albeit short, ghrelin rose slightly with time and took longer to decrease.

Paradoxically, with a longer fasting window of 24 hours, ghrelin doesn’t seem to get any higher. However, after eight weeks of ADF, it’s a different story. One study found a significant increase in ghrelin levels, even with no change in actual hunger feelings.

It has been said that chronically elevated levels of ghrelin are associated with changes in body fat. Further research linked high ghrelin to positive feedack loops in the brain’s reward circuitries and emotional eating.

It would seem, though, that issues with high ghrelin can be at least partly mitigated by eating a diet containing the right balance of nutrients.

The composition of the mealthat breaks the fast seems to matter to keep ghrelin levels in balance. Ghrelin is least affected by fats, whilst carbs often cause a rebound hypersecretion in ghrelin.

Increasing proteinespecially is one way to limit hunger and may even maintain satiety in the face of high ghrelin. Ghrelin is also higher when sodium intake is low, so maintaining a good salt intake could make a difference.

Fasting and leptin

Leptin is a master-hormone with downstream effects on other hormones related to appetite. A drop in leptin increases hunger and is thought to cause weight loss plateaus.

People who are overweight or obese typically have high leptin but are resistant to it while leaner individuals may have lower levels but higher sensitivity.

As leptin communicates signals to the brain that regulates satiety, a lack of leptin means the brain won’t necessarily register satisfaction and hunger could remain elevated.

Extended periods of fasting almost invariably lead to a loss in body fat. As soon as this happens, the number of factories for leptin production (remember leptin is derived from fat cells) will decrease, and therefore so will the absolute amount of leptin in circulation.

According to research, leptin levels can rapidly decline in the fasted state and one possible result of this is an increase in appetite. One study reviewing leptin’s role in weight loss has also found that this rapid fall in response to fasting may push people toward overeating.

The magnitude of the decrease in leptin varies. A short-term study found that college women at the University of Virginia who fasted for two days experienced a 75 per cent drop in lepti, although this was probably compensated by a big boost during refeeding.

Another trialin obese participants, which involved eight weeks of ADF, found that leptin concentrations were reduced by 21 per cent by the end of the experiment.

There’s also contradictory evidence. Women fasting during Ramadan actually show a big increase in leptin. The fasting also decreases neuropeptide-Y, a hormone that stimulates hunger.

It is worth mentioning that the study failed to address effects of late night eating on the body’s natural circadian rhythm to hunger, which is inherent in our genetic makeup and affects both leptin and ghrelin independently of the eat/fast cycle.

As far as leptin goes, it appears that the typical modern lifestyle (fast food, little or no exercise, too much stress and not enough sleep) greatly contributes to its disruptio, with lack of sleep being a major culprit.

Research done at Laval University in Quebec found that 7-8 hours of sleep each night, on average, was associated with relatively higher levels of the hormone. In contrast, getting less sleep meant lower levels of leptin and higher body mass indexes in the study.

So sleep the recommended 8 hours a day if possible. If not, sleep more in general as your body makes less leptin if you don’t get enough rest. If you’ve ever felt hungrier the day after a poor night’s sleep, this could be because your leptin levels have dropped.

Summing up fasting and hunger hormones

Hunger is not so simple as ‘the longer you don’t eat, the more hungry you’ll be’. There are many more subtle inputs and a complex hormonal regulation of hunger is at play.

Hormones that trigger eating, like ghreli, may get amplified in response to fasting. However, ghrelin usually peaks at day 1-2 and then steadily falls.

This aligns with what is seen clinically, where hunger is the worst problem at onset. Many people on longer fasts report that hunger typically disappears after day 2.

Fasting can also suppress hormones inducing satiety, like lepti, but probably more so if calories are restricted during the eating window and when leading an unhealthy lifestyle or not sleeping well.

Individual variability in the fasting response

Fasting impacts everyone differently and the negatives with regard to appetite regulation are more likely in lean and very active individuals, those with high stress levels and poor eating habits or with protocols that take fasting to the extreme.

One population that probably shouldn’t use IF’ing* are those with a previous eating disorder, like bulimia or anorexia, or even those with a tendency towards disordered eating patterns.

In this situation, IF could put them right back into the type of alternated restriction and binge cycles that they had in the first place and cause them to lose total food control when fasting.

*People who are severely underweight, pregnant or breastfeeding women, and children and adolescents should also not fast. And anyone on medications should check with their doctor that these are not contraindicated.

How to conquer hunger pangs while fasting

As with all things, certain individuals may be more susceptible than others to the loss of control over food intake when breaking the fast. But it seems overall truer for those attempting to limit themselves to a single meal.

For most people, a moderate approach to IF like the 16:8 method, can convey the benefits of fasting (fat burning and insulin sensitivity) without significantly affecting hunger hormones or causing uncontrolled eating.

Finally, becoming aware of the emotions behind eating and making the distinction between a hedonistic drive to eat, i.e, being interested in food and bored, and true hunger can make the transition to fasting easier.


Further reading 

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