What is IF, or intermittent fasting?
IF is nothing more than intermittent fasting. This eating pattern is divided into periods called “eating window”, in which you can eat normally, and periods called “fasting”, in which it is forbidden to take foods and liquids that provide energy. During fasting, you can drink water, as well as calorie-free drinks: black coffee (sugar-free), infusions and teas.
There are different IF schedules which are most often divided according to the duration of the eating window.
TRF intermittent fasting
Time restricted feeding (TRF) is an approach that divides the daily routine into fasting and eating window of different lengths:
- 14:10 – 14 hours of fasting and 10 hours of eating window. The least restrictive variation of IF.
- 16:8 – 16 hours of fasting and 8 hours of eating window. The most popular variant of IF, also known as the “8-hour dieting schedule”.
- 18:6 – 18 hours of fasting and 6 hours of eating window. Considered restrictive.
- 20:4 – 20 hours of fasting and 4 hours of eating window. Drastic variant of intermittent fasting.
The exact hours of the eating window are adapted to the individual daily schedule and needs. IF normally involves skipping breakfast and eating the first meal in the second half of the day. The most popular variant of IF, i.e. 16:8 schedule, may look like this: “You eat your first meal at noon and finish your last meal before 8 PM.” The eating window lasts from noon to 8 PM – exactly 8 hours.
You may find yourself following a less restrictive variant of IF even unconsciously if you skip breakfast and make sure not to eat until about 4 hours before bedtime.
ADF intermittent fasting
ADF (alternate day fasting) intermittent fasting involves using eating windows on a daily basis. It can be used in the following variants:
- 2:1 – 2 “normal” days and 1 fasting day.
- 5:2 (also called the 5:2 diet) – 5 “normal” days and 2 fasting days.
- 1:1 – 1 fasting day, 1 day of normal eating pattern.
On non-fasting days, you eat according to your appetite, and on “fasting” days, you limit your food intake to a maximum of 25% of your daily requirement – in practice, this means only one small meal a day. In restrictive variants, during the “fasting day” you drink only water and abstain from taking any food.
Such a strategy is to introduce an energy deficit on a weekly basis and in this respect it can work. However, the ADF schedule is less popular than TRF.
Does intermittent fasting help reduce body weight?
Intermittent fasting is primarily used as a way to reduce body weight. Body weight reduces only when a caloric deficit is achieved and maintained. Measuring the effectiveness of the IF diet is therefore simple in theory – if maintaining an eating window helps you maintain an energy deficit, IF can help you achieve results. However, if you consume as many calories as you actually consume (or more) during the eating window, you will not achieve the expected results.
As usual, weight loss (with or without IF) comes down to simple mathematics and is based on energy balance.
Read also: 6 non-obvious reasons that make weight reduction difficult
Who will the IF diet work for?
Intermittent fasting is certainly not for everyone, but it can work for those who:
- consume especially many calories in the evenings – shifting the eating window so that it ends about three hours before bedtime can naturally support a negative calorie intake.
- snack between meals – “fasting” can be a good mental barrier that will protect you from following cravings impulsively, snacking and exceeding caloric needs.
- have well-established healthy eating habits – fasting can be a good way to limit the number of meals consumed and introduce an energy deficit, but only on the condition that the meals consumed during the eating window are valuable and “healthy”.
- are frustrated with a traditional reduced-calorie diet – the eating window can make it easier to maintain an energy deficit for those who are often hungry or think a lot about food on a traditional reduction diet. Imposing specific time limits on which meals are taken can reduce the mental burden of “dieting”.
- like eating larger portions of food at once – eating five small meals will not work for everyone. For many people, this can be frustrating, especially with low calorie content, when meals tend to be small in volume. Those who prefer to eat fewer large meals can benefit from the IF method.
Who should not follow the IF diet?
The IF diet approach is not recommended for those:
- With a regulated lifestyle and a healthy relationship with food, intermittent fasting can cause problems that have not occurred before, driven by an external “restriction”. For example, you don't usually have a late dinner, but because you're on the IF diet and you're “forbidden to eat after 7 p.m.”, you start thinking about food.
- Eating regularly and thus feeling well – if you feel good in a schedule where you eat many small meals every day, do not switch to IF. The evidence for the effectiveness of this method is not so great that it would be good for you to deregulate your current schedule.
- Who do not have a problem with excessive snacking – the IF diet mainly solves the problem of snacking between meals (especially in the evening). If the problem of snacking does not concern you, the IF diet will most likely not help you achieve results in terms of body weight.
- Who want to eat processed food and expect the IF to help them reduce their body weight – even if you maintain the eating window, long-term processed food will not have a good effect on your body, figure, health and well-being. It is possible to generate a caloric deficit by eating processed and low-quality food (and thus reduce weight), but this does not mean that it is worth considering.
- Who do not have well-established healthy eating habits and expect the IF to be the first step towards a healthy diet for them – in healthy eating and weight reduction, the most important thing is to learn how to properly compose meals and build permanent eating habits. In the beginning, what you eat is much more important than when you eat.
- Who find it difficult to tolerate long breaks between meals – some function much better when they eat smaller portions more often. Hunger makes it harder for us to stay in control, so we’re more prone to reaching for low-nutritional meals and snacks. If you feel worse with longer breaks between meals, the IF diet is definitely not for you.
- Who feel bad during fasting – if you don't feel well following the IF diet (and it can happen – it's not a schedule for everyone to follow!), don't torture yourself. There are other effective methods to stay in a calorie deficit.
- Suffering from gastrointestinal problems – the eating window usually requires eating more food in a shorter period of time, and this can exacerbate the symptoms of gastrointestinal problems, e.g. IBS, Gastroesophageal Reflux Disease, SIBO. Most people with gastrointestinal problems will not benefit from following an IF strategy on a daily basis (a 5:2 diet could possibly be considered).
- Who have a very active lifestyle and practice sports – with high energy demand, the IF diet can be difficult to maintain due to the need to consume large portions in a short time and the associated digestive discomfort. An additional argument against the use of IF in athletes (amateur and professional) is the potential energy deficiency, which can cause, for example, a decrease in training and recovery.
Intermittent fasting is also categorically contraindicated in children, pregnant and lactating women. IF should also not be followed by diabetics with impaired glucose levels.
The IF diet from scientific point of view
Scientific research on the effectiveness of intermittent fasting in the context of weight loss is inconclusive. There are many individual studies involving volunteers that have achieved the desired results and the IF diet has proved to be effective. Other studies, on the other hand, refute the effectiveness of the eating window.
In the latest meta-analysis (i.e. a publication collecting and summarizing the results of various scientific studies), researchers draw the following conclusion – the IF diet is more effective in reducing body weight, waist circumference and body fat than the “ad libitum” diet (without any modifications or restrictions on food). Compared to a reduction diet (a reduced-calorie diet), the benefits of following IF completely pale into insignificance. In the long term, the effects of the reduced-calorie diet alone are very similar to those of the same diet used in the IF model.
Scientific research gives a fairly clear answer to the question “Can IF help to reduce weight?”. Perhaps. However, they do not answer the question “Is IF more effective in weight reduction than a regular reduction diet?” Most likely, it is not.
IF vs. weight reduction – is it a good choice?
Promoting the IF diet as a way of eating that allows you to reduce body weight “by eating whatever you want in any amount” is a clear abuse. Intermittent fasting is simply one of the methods to control calorie intake. However, it does not guarantee success in weight reduction, let alone a guarantee of better well-being. What’s more, when following the IF diet you’re likely to gain weight as well – if you consume more calories than you use during the eating window.
The issue of combining a healthy diet with an eating window is also very important. The IF diet is often presented as one that has no rules except for one – maintaining a specific fasting time and eating window. This approach can be misleading and lead to a rejection of the general principles of a healthy diet in favor of prioritizing meal times. The principles of a truly nutritious diet remain unchanged – well-balanced meals (rich in vegetables and fruits, protein sources, healthy fats and unprocessed carbohydrates) are necessary to maintain health. The IF diet itself will not bring you closer to this.
Potential benefits of intermittent fasting
The most common reason for taking up the IF diet is the desire to regulate body weight. Intermittent fasting also has other potential benefits that have been revealed in some (though usually single) studies:
- more effective loss of abdominal fat,
- improvement of glycemic parameters (insulin level, cell sensitivity to insulin, fasting glucose level),
- improvement of lipid parameters (cholesterol level, triglyceride level, LDL level),
- modulation of inflammation,
- reduction in blood pressure,
- support for cognitive functions.
Most of these advantages, however, are closely related to weight loss and coincide with the general advantages of fat loss, regardless of which method it was achieved. It is therefore an abuse to attribute these advantages to the IF model, without adding that similar effects can be achieved with a calorie-restricted diet, without specific restrictions on meal times.
When listing the benefits of the IF diet, its proponents often refer to the process of autophagy, understanding it as a method of “cell renewal”. This is a misinterpretation of this metabolic process. It is true that intermittent fasting can increase cellular autophagy (although studies on this topic have been conducted on rodents), but it is a complicated metabolic process that is not at all unequivocally positive. Autophagy is a metabolic curiosity and should not be the main motivator for taking up the IF diet.
Intermittent fasting – impact on women and men
Some studies suggest that intermittent fasting may have different impact on women (especially women of childbearing age) and men. Caloric restriction significantly affects female hormones, and following an IF schedule without proper knowledge may cause the hormonal cycle to be dysregulated.
The entire menstrual cycle is strictly dependent on many hormones. Gonadotropin-releasing hormone (GnRH) is a hormone that indirectly regulates the levels of progesterone and estrogen in the menstrual cycle. GnRH is very sensitive to all environmental factors, such as stress, fatigue or an altered circadian rhythm. The factors that strongly affect this hormone also include the supply of food – fasting can dysregulate the menstrual cycle in this mechanism. Fasting (combined with caloric restriction) is a stress factor for the body and a signal that it is not the optimal time for potential offspring (and ovulation). This can block the release of estrogen and progesterone, which are essential for ovulation and a normal, continuous menstrual cycle.
IF can also affect other hormones, such as leptin (known as satiety hormone) level which, if reduced, directly affects the hypothalamic-pituitary-ovarian axis. This can additionally cause the menstrual cycle to be dysregulated.
How to avoid the potentially negative effects of IF in this regard?
- Do not follow any IF schedule during the luteal phase of the menstrual cycle, i.e. from the moment of ovulation to the day before the next menstrual period. The safest time to follow the IF diet by women of childbearing age is from the second day after menstruation to a maximum of one week after menstruation. During this time, female hormones (estrogen, progesterone) are the least sensitive to the side effects of fasting.
- Choose less restrictive IF variants. 14:10 and even 16:8 variants are less restrictive that, with a properly balanced diet, should not lead to hormonal problems. Avoid significant calorie restrictions and fasts lasting more than 16 hours.
- Pay attention to the quality of your diet. Remember that although IF does not have specific dietary “don’ts” and “do’s”, the rules for composing a healthy and balanced diet still apply. Ensure that you take an adequate amount of healthy protein, fat, fiber and a balanced portion of carbohydrates.
Fasting also leads to a decrease in the activity of thyroid hormones. In less restrictive variants of IF (14:10, 16:8), fasting should not have a significant negative impact on this body organ. All more restrictive variants of IF can dysregulate this sensitive organ in both women and men.
References:
- Cienfuegos S., Corapi S., Gabel K., Ezpeleta M., Kalam F., Lin S., Pavlou V., Varady K. A., Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials, Nutrients, 2022 Jun 3;14(11):2343, doi: 10.3390/nu14112343. PMID: 35684143; PMCID: PMC9182756.
- Gu L., Fu R., Hong J., Ni H., Yu K., Lou H., Effects of Intermittent Fasting in Human Compared to a Non-intervention Diet and Caloric Restriction: A Meta-Analysis of Randomized Controlled Trials, Front Nutr. 2022 May, 2;9:871682, doi: 10.3389/fnut.2022.871682. PMID: 35586738; PMCID: PMC9108547.
- Lowe D. A., Wu N., Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity, JAMA, 2020.
- Jamshed H., Steger F. L., Bryan D. R., et al., Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial, JAMA Intern Med., Published online August 08, 2022, doi:10.1001/jamainternmed.2022.3050
- Varady K. A., Lin S., Oddo V. M., Cienfuegos S., Debunking the myths of intermittent fasting, Nature Reviews Endocrinology, 2024, doi: 10.1038/s41574-024-01009-4