Intermittent Fasting and Time Restricted Feeding Overview
Updated: March, 16th, 2024
In this article, I cover what Intermittent Fasting and Time Restricted Feeding is and how it works based on the evidence we have to date.
Intermittent fasting has been a popular dietary practice for some time now. Since 2010 it has been gaining in popularity with a peak in 2012 and following in recent years.
Although Intermittent fasting is connected with different benefits, most people do it because of aesthetic reasons – weight loss.
Interest over time

What is Intermittent fasting / Time restricted eating?

Intermittent fasting is not a diet. It is a meal timing schedule. Naturally, we have periods of time when we eat and periods when we don’t.
Key terms:
  • feeding/eating window – time during, which you consume food
  • postprandial state – time referring to after food has been consumed
  • post-absorptive state – time after a meal has been eaten and ingested, usually 8-12h after the meal
  • fasting/ fasted state – a period of time during which you voluntarily or involuntarily abstain from food, mostly used in religious practices like Ramadan.
Most people eat several meals spread throughout the day with some snacks in between the main meals. With food being available everywhere, people can easily be in feeding or postprandial state for the most of the day. That can be problematic as your body is constantly working on processing the food, your blood glucose is constantly elevated and you are likely in positive energy balance. Story told short, you gain weight.
Intermittent Fasting and Time Restricted Feeding (Eating) refer to having scheduled periods of day during which you eat and during which you fast. The most popular schedule is 16:8, which allows for 8 hours of eating/feeding time. For example from noon to 8 p.m. Then you fast for the rest of the day (16h).
Although 16:8 is the most popular fasting schedule, there are several protocols:
  • 16:8
  • 20:4 also called a warrior diet
  • OMAD (One Meal A Day)
  • 5:2 consisting of two non-consecutive days of low calories (500-600) and five days of higher calorie days
  • Extended fasts lasting 24+hours
  • Alternate Day fasting  – alternating between 24h eating and 24h fast
Time Restricted Eating usually refers to shorter periods of feeding time (6-10h) during the day. The idea is that eating during daylight is aligned with our circadian rhythm and has more benefits for our health.
Most people prefer eating from noon to 8 p.m. due to ease of implementation.

Benefits of Intermittent Fasting and Time Restricted Feeding

Some studies show IF may lead to:
  • weight loss,
  • stabilized blood sugar,
  • reduced inflammation,
  • improvements in memory and stress resistance,
  • slowed aging, and longer lifespan,
  • improved metabolic health,
  • improved hormonal health,
  • make better eating choices.
Besides that, having fewer meals throughout the day can help you:
  • be more productive,
  • get more time that would otherwise be spent preparing food and eating,
  • it can help you make better nutritional choices
  • experience the real hunger,
  • reveal problems like food addictions.

Potential negatives of Intermittent Time Restricted Eating

Just like any other restrictive dietary approach, there are potential downsides to time restricted eating:

  • It can lead to chronically low energy intake,
  • Malnourishment,
  • Exacerberate disordered eating tendencies.
  • Lower performance in athletes, once again due to low calorie intake
  • Dogmatic adherence to fasting can mask other issues,
  • Digestive issues if you cannot handle bigger meals,
  • Hormonal disregulation, especially in women,
  • Can lead to restriction -> binge eating cycle,
  • Over long term can cause blood sugar issues in some people,
  • It is not a ‘get out of jail free card’ – you should still eat nutritious foods.
There is a myriad of benefits linked to fasting practices, which often lead to misinterpretations, overexagerated information, and false beliefs.
Most people who try fasting with positive results also start exercising and make better choices about what they eat.

Some of the changes in body induced by fasting

Insulin
After eating food, insulin increases at first to manage the influx of blood glucose. Several minutes later (depending on composition and quantity), your insulin drops as it is no longer needed to ‘clean’ blood glucose from blood. If your meal is high in fat and low in carbohydrates, insulin will be secreted in small doses and vice versa.
Why is lowered expression of insulin important? If we constantly keep producing insulin in excessive amounts, our body becomes resistant to it – the body stops responding to insulin and we need to produce higher doses, which leads to pre-diabetic state.
However, it happens only in the state of positive energy balance (when you have more energy from food available than what you expend).
If you eat whole foods and stay active, your risk of developing type 2 diabetes will be greatly diminished regardless of your eating window.
Insulin causes weight gain in diabetic patients “…insulin simply restores healthy blood glucose levels and allows us to weigh what we should based on our diet and exercise patterns.” (source)
  • Physical activity helps with blunting insulin and blood glucose after meal.
  • Eating protein, fiber, and fat with carbohydrates helps slower the rise of blood glucose blood glucose and therefore insulin, after the meal.
  • An Intermittent Fasting diet may provide a significant metabolic benefit by improving glycemic control and insulin resistance in general population. (source)
Growth Hormone
Growth hormone is upregulated during fasted time, which lead to a big myth that fasting is anabolic (builds muscle). Fasting cannot be anabolic by definition as it is a catabolic process.
Increased GH serves to preserve tissues. It is increased by sleep, exercise, or trauma. There has not been proven increases in performance or strength when GH is supplemented. (source)
Growth hormone (GH) has no effect on human muscle size or MPS (source) but do have a positive effect on strengthening the collagen matrix in musculotendinous tissue (source).
Inflammation
Fasting can help to reduce systemic inflammation.
Why is it important to manage inflammation? We experience acute increase in inflammation after exercise or when fighting infections. However chronic inflammation leads to heart disease, gut problems, digestive issues, diabetes or joint issues. It is caused by the modern lifestyle when we are constantly exposed to environmental and mental stress.
Some of the new research suggests that chronic inflammation can be a cause of lowered mental performance resulting in slugishness and brain fog. (source)
Chronic inflammation can be downregulated by practicing fasting and time restricted feeding by restriction of caloric intake. (source)
Metabolic Flexibility
Metabolic flexibility refers to the body’s ability to effectively switch between glucose and fat metabolism. For example, obese people can be metabolically inflexible and their body’s rely on glucose and can’t access stored fat.

In humans, six months of a 25% CER has been shown to improve metabolic flexibility, as evidenced by increased shift in fasting-to-postprandial concentrations of acyl carnitine (important for transfer of fatty acids into the mitochondrion prior to oxidation). There are currently no data of the effects of IER on metabolic flexibility in humans. (source)

I suppose the effects of caloric restriction, whether continuous or intermittent will lead to the same metabolic flexibility.

Cognitive functions
Caloric restriction such as that induced by fasting or has been shown in animal models to reduce the risks of Alzheimer’s, Parkinson’s and Huntington’s diseases and stroke.
Moreover, it can stimulate the production of new neurons from stem cells and can enhance synaptic plasticity, which may increase the ability of the brain to resist aging and restore function following injury. (source)
Some people also report “clear”thinking when they are in the fasted state. Eating food and substances one is sensitive to, big volume of food and blood glucose fluctuations may impair cognitive performance. This would explain why some people feel benefit when they fast as opposed to when they eat food. Some studies show no impairment in cognitive abilities while fasting, while the others do. But neither show benefit of fasting for cognitive tasks. (source)
Human brain uses 120g glucose a day for proper functioning, unless in nutritional ketosis. A systematic review of studies in children and adolescents show that skipping breakfast is more likely to have negative effects on academic performance than eating breakfast. Breakfast consumption is more beneficial than skipping breakfast, but this effect is more apparent in children whose nutritional status is compromised. There is a lack of research comparing breakfast type, precluding recommendations for the size and composition of an optimal breakfast for children’s cognitive function. (source)
A recent (2020) small study in students (unknown age) during ramadan fasting shown no differences in mathematics and memory during ramadan fasting compared to after ramadan. (source)

How does fasting affect sex hormones?

Intermittent fasting is a popular diet but there have been concerns about its effect on hormonal health. A meta-analysis from 2022 looked at how fasting affects sex hormones in men and women.

The studies covered in this meta-analysis were predominantly obese, premenopausal women (18-45 y.o.) and included 5:2 fasting protocol as well as 16:8 fasting protocol. One study examined the distribution of energy intake during the day, where more than 50% of calories were consumed by participants either in the morning or in the afternoon.

Studies in men were on young, healthy men 19-35 y.o., all using 16:8 fasting protocol.

Female hormones

The researchers studied testosterone, DHEA-S, Androstenedione, FAI, estradiol, and Sex hormone-binding globulin (SHBG). Without going into too much details, there made some interesting findings.

Estradiol

Estradiol is the main female reproductive hormone and is involved in the development and maintenance of female reproductive tissues and the regulation of the menstrual cycle. We can observe, that obese and overweight women have higher levels of estrogens.

Women with PCOS, who ate >50% calories in the later part of the day had higher production of estrogens that lead to HPA impairment. HPA dysregulation is connected to lacking or absent ovulation.

Androgen levels in women

Increased androgen levels in females is portrayed by hirsutism (i.e., excessive hair growth), seborrhea (scaly patches on the body and scalp), and disorders in the menstrual cycle. It is also connected t insulin resistance, fat storage, and decreased ability of the body to handle glucose.
A study comparing women with PCOS eating >50% of calories at later part of the day shown unfavorable results. Androgens FAI, DHEA-S, and androstenedione decreased significantly in the breakfast group relative to the dinner group. These changes occurred without weight loss.
These findings suggest that fasting can significantly decrease androgen markers in premenopausal females and those with PCOS, especially when calories are consumed earlier in the day, leading to health improvements.

Sex hormone-binding globulin (SHBG)

Observational studies show that low levels of SHBG are associated with an increased incidence of insulin resistance and type 2 diabetes, independent of sex hormone concentrations.
Again, these preliminary findings suggest that intermittent fasting regimens may produce beneficial increases in SHBG concentrations in premenopausal females and those with PCOS, particularly when most of the food is consumed in the morning or afternoon.
Intermittent fasting seems to have benefits for women who are overweight, obese, and those suffering from PCOS independent of weight loss, especially when the foods are consumed in the first part of the day. People who do intermittent fasting typically do the opposite – they skip breakfast and have a large dinner.

Note that these beneficial results were obtained in overweight, obese, premenopausal women and women with PCOS and will likely not be the same for healthy and active women. Active women and athletes often struggle with insufficient energy intake, which poses risk to health, including hormonal health.

Male hormones

The researchers studied testosterone and SHBG.

Testosterone

Testosterone is the major steroid hormone in adult males and is responsible for maintaining sperm production, libido, motivation, and it stimulates muscle protein synthesis, thereby increasing muscle mass.
Most studies combined 16:8 fasting protocol combined with resistance training.
All studies showed a lowering effect of time-restricted feeding on total and free testosterone suggesting negative effects on male health, but the results shown improvements on body composition (reduction of fat mass).

Sex hormone-binding globulin (SHBG)

SHBG is a glycoprotein that serves as a major carrier of testosterone in blood circulation—carrying 40–45% of bound testosterone— When blood testosterone levels are higher, SHBG levels are higher as well. Obese individuals have lower testosterone and SHBG levels.
16:8 intermittent fasting does not seem to affect SHBG levels.
The studies on lean, young, physically active men shown decreased testosterone levels, without negatively impacting strength or body composition. The studies, unfortunately, did not measure sleep quality, motivation, or other effects of testosterone. What other studies suggest is that 16:8 fasting does not have a negative effect on men, if they have sufficient energy intake, which can be hard to achieve for highly active men such as athletes.

Is it fasting / time restricted eating or calorie restriction that drives fat loss?

It is a well established fact that caloric deficit is the primary drive leading to weight loss and improved health outcomes. Some people claim that increased time without food leads to improvements of body composition, improved health markers and weight/fat loss even without reduction in caloric intake. Is is true?

“Intermittent fasting is an equivalent alternative to continuous energy restriction for weight loss.”

Some studies concluded that continuous energy restriction and intermittent energy restriction give similar results (source, source), but fat loss happens as a result of caloric deficit. (source).

Our results suggest that an intermittent fasting program in which all calories are consumed in an 8-h window each day, in conjunction with resistance training, could improve some health-related biomarkers, decrease fat mass, and maintain muscle mass in resistance-trained males.
But there are cases and studies that show that you can lose fat even when you don’t restrict calories (body recomposition).
For example this randomized cross-over study compared 10 female and 5 men, 40-50y.o. with normal body weight at maintenance calories eating three meals a day vs. one meal a day. One meal a day group lead to weight loss (-1.4kg) with no changes in fat-free mass, while three meals group did not lose any weight. (source) The authors noted that there was a small caloric deficit of 65kcal per day in the one meal per day group. in conjunction with measurement imprecision, this may explain the differences in body fat measurements. They also reported an increase in hunger throughout the day!
This study in trained athletes shows it is possible to employ time-restricted diets to reduce body fat, while maintaining muscle and performance. (source)
In obese people, “Intermittent fasting without calorie restriction can enhance health and cellular resistance to disease without losing weight and those effects may be attributed to different signalling pathways and circulating ketones during IF” (source).
But intermittent fasting in free-living conditions (in absence of other interventions, resistance training and calorie control) is not more effective in weight loss than eating throughout the day. More often than not, without tracking macros and calories, people adhering to the intermittent fasting alone can lose more muscle mass. (source).
A controlled clinical trial (high-value of evidence) in 2021 sought to sought to determine how alternate day fasting (ADF) affected weight loss, fat loss, lean body mass, metabolic rate, and various other metrics in lean (BMI<25), free living people compared to people who engaged in either intermittent fasting without weight loss, or continuous caloric restriction (CCR).
While CCR and ADF groups both lost weight, the weight lost in the ADF group was 50% muscle mass, while the CCR group lost weight mostly from fat! (source).
The benefits of IF regimens for weight/fat loss occur thanks to energy restriction.

Intermittent fasting / time restricted feeding and lifespan

As I discussed above, fasting, caloric restriction, certain foods, and exercise upregulates autophagy, which is connected to lifespan in several species. The benefits of fasting thus boil down to improved metabolism and overall improved health through weight loss, with no additional benefits for healthy individuals.

Caloric restriction (CR) extends healthspan and lifespan in multiple organisms (,).

image

Escobar, KA, Cole, NH, Mermier, CM, VanDusseldorp, TA. Autophagy and aging: Maintaining the proteome through exercise and caloric restriction. Aging Cell. 2019; 18:e12876. https://doi.org/10.1111/acel.12876

Conclusion

To conclude, timerestricted eating strategies are viable options for weight loss, improvement of health markers, and prevention of several neurodegenerative illnesses.

More long-term studies on humans need to be made to come to viable conclusions, which could prove some of research to be at least partially wrong.

IF and PF, as well as TRF have emerged as potential strategies for avoiding major dietary changes while achieving strong effects not just for one diseases risk factor but for an array of factors that constitutes the foundation for metabolic syndrome, cardiovascular disease, cancer, and possibly neurodegenerative diseases. (source)

However, if time restricted eating works for your life style, it is a good tool to keep your energy balance in check and possibly reap other benefits as well. As we can see in the studies presented, if you are overweight, practicing IF can improve your metabolic health. If you are lean however, you will benefit from a sustainable eating pattern with more frequent feeding pattern.

There are many fasting protocols to choose from, so it is up to your preference if you choose to change your eating schedule. There might be also differences between genders that need more research. Generally, men seem to be less prone to negative effects of fasting.

There is much more to cover

There is much more to cover like how to break your fast or if it matters what time of day you fast and for how long or fasted training.

I will cover these topics in future articles as well as provide my experience with fasting.