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Intermittent Fasting: Good strategy or the villain?

by Rennan Natalino, Bacharel in Nutrition - Universidade de Taubate

What is fasting?

Fasting can be defined as the state in which the body uses its own sources of energy in addition to diet, after long periods without eating. From the lack of nutrients from external sources, the body uses the fat stored in adipose tissue. When all the fat source of the adipose tissue is used, glucose starts to be used, for example: as energy, and to use muscle and liver glycogen more intensely when causing the carbohydrates from the diet. In this situation, there is also a breakdown of muscle tissue proteins into amino acids for energy production. The body starts to use these intrinsic sources (fat, glucose, glycogen, amino acids), because during fasting the body does not receive nutrients from food for energy production.

Many people, when they hear the term “fasting”, think of situations of extreme hunger, loss of muscle mass, weakness, among other situations. But this will only occur in very long periods of fasting. Intermittent fasting, on the other hand, works based on other aspects.

Intermittent fasting

Intermittent fasting is a form of “programmed” fasting. As the name suggests, it does not remain permanently, but at predetermined time intervals - it presents periods of fasting alternating with periods of food intake.

Intermittent fasting can be done in several ways: practiced every day, every other day, every 3 days, once a week, among other ways. It can last 12, 16, 24 hours, etc ...

The intervals most commonly practiced for intermittent fasting are daily and practiced every other day. Regarding the fasting period, it usually does not exceed 1 full day (24 hours).

Among the most common intermittent fasting practices is a 12-hour fast. In it, the individual eats before sleeping in a day, has an 8-hour sleep and still remains fasting after waking up for another 4 hours. Basically he has his last meal at dinner for one day and will only eat again at lunch the next day, removing from his routine breakfast or any other meals he usually had until lunch. This practice is quite common since it already uses most of the 8 hours of sleep and also a good part of people do not feel much hunger in the morning.

Let's look at the following studies that show results on the effect of intermittent fasting:

Heilbron et al (2005a) 1

Healthy individuals underwent intermittent fasting for 24 hours, every other day. After 22 days, there was a weight loss of 2.5% and a loss of body fat of 4%. Other positive results: maintenance of the resting metabolism rate; increase in the fat oxidation rate; insulin levels reduced by 57%; the women who participated showed an increase in HDLc and men showed a reduction in the level of triglycerides. In this study, participants were instructed to double their usual food intake on days that they did not fast, in order to maintain their initial weight. Even with this practice, the participants experienced weight loss.

Heilbron et al (2005b) 2

The protocol of this study was the same used in the research reported above. The average weight loss was 2.1 kg, leading to the belief that on days when there was free food intake, individuals were not able to “compensate” for the energy restriction they had practiced on fasting days, as in the previous study. The men had reduced insulin levels at the end of the study. Some genes related to the use of fat as an energy substrate (SIRT1 and CPT1) showed increased expression after the fasting period, suggesting that these molecules were with increased activity, which would favor the loss of body fat.

Varady et al (2013) 3

Individuals with a Body Mass Index between 20 and 29.9 kg / m² were divided into 2 groups: intermittent fasting and control. The fast was carried out every other day, and on fasting days the participants ingested 25% of their daily energy needs, and on days without fasting they could eat without restrictions. The control group maintained its usual dietary intake. The group that practiced fasting presented the following results: average weight reduction of 5.2 kg, also reduction in LDLc, triglycerides, blood pressure (systolic and diastolic) and leptin (hunger regulating hormone that increases with weight gain) and decreases with weight loss). The control group showed no change in any of the parameters evaluated in the study.

In summary, the following benefits were observed:

Weight loss and body fat.

Reduction in LDLc and triglyceride levels.

Maintenance of HDLc levels - which is important, given that diets for weight loss usually lead to a reduction in HDLc4 (low-carb diets, on the other hand, increase HDLc5,6,7).

Increase in the size of LDL particles.

Reduction in insulin levels.

Maintenance at resting metabolism rate.

Absence of adverse effects.

The effectiveness and benefits found in the practice of intermittent fasting are given through several mechanisms, as we can see below:

Greater secretion of GH hormone produced by the body8, associated with increased lean mass9 and reduced body fat10;

Greater use of fat stored in adipose tissue11,12;

Higher rate in the thermogenesis (heat production) of the organism12,13;

Greater sensitivity to the hormone insulin14,15

Physical activity x Fasting

The relationship between the practice of fasting and physical exercises is very delicate and a subject that does not have concrete conclusions, but in view of the scientific bases observed, initially the practice of aerobic exercises in fast with the objective of "burning fat" is not effective, bearing in mind that studies found do not suggest that such a practice would be more effective in losing body fat than exercising after a meal16.

The practice of intermittent fasting inserted in a planned context of training and adequate nutrient intake does not demonstrate to affect performance in practitioners of physical activity17,18,19,20. The correct adaptation to training and diet can bring good results regarding the more efficient use of carbohydrates and fats for the production of energy during exercise21, but it is important to pay attention to the composition of the diet, as an essential step in the process.

It is important to recommend that the practice of activities is not carried out in the interval that fasting occurs. Perform your physical activity safely and, preferably, after you have eaten.

Lean mass x Fasting

A common concern regarding the practice of intermittent fasting is whether there may be loss of lean mass, since one of the main objectives of the practitioner is the greatest possible reduction in body fat concomitant with the maximum preservation of lean mass.

It was possible to notice in some studies that there is no statistical difference between the initial and final lean mass of individuals who practiced fasting22, 23, 24,25. When there was a decrease in lean mass21, it was relatively small, being equal to or less than the reduction in lean mass observed in any other conventional strategy for weight reduction.

It is important to note that research usually evaluates lean mass in general and not just muscle mass. The expression “lean mass” encompasses all weights of tissues that are not fat.

Bearing in mind that fasting, like diets reduced in carbohydrates, will normally reduce the levels of glycogen and, consequently, of water (since glycogen is stored in the muscle and liver tissues linked to water molecules), a considerable part of the mass reduction lean observed in these surveys can be caused by water loss and not essentially a loss of muscle mass.


There are several ways to perform intermittent fasting. The most important thing is that the person feels in a comfortable state, and not in a state of deprivation or still feels obliged to fulfill the fasting period. Any and all health-related practices must be accompanied by maximum comfort and pleasure.

In any and all situations, intermittent fasting must be very well planned. Always be accompanied by a health professional who understands the subject, such as a good nutritionist, to guide you about the need or not and the possible benefits and losses that this practice can provide you. Always remember that everything in excess becomes harmful, in the same way, living constantly practicing fasting, with low intake of nutrients can bring you the opposite effect to the benefits you are looking for.

As with many controversial subjects in the area of ​​Nutrition and health, always keep in mind that there is never an absolute truth. Have curiosity, common sense, listen to opinions and make decisions based on what science shows. Do not let yourself be carried away by fashions and “guessing”.


Heilbronn LK, et al. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005; 81 (1): 69-73.

Heilbronn LK, et al. Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obes Res. 2005; 13 (3): 574-81.

Varady KA, et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutr J. 2013; 12 (1): 146.

Katan MB. Effect of low-fat diets on plasma high density lipoprotein concentrations. Am J Clin Nutr. 1998; 67 (3 Suppl): 573S-576S.

Bueno NB, et al. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomized controlled trials. Br J Nutr. 2013; 110 (7): 1178-87.

Schwingshackl L, Hoffmann G. Comparison of effects of long-term low-fat vs high-fat diets on blood lipid levels in overweight or obese patients: a systematic review and meta-analysis. J Acad Nutr Diet. 2013; 113 (12): 1640-61.

Schwingshackl L, Hoffmann G. Comparison of the long-term effects of high-fat v. low-fat diet consumption on cardiometabolic risk factors in subjects with abnormal glucose metabolism: a systematic review and meta-analysis. Br J Nutr. 2014; 111 (12): 2047-58.

Ho Ky, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988; 81 (4): 968-75.

Liu H, et al. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med. 2008; 148 (10): 747-58.

Berryman DE, et al. Growth hormone and adipose tissue: beyond the adipocyte. Growth Horm IGF Res. 2011; 21 (3): 113-23.

Nielsen TS, et al. Fasting, but not exercise, increases adipose triglyceride lipase (ATGL) protein and reduces G (0) / G (1) switch gene 2 (G0S2) protein and mRNA content in human adipose tissue. J Clin Endocrinol Metab. 2011; 96 (8): E1293-7.

Zauner C, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000; 71 (6): 1511-5.

Weigle DS, et al. Elevated free fatty acids induce uncoupling protein 3 expression in muscle: a potential explanation for the effect of fasting. Diabetes. 1998; 47 (2): 298-302.

Long DV, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014; 19 (2): 181-92.

Mattson MP, et al. Meal frequency and timing in health and disease. Proc Natl Acad Sci U S A. 2014; 111 (47): 16647-53.

Phinney SD. Ketogenic diets and physical performance. Nutr Metab (Lond). 2004 17; 1 (1): 2.

Chaouachi A, et al. Effec



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