Fasting has become increasingly popular due to its potential health benefits, such as aiding weight loss, boosting metabolism, and possibly promoting longevity. However, one of the most common questions is: what happens to your blood sugar when you fast? During fasting, the body undergoes key metabolic changes. Initially, blood glucose levels drop as stored sugar is used for energy. Eventually, the body begins burning fat, helping to stabilise blood sugar levels. Understanding the effects of fasting on blood sugar is especially important for those managing insulin resistance, prediabetes, or type 2 diabetes, where glucose control is crucial for health.
How fasting affects blood sugar levels
When you fast, you abstain from food for a period, this can range from intermittent fasting (12-16 hours) to longer fasts lasting 24 hours or more. During this time, your body undergoes several changes:1. Initial drop in blood sugarIn the first few hours after your last meal, blood sugar begins to fall as glucose is absorbed by your cells for energy. Without new food intake, glucose levels naturally decline.2. Glycogen breakdownTo maintain energy supply, your liver breaks down stored glycogen (a form of glucose stored in the liver and muscles) into glucose, releasing it into the bloodstream. This process helps keep blood sugar levels within a normal range during fasting.3. Shift to fat metabolismAs fasting continues and glycogen stores deplete (usually after 12-24 hours), the body switches to burning fat for energy. This process produces ketones, which can also be used by the brain and muscles. Blood sugar levels stabilise at a lower, steady state.
Blood sugar levels during different types of fasting
Intermittent fastingDuring intermittent fasting, blood sugar typically decreases modestly but remains within normal ranges. This is due to the balance between glycogenolysis (glycogen breakdown) and gluconeogenesis (creation of glucose from non-carbohydrate sources).Extended fasting (24 hours or more)Extended fasting leads to lower blood sugar levels, as the body relies more heavily on fat metabolism and ketone production. Blood glucose usually remains stable and low, but not dangerously so in healthy individuals.Water fastingConsuming only water results in similar changes, with blood sugar dropping initially and then stabilising at lower levels as ketone bodies increase.
Fasting and blood sugar : What does science say?
Multiple studies have explored how fasting impacts blood sugar regulation:
- A 2019 review published in The New England Journal of Medicine highlights that intermittent fasting can improve insulin sensitivity, meaning your body uses insulin more effectively to lower blood sugar levels.
- Research from the University of Southern California showed that fasting improves metabolic markers, including blood sugar, by reducing insulin resistance and lowering fasting glucose levels.
- The British Dietetic Association notes that fasting may help reduce blood sugar spikes after meals, particularly beneficial for those at risk of type 2 diabetes.
Is fasting safe for people with diabetes
If you have diabetes, especially type 1 or insulin-dependent type 2, fasting can be risky due to potential hypoglycaemia (dangerously low blood sugar). However, under medical supervision, some people with type 2 diabetes can fast safely and even improve their blood sugar control.Important tips:
- Always consult your healthcare provider before starting any fasting regimen.
- Monitor blood glucose frequently during fasting periods.
- Adjust medication dosages as advised by your medical team.
Benefits of fasting for blood sugar control
Fasting has been associated with several benefits related to blood sugar:
- Improved insulin sensitivity: Reduces insulin resistance, helping the body regulate blood sugar better.
- Lower fasting glucose levels: Helps maintain blood sugar in a healthier range during non-eating periods.
- Weight management: Reduces body fat, a major contributor to insulin resistance and high blood sugar.
- Reduced inflammation: Chronic inflammation is linked to impaired blood sugar control; fasting can lower inflammation markers.
Potential risks and considerations
While fasting can be beneficial, it’s not suitable for everyone. Risks include:
- Hypoglycaemia, especially in diabetics on insulin or certain medications.
- Nutrient deficiencies occur if fasting periods are too long or poorly planned.
- Dizziness, fatigue, or irritability during initial fasting days.
Tips for managing blood sugar while fasting
- Stay hydrated with water, herbal teas, or black coffee (without sugar or cream).
- Breakfast gently with balanced meals that include fibre, protein, and healthy fats.
- Avoid high-sugar or high-carb foods immediately after fasting.
- Monitor your blood sugar regularly, especially if you have diabetes.
- Listen to your body; stop fasting if you experience severe symptoms like dizziness, confusion, or weakness.
Also read | Can drinking coffee daily affect your blood sugar levels? Here’s what you need to know