Eating every other day is a common term for Alternate-Day Fasting (ADF), a rigorous form of intermittent fasting. This eating pattern involves alternating between a “fast day” and a “feast day” throughout the week. On the fast day, individuals either consume zero calories or significantly restrict intake to about 500 calories, which is roughly 25% of their typical daily energy needs. The following “feast day” generally allows for unrestricted eating.
The Body’s Metabolic Shift During Fasting
When the body goes without food for an extended period, it must switch its primary energy source from glucose to stored fat, a process known as the metabolic switch. Initially, during the first 12 to 24 hours of a fast, the body relies on stored glucose in the liver, called glycogen. As blood sugar levels drop, the hormone insulin decreases significantly. Once glycogen stores are depleted, the body increases glucagon production, signaling the need for an alternative fuel source. This shift triggers lipolysis, the breakdown of stored triglycerides into free fatty acids.
These fatty acids are transported to the liver, where they are converted into ketone bodies, such as beta-hydroxybutyrate, through a process called ketogenesis. Ketones then become the main energy source for the brain and other tissues, providing a steady fuel supply. This metabolic state helps preserve muscle mass, which is a significant adaptation in times of nutrient scarcity. The human growth hormone (HGH) also experiences a surge during longer fasts, further supporting the preservation of lean muscle tissue.
Measured Health Effects of Alternate-Day Eating
A consistent finding of Alternate-Day Fasting (ADF) is a reduction in body weight and fat mass, with participants typically losing 3–7% of their body weight over two to three months. Studies indicate that ADF is effective for weight loss, sometimes comparable to daily calorie restriction. While ADF leads to reductions in fat mass, it can also result in a modest loss of lean mass. This suggests that careful attention to protein intake on non-fasting days may be necessary to mitigate potential muscle loss.
ADF has been documented to improve several cardiometabolic risk factors. Research shows that ADF can lead to significant reductions in total cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides. For individuals with insulin resistance, ADF has been shown to produce greater improvements in fasting insulin levels and insulin resistance compared to simple daily calorie restriction. Furthermore, ADF has been linked to reductions in both systolic and diastolic blood pressure.
Prolonged fasting periods can also induce a cellular process called autophagy. Autophagy is a natural “cellular cleanup” mechanism where the body removes damaged cells and recycles waste components to regenerate healthier ones. Although the extent of autophagy activation in humans on ADF is still being studied, it is thought to be one of the underlying benefits that contributes to improved cellular function.
Safety Considerations and Practical Implementation
Alternate-Day Fasting (ADF) is a demanding dietary approach that requires careful consideration. Individuals who are pregnant, breastfeeding, underweight, or have a history of an eating disorder should not attempt ADF. People with Type 1 diabetes or those taking specific medications that affect blood sugar must consult a healthcare provider before starting any fasting regimen.
When beginning ADF, some individuals may experience acute, temporary side effects, such as headaches, fatigue, dizziness, and heightened hunger. These symptoms often relate to dehydration, electrolyte imbalances, or the initial drop in blood sugar. Ensuring adequate fluid and electrolyte intake, often through water, black coffee, or unsweetened tea, is important for mitigating these initial discomforts.
A common pitfall of ADF is compensatory overeating on non-fasting days, which can negate the calorie deficit created during the fast. To promote sustainable health outcomes, the unrestricted “feast” days should focus on nutrient-dense, whole foods. Working with a healthcare professional can help tailor the fasting schedule and nutritional plan to individual needs and health status.