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Intermittent Fasting, How to Do It Correctly?

Intermittent Fasting, How to Do It Correctly?

Published: 2 February, 2024 | 9'

What is intermittent fasting?

Intermittent fasting (IF) is a dietary pattern that involves alternating regular periods of unrestricted eating with prolonged periods of fasting. It is important to note that due to caloric restriction, it is recommended that this type of diet be implemented and monitored by a medical specialist.

The mechanisms by which intermittent fasting produces not only weight reduction but also metabolic benefits are:

  • Regulating enzymes involved in ketogenesis and fatty acid oxidation, and thus efficiently utilizing them as a source of energy.
  • Reducing caloric intake leading to an energy deficit, which results in weight and fat mass loss over time.
  • Improving insulin sensitivity, reducing inflammation, and promoting cellular repair.

Benefits of intermittent fasting

There is extensive clinical data on the benefits of intermittent fasting on health indicators. Let's take a look at some of them.

  1. Weight and adiposity reduction. Health organizations have determined that a weight reduction of 5% to 10% decreases the risk of cardiovascular and metabolic diseases. Studies implementing the IF diet demonstrate a reduction in body mass index (BMI) of 4.3% and waist circumference by 3-8 cm12. Additionally, visceral fat is significantly reduced, while lean mass is maintained3,7.
  2. It decreases total cholesterol, LDL, and increases HDL (good cholesterol)7,11.
  3. Clinical evidence has shown that it improves insulin resistance, reduces fasting glucose, and inflammatory markers7,10,12. It is important to note that in diabetic patients, the use of this diet may cause hypoglycemia, so they must consult their physician to assess the suitability of fasting and adjust their antidiabetic medication.
  4. Circadian rhythm. Implementing intermittent fasting has shown positive effects on the circadian rhythm of eating. Limitations in the feeding windows allow for synchronization and have a positive effect on glucose levels, pancreatic function, body weight, metabolism, and oxidative stress11.
  5. Cognition. The regulation of inflammatory markers has shown that intermittent fasting can have a neuroprotective effect. By influencing metabolic mechanisms, it optimizes neuronal bioenergetics, plasticity (capacity to learn), and stress resistance, thereby maintaining or improving cognitive performance. In fact, studies have shown that intermittent fasting improves memory, executive function, and working memory2,3,7.
  6. Due to its action in improving cardiovascular health indicators, metabolism, adiposity, cellular repair, and cognition, intermittent fasting directly affects life expectancy. Furthermore, it has been found to stimulate the increase of growth hormone levels2,7.

Types of fasting

Intermittent fasting is characterized by three modalities: (1) alternate-day fasting (ADF) and modified alternate-day fasting (mADF); (2) the 5:2 diet; and (3) time-restricted eating (TRE). Let's review each of these and their characteristics in detail.

24-hour fasting or alternate-day fasting (ADF and mADF)

This type of fasting is characterized by two modalities:

  • ADF regimen consists of a fasting day during which no food or energy drinks are consumed, alternating with a day on which participants can eat whatever they want (ad libitum).
  • Modified alternate-day fasting (mADF) allows for caloric intake of 20-30% of normal intake on fasting days, followed by ad libitum eating the next day.

5:2 fasting. 

This type of fasting involves fasting or reducing caloric intake (<500-600 calories/day) for 2 consecutive or non-consecutive days a week, followed by 5 days of ad libitum, normal, or hypocaloric diet, depending on the individual's requirements.

Time-restricted eating: 12/12, 16/8, and 14/10.

This modality of IF focuses more on a consistent daily feeding window than on energy restriction per se, alternating between eating periods (usually ad libitum) and fasting periods.

Thus, there is the 12/12 regimen, which equates fasting and eating hours; the 16/8 regimen where food is consumed during an 8-hour window, followed by a 16-hour fast; and finally the 14/10 regimen with a 10-hour eating window and 14-hour fast. It is recommended that meal windows be in the first half of the day, without skipping breakfast; and during fasting hours, proper hydration should be maintained with calorie-free beverages.

How to do intermittent fasting

The main recommendation is that any type of diet should be evaluated, implemented, and monitored by a medical specialist. This evaluation determines the general health status, the presence of complications or contraindications for intermittent fasting, lifestyle, dietary preferences, physical activity, among others.

Intermittent fasting diet

It is advisable to transition to fasting at a rate that our body can handle. For this, it is suggested to start with the 16/8 protocol, which is easier to adapt to metabolically. It is important to note any symptoms of fatigue, tiredness, or headaches, as the absence of these symptoms is an indication that the same pace can be maintained or, conversely, that another diet modality should be considered.

As mentioned earlier, it is important to stay hydrated with calorie-free beverages during fasting. During meal times, the recommended intake should mainly consist of proteins, complex carbohydrates, fruits, and vegetables. The fats that are recommended to be consumed come from seeds or nuts, or from vegetable oils containing omegas, all in limited quantities. The attending physician may suggest the best suitable combination based on individual requirements. An example of a meal that does not exceed 1400-1500 calories/day could be:

First meal: 1 piece of fruit (banana, mango, apple) or honey-sweetened oatmeal.

Second meal: fish (salmon, hake) combined with vegetables (broccoli, carrot), dressed with olive oil.

Third meal: a salad that can contain various vegetables (avocado can be included), dressed with olive oil or flaxseed oil.

How to control hunger?

The sensation of hunger follows a natural circadian rhythm. The hormone ghrelin is responsible for hunger, and its peak occurs around 8 p.m. When intermittent fasting begins, ghrelin levels increase during the first two days and then stabilize. This is because this type of diet synchronizes the circadian rhythm of hunger/satiety.

During those first two days, it is recommended to slowly consume abundant liquids in the form of sugar-free herbal infusions or "light" beverages (with sweeteners) to help alleviate hunger. Additionally, choose foods that provide greater satiety during meals by including more fiber, vitamins, and minerals. Engaging in physical activity such as yoga helps with relaxation and controlling the feeling of hunger.

How long can intermittent fasting be done?

The duration of intermittent fasting, like any other diet, depends on individual goals and the guidelines provided by a healthcare professional.

If the goal is weight loss, it is a fact that the first results will be evident between 2 to 4 weeks, and reaching the estimated goal will depend on the choice of foods, the duration of fat reduction, fasting consistency, exercise, and genetics.

Once the required weight has been achieved, intermittent fasting can be continued for weight maintenance, of course, making adjustments according to the nutritional recommendations of a medical specialist. Intermittent fasting, but not prolonged fasting or water fasting, can be made into a long-term lifestyle, always with specialized supervision.

Who is it not recommended for?

IF is not recommended for the following groups of people:

  • Children and adolescents
  • Pregnant or lactating women
  • Diabetics
  • Those with kidney disease
  • Those with a history of eating disorders
  • Those taking certain medications.

Is supplementation necessary?

If required, supplementation should be taken during feeding periods, never during fasting. Ingredients that can be beneficial and commonly used, as long as determined by your healthcare professional, include:

Myths and Misconceptions about Intermittent Fasting

Does fasting mean going hungry?

It is important not to confuse fasting with feeling hungry or starvation. "Feeling hungry" is an involuntary way of not meeting the nutritional needs required by metabolism for an extended period, causing our body to consume its energy reserves – including stored fat – resulting in a general failure or imbalance of the organ system. When fasting, on the other hand, the hours of intake are limited, but food is not restricted, which always includes an energy balance tailored to the individual. Intermittent fasting balances our biology with periods of eating and not eating.

Can I experience adverse effects?

Clinical data shows that the majority of adults can implement intermittent fasting with few or no unwanted side effects. However, it is important to note that lifestyle and dietary habits may influence the fasting regimen, so professional guidance is recommended. In general, during the first few weeks, you may feel a bit fatigued, dizzy, or tired. If these symptoms persist or any other imbalances occur, it is important to inform your healthcare provider to make the necessary adjustments.

Does fasting reduce the metabolic rate?

No. This is associated with starvation, but fasting is different. When fasting, the body undergoes a metabolic change, as it starts using stored fats for energy instead of carbohydrates. This change leads to an elevated metabolic rate, resulting in weight reduction, similar to other diets.

Can I experience nutritional deficiencies?

One of the most common misconceptions about fasting is that the lower calorie intake results in nutrient deficiencies, which could harm overall health. Like any other diet, the key is to maintain a balanced diet during feeding windows. Deficiencies commonly occur when meals are not properly planned and do not include nutrient-rich foods that provide a variety of vitamins and minerals.

Intermittent fasting is only useful for weight loss.

Although it is a fact that intermittent fasting leads to weight reduction, it is not the only benefit. As mentioned before, intermittent fasting can be implemented as a measure to enhance cardiovascular or mental well-being.

Does fasting make us overeat or indulge in junk food?

Indeed, after fasting, you may feel hungry and may consume a bit more food; however, while intermittent fasting does not restrict the types of food that can be eaten, it is important to balance the proportion of ingredients (proteins/carbohydrates/fats) consumed. Including filling ingredients like vegetables, greens, or high-fiber foods can help prevent the temptation to consume unhealthy sugars and fats, which are readily available and more "palatable." Additionally, it is important to note that the amount of food consumed on non-fasting days is not enough to compensate for the previous fasting period. This has been demonstrated in various studies, where after more than 36 hours of fasting, consuming a larger amount of food does not make up for the nearly 2000-calorie deficit from fasting.

Can I exercise? Will I lose muscle mass?

As with any other diet, the type of exercise performed during fasting plays a key role in maintaining non-fat body mass. The type of exercise to be performed (preferably strength or resistance training) depends on habits, the type of fasting, post-fasting food intake, previous training, and specialist advice. During the period of adaptation to using fat instead of carbohydrates for energy, you may notice a decline in physical performance, which typically lasts for about two weeks. After this period, performance, strength, aerobic capacity, and resistance return to normal. In fact, various studies confirm that fasting preserves lean muscle mass and even stimulates muscle growth.

Intermittent fasting is a universal method.

Intermittent fasting is neither suitable for everyone nor a miracle method. The lifestyle, preferences, and health status of each individual determine which type of fasting would be most effective, or if an alternative dietary approach is needed. The goal is to consume fewer calories than the body "burns," as that is what leads to weight loss. Therefore, fasting windows, like eating windows, should be balanced with a diet that provides key nutrients and promotes satiety.

I've heard that intermittent fasting causes mental irritability.

All diets require an adjustment period; however, if the eating regimen aligns with individual requirements and is controlled by a specialist, episodes of irritability are highly unlikely. Generally, this occurs when we implement excessively restrictive diets without sufficient clinical control, which can lead to feelings of distress.

Will fasting affect my memory or ability to concentrate?

No. You shouldn't experience any memory loss or lack of concentration during fasting. In fact, clinical evidence suggests that intermittent fasting can optimize cognitive performance.

Tips for Intermittent Fasting

  • Always consult a medical specialist before starting any restrictive eating plan.
  • Drink water at regular intervals, as it helps with digestion and increases the feeling of fullness or satiety.
  • Do not deprive yourself of water or calorie-free beverages (such as plain tea and coffee), which are allowed during fasting.
  • The adaptation period lasts approximately one month, so be prepared and do not get discouraged.
  • Maintain physical activity throughout the day to build muscle tone and "burn" fat. Exercise helps effectively digest and metabolize nutrients.
  • Learn to practice mindful eating by consuming meals slowly and thoroughly chewing your food.
  • After breaking a fast, focus on healthier and nutritious foods like vegetables, fruits, whole grains, and lean sources of protein.
  • Try to eat earlier in the day, rather than at night before going to bed.
  • Maintain a regular sleep schedule, as sleep helps your mind and body regain normal energy levels.

Content drafted and reviewed by specialists in the Scientific Information department of MARNYS. This article is for informational purposes and does not replace consultation with a specialist.