How Long Can We Go Without Eating?

Today, we're going to tell you how long a person can go without eating and what physiological changes occur during the calorie deprivation process.

Written and verified by el nutricionista Saúl Sánchez.

Last update: 18 June, 2023

Spending time without eating has become a fad. It’s a protocol known as intermittent fasting. It has shown benefits in terms of health and weight loss. However, there’s a maximum number of hours that this condition can be endured.

First, it must be taken into account that the human body lasts several days without eating food. Now, from a certain point in time, you start to consume muscle, as well as important amino acids.

The physiology of fasting

To fully understand how long you can go without eating, it’s important to make an introduction to the physiology of fasting. When a certain number of hours are spent without eating, there’s a depletion of glycogen stores. From here, a greater number of lipids begin to be oxidized for energy production.

This scenario is ideal in the context of intermittent fasting. Fats are used and their mobilization is increased.

For this reason, the loss of muscle in less than a day will be minimal. This is a good thing for body composition. It’s a process marked by the production of growth hormone. The synthesis of this element is closely related to the secretion of ghrelin.

However, after 24 hours, the muscle’s protective mechanisms begin to fade. At this time, the body sees the need for the catabolism of lean tissue to obtain amino acids that satisfy energy needs.

The first 24 hours of fasting

At this time, circulating and stored glucose is consumed, thus reducing glycogen stores. There’s also an increase in glucagon production and a decrease in insulin synthesis. However, there’s hardly any muscle proteolysis.

It’s a phase in which weight loss isn’t even experienced unless there’s an associated physical activity. The body uses the 600 calories it stores in the form of glycogen, in addition to triglycerides and fatty acids circulating in the bloodstream.

On the other hand, it should be noted that tissues undergo an adaptation process by which it’s easier for them to use ketone bodies for energy production. In this way, glucose is saved, which will be directed to the central nervous system.

From the third day

At this time, hypoglycemia begins to be the protagonist. Obtaining energy comes mainly from lipids. Protein consumption is increasing, although the brain begins to use ketone bodies to function. So, it tries to save protein.

From this point on, a greater amount of growth hormone is released at the hypothalamic level with the aim of reducing glucose consumption by the cells. This is with the intention of increasing lipolysis or oxidation of fats in order to generate energy. Insulin is at a low and glucagon synthesis continues to increase.

Long fasts

When you go beyond 3 days without eating, you go from intermittent fasting to a long fast. At this time, mood swings, inability to sleep, irritability, and weakness may be experienced. All the mechanisms responsible for saving energy and producing glucose from other immediate principles are activated.

However, the reserves are starting to run out. Still, lipids are capable of providing enough calories to survive almost a month without eating. It’s something that depends on the previous state of body composition of the person.

The determining factor at this time is the electrolyte balance. You can go on for more days without eating, but not without drinking. If essential fluids and minerals aren’t provided, a situation of dehydration or the impossibility of nerve impulse transmission will be experienced.

In a scenario in which the replacement of liquids and mineral salts is ensured, it’s possible to endure almost a month without consuming energy foods. However, after 30 days, the amino acids necessary for brain function, which are the most essential, will begin to be destroyed.

Some cases have been documented in the scientific literature of people who haven’t eaten for a few months in order to lose weight. However, we’re talking about a starting point located within obesity. Also, a solution with glucose and salts was administered relatively continuously, so it wasn’t a strict fast.

As a general rule, fasting tolerance is estimated to be 40 to 60 days, depending on previous body composition status. It’s the maximum time that one could survive without eating solid or energetic food.

Likewise, it must be taken into account that there’s evidence that the practice of long fasts generates a negative impact on the metabolism. Alterations in hormonal production may be experienced that condition future quality of life.

Compensation mechanisms when you go several days without eating

During the fasting state, a state of metabolic acidosis is normal. For this reason, a series of compensations are generated in the body to avoid imbalances that can be counterproductive or fatal.

At the respiratory level, there’s an increase in ventilation, which is why more carbon dioxide is released. In this way, the concentration of the substance in the blood is reduced.

An effect is also experienced at the renal level, with the acidification of the urine to stimulate the elimination of substances. This allows a balance of acid/base values at the blood level.

The effects of spending time without eating

When you go without eating for a while, you can experience several health benefits, as long as the period is controlled and certain limits aren’t exceeded. Until the 2nd day of caloric restriction, the period known as intermittent fasting would be entered.

The first of the benefits is improved body composition, as established in a study published in the journal Canadian Family Physician. As the mobilization and oxidation of lipids increase, there’s a progressive loss of adipose tissue. In addition, as protein is saved during the first hours, the lean mass remains unchanged.

At the same time, we must highlight the increase in metabolic flexibility derived from periods without eating. The deprivation of energy causes a reduction in the release of insulin, which causes increased sensitivity of the cells toward the hormone. Thanks to this, the risk of type 2 diabetes can be reduced. This is evidenced by research published in Nutrients.

It could also be a good strategy for treating type 1 diabetes. However, the application method here is simpler. Evidence has been found to support the protocol, but it’s crucial to start from a situation of glycemic control.

Ideally, these types of protocols are managed by a nutritionist. In this way, problems and surprises that may be unpleasant or that put health at risk are avoided.

Does going time without eating reduce the incidence of cancer?

It has been suggested that periods of fasting could reduce the risk of developing cancer. This is due to a mechanism that’s generated when you spend a certain time without eating, and that is autophagy. Through this process, cells and organelles that aren’t working are destroyed, while efficient ones are replicated. This is evidenced by a study published in Clinics.

Thanks to this phenomenon, there’s an elimination of cells that can mutate or replicate uncontrollably. In this way, the risk of tumor mass formation would be reduced.

However, intermittent fasting isn’t an equally powerful preventive mechanism for all types of tumors. Above all, it has proven effective on those related to the digestive tract.

Despite this, the application of periods without eating during cancer treatment is difficult on many occasions. If cachexia is experienced, such a restrictive protocol may not be appropriate.

To this day, there’s still a lack of consensus in the scientific community on how to deal with this type of case. It has been suggested that the partial restriction of carbohydrates and the implementation of fasting can help, but there’s no systematization.

Can athletes go without eating for a long time?

Classically, it’s understood that athletes have higher energy requirements. However, they can benefit from certain periods of time without eating. On the one hand, they experience an improvement in the state of body composition. On the other, they can increase performance.

When moderate-intensity aerobic exercise is performed in the context of fasting, fat oxidation mechanisms are stimulated. In this way, the body becomes more efficient using lipids as an energy source, being capable of saving glycogen.

This situation is advantageous to later transfer to competitive land. If the athlete is able to increase energy production from fat at higher physical intensity, they’ll have glucose for the decisive moments of the competition. This type of metabolic training is often used in the context of endurance sports.

Now, in the framework of strength exercises, the question changes. Although calorie restriction can be used to improve body composition, it’s never a good idea to play sports without eating.

When doing maximal or submaximal efforts, glucose and glycogen are needed. If both aren’t available, performance suffers, in addition to increasing the risk of muscle injury.

The body is able to endure many days without eating

As you’ve seen, the body’s tolerance to fasting is high. A person can go up to 60 days without food and still be alive. However, their functions will be diminished, as well as their capabilities.

In any case, it must be borne in mind that the same doesn’t happen with liquids. If you start a deprivation of water or electrolytes, you can only last about 3 days, depending on the weather conditions.