Alternate day fasting (ADF) is an intermittent fasting approach that involves a cycle of one “fasting” day followed by one “eating” day, restricting food intake every other day. It is a structured dietary method focused on the timing of food consumption rather than specific food types.
How Alternate Day Fasting Works
Alternate day fasting can be practiced in two main ways: a strict approach or a modified approach. In the strict version, no calories are consumed on fasting days, with only calorie-free beverages like water, black coffee, or unsweetened tea allowed. The modified approach, which is more commonly studied and considered more sustainable, permits a limited calorie intake on fasting days.
On modified fasting days, individuals typically consume around 500 calories, or approximately 20-25% of their usual daily energy needs. These calories can be eaten at once, perhaps as a small meal, or spread throughout the day as smaller snacks. Foods often include lean proteins, vegetables, and healthy fats, such as Greek yogurt with berries or a salad with grilled fish.
On “eating” days, also known as “feast” days, individuals return to a normal, healthy eating pattern without intentional calorie restriction. This does not mean consuming unhealthy foods excessively; the goal is to maintain a balanced and nutritious diet. This alternating pattern results in a weekly calorie reduction of approximately 50% compared to daily eating.
Potential Health Outcomes
Alternate day fasting has been investigated for its potential health outcomes, primarily weight loss and fat reduction. Studies show that individuals engaging in ADF may lose 3–8% of their body weight over 2–12 weeks. This method can lead to a reduction in fat mass, similar to the effects seen with continuous daily calorie restriction.
Beyond weight management, ADF may offer other metabolic improvements. Research indicates it can enhance insulin sensitivity, decreasing fasting insulin levels. Studies suggest ADF may also reduce levels of total cholesterol and triglycerides. ADF has also been observed to stimulate autophagy, a cellular process where the body cleans out damaged cells and recycles old components.
Common Side Effects and Considerations
Individuals beginning alternate day fasting may experience several common side effects during the initial adaptation period. These often include intense hunger, fatigue, irritability, and headaches. Other reported symptoms include difficulty concentrating, lightheadedness, and sleep disturbances, though these often lessen as the body adjusts over a few weeks. Bad breath and occasional gastrointestinal issues like constipation can also occur.
ADF is not suitable for everyone. Certain populations should avoid this restrictive diet or only undertake it under strict medical supervision, including:
- Pregnant or breastfeeding individuals.
- Children and teenagers under 18.
- Those with a history of eating disorders, as ADF may not provide adequate nutrients or could worsen disordered eating behaviors.
- Individuals with type 1 diabetes, due to the risk of unsafe blood sugar levels.
- Those with type 2 diabetes, who should consult a doctor regarding medication adjustments.
Comparing Fasting Schedules
Alternate day fasting is one of several intermittent fasting methods, each with a distinct schedule and intensity. The 16:8 method, a form of time-restricted eating, involves consuming all meals within an 8-hour window daily and fasting for the remaining 16 hours. This often means skipping breakfast or eating an early dinner.
Another popular method is the 5:2 diet, where individuals eat normally for five days of the week and restrict calorie intake to 500-600 calories on two non-consecutive days. While ADF may offer a larger caloric deficit over time due to more frequent fasting days, potentially leading to faster initial weight loss results, it can also be more intensive than 16:8 or 5:2. The 16:8 method is generally considered a gentler introduction to intermittent fasting, offering daily flexibility. The 5:2 diet provides flexibility in choosing fasting days but requires stricter calorie counting on fasting days. The choice among these schedules often depends on individual lifestyle, adherence, and specific health goals.