Is fasting a safe and effective means to lose weight?

Fasting is a very useful tool for those wanting to achieve healthy weight loss, since the process encourages a reduction in fat.

During the fasting process a series of molecular changes that lead to weight loss occur: consumption of carbohydrates (which leads to a decrease in insulin levels), production of glucagon (the hormone responsible for controlling blood glucose levels and that is involved in the process of burning fat), gluconeogenesis production, elevation of growth hormone levels (that lead to lipolysis) and formation of ketone bodies.

Estefanía Schoendorff – Neolife Nutrition and Nursing Unit

The fasting method that is used most frequently today is intermittent fasting, which consists of the absence of food intake for a period of time.

What is fasting? What is involved in fasting? What happens to our body when we undergo a fast? What hormonal and molecular changes are produced in our body when fasting? What is the most effective type of fasting for losing weight? Can we perform fasting without medical or nutritional supervision?

A fast is defined as “the act of abstaining totally or partially from the ingestion of liquids or foods for a period of time” (1). During the fasting period, our body consumes our own reserves and provides us with the necessary energy to carry out the activities of daily living (ADL), fat is used as the main energy source (one of the main objectives in weight loss and nutritional re-education).

Intermittent fasting. Neolife

A fast consists of three phases:

  • During the first phase there is glucose consumption of approximately 1200 Kcal during the first 24 hours.
  • Then we enter the second phase, the most important, where the hypothalamus (central part of the brain that controls the functioning of the nervous system) and the pancreas, along with the rest of the body, begin to use the fat as a source of energy.
  • Finally, in the third and last phase, your appetite returns. It is then that a food intake should be reintroduced to avoid starvation – “extreme physical weakness caused by lack of food” (2) -.

This means that during a fasting process the body uses carbohydrates as an energy source during the first 24-48h to maintain bodily functions. After this time the body resorts to using fat reserves, which correspond to 75% of the energy produced; before finally, as a last resort, the body absorbs the energy generated by the proteins of their musculoskeletal system, which make up the remaining 10-20%.

During the fasting process, a series of molecular changes take place (3):

  • The consumption of carbohydrates during the first phase leads to a decrease in the levels of insulin which leads to a reduction in fat storage.
  • During this process, the pancreas produces increased levels of glucagon, a hormone responsible for controlling the levels of glucose in blood and encourages theburning of fat.
  • In the third phase of the fasting process there is a gluconeogenesis, which consists of the production of energy (glucose) from the amino acids that are found within the proteins of the muscles. During this phase it is very important to remain under medical and nutritional care and supervision in order to avoid a loss of muscle mass.
  • Throughout the fast, growth hormone levels that encourage the burning of fat (lipolysis) rise.
  • The burning of fatty acids through oxidation produces ketone bodies, which provides medical practitioners with a means to measure and check that the correct practice is being followed during the fast. It is essential that the ketone bodies are monitored and checked regularly throughout the fasting process. This should be undertaken by a doctor or nutritionist.

Currently the fast is an effective and safe tool for those wanting to achieve healthy weight loss, since all the processes lead to the loss of fat, as well as weight control.

Although there is a wide variety of fasts it is best to opt for one which is best suited to the health goals set for each patient. In this article we are going to focus on the most well-known and prominent, intermittent fasting:

Intermittent fasting consists of the absence of food intake for a period of time (during which only water intake is permitted) (4). The most commonly used are:

  • Fasting of 12/12: consists of a nocturnal rest of about 12 hours without any food intake and a feeding window for the remaining twelve hours. In such a way that, if we have dinner at 21:00 and go to bed and do not eat again until 09:00 in the morning the 12-hour fast has been completed. A suitable intake should be maintained during the feeding window at usual intervals every 3-4 hours.
  • Fast: 16/8: consists of 16 hours of fasting and 8 hours of feeding, where you can eat without counting the number of calories, but should take into account its nutritional value. During this type of fasting, you can have a snack around 19:00, with a good nutritional value, and then not eat again until 12:00 the next day.
  • 24-hour fast: is widely used in ketogenic diets. At Neolife we ​​recommend carrying out this type of fasting under medical supervision. Consists of a fast for 24-hours: the total absence of food intake whilst maintaining hydration.
  • Fast on alternate days: this consists of alternating days between a normal diet without counting calories, but preserving a good nutritional value and a limited food intake at 500-800kcal on the alternate days. There is no total restriction on food intake, but there is a reduction.
  • Paleolithic fasting: consists of skipping meals from time to time, without counting the number of times or the quantity, as happened in the Paleolithic period.

At Neolife we use intermittent fasting if necessary for periods of weight loss. It should always be carried out only with periodic medical checks and the correct use of nutritional supplements. If so, this can be an effective and safe method for loss of body fat whilst maintaining muscle mass.


(1) RAE, Real Academia Española.

(2) RAE, Real Academia Española.


(4) Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?