How to fast safely

The body metabolizes food to create glucose, its preferred source of energy, which is used to carry out the everyday energy demands of life. Glucose can be measured in the blood and that measurement is commonly referred to as one’s blood sugar. Glucose metabolism (digestion and use for energy) can last for up to 5 hours after the last meal.

After readily available glucose from meals recently eaten becomes depleted, energy demands are met by pulling from the stored form of glucose — also known as glycogen — in the liver and muscles. This process can sustain the body for up to a day but varies based on a person’s caloric intake prior to fasting, which ultimately determines how much glycogen has been stored.

If glucose isn’t readily available from carbohydrate-rich meals — or similarly, during prolonged periods of fasting — the body switches its fuel source from glycogen to free fatty acids stored in body fat, and then to protein stored in muscles. Each fuel source becomes metabolically less efficient when fasting or a caloric deficit occurs. This is because the body perceives the deficit as a state of starvation and attempts to preserve its most metabolically active tissue in muscle.

Additionally, it is crucial to minimize external stressors when fasting. Fasting in itself is a significant amount of stress on the body.

To prevent dehydration, it’s crucial to drink an adequate amount of fluids while fasting, as the body is not receiving any fluid from food. People who are fasting should also avoid hot and humid environments and avoid diuretics, such as caffeine and alcohol, around the time of starting the fast.

Consider adding electrolytes — sodium, potassium and magnesium — to your beverages, if permitted. And discuss your individual fluid needs with a dietitian or doctor prior to fasting if you have preexisting medical conditions that could affect your fluid balance.

Fasting is not recommended for pregnant or nursing women, children, the elderly; and people with diabetes, recovering from surgery or who are underweight — unless under a doctor’s supervision. Additionally, people with a history of eating disorders should avoid fasting.

To break a fast, eat a balanced meal complete with carbohydrates that contain fiber, lean protein and a small amount of healthy fat. Complex carbohydrates that contain fiber — fruit with the skin, whole grains or legumes — replenish the body’s stores of glucose. Lean protein — fish, chicken or turkey — aids the body in restarting muscle protein synthesis, the process of building new lean body mass. And healthy fats help to keep us satiated and aid in cell signaling (the process where a cell responds to substances outside of it) fat-soluble vitamin absorption and hormone metabolism.

Fasting, when practiced incorrectly or unsupervised by a medical professional, can lead to deficiencies in certain micronutrients. Long-term fasting can also have detrimental impacts on metabolism and immune function if not practiced safely.

Side effects of fasting can include light-headedness, dizziness, fatigue and nausea. Not all fasting plans are the same. And the same fasts can create different effects on the body, based on the amount of stored calories and nutrients.