An empty stomach can potentially raise blood sugar levels in some people. When you haven’t eaten for several hours, your body may start breaking down glycogen stored in the liver and muscles for energy. This process releases glucose into the bloodstream, which can cause a rise in blood sugar levels (1). However, the rise is usually small and temporary for people without diabetes. Those with diabetes need to be more cautious about prolonged periods without food to prevent spikes in blood sugar.
What happens during fasting?
When you fast or skip meals, your body goes into a metabolic state called ketosis. This means your body starts burning its stored fat for fuel through a process called ketogenesis. Ketones are produced as a byproduct of breaking down fat. After several hours of fasting, glycogen stores become depleted. Glycogen is the stored form of quick energy (glucose) found in the liver and muscles (2).
Once glycogen is used up, the body releases hormones like glucagon and epinephrine. These hormones signal the liver to start converting non-carbohydrate molecules into glucose through gluconeogenesis. The liver breaks down glycerol, lactate, and amino acids into glucose to provide fuel for the body (3). This process causes blood sugar levels to start rising after prolonged fasting, usually after 12-16 hours without food (4).
In non-diabetics, these mechanisms help maintain normal blood sugar levels even with inconsistent meals. However, those with diabetes may experience exaggerated spikes in blood glucose when fasting for long intervals. This is because the pancreatic beta cells cannot produce enough insulin to compensate for the glucose release caused by glycogenolysis and gluconeogenesis (5).
How long should you fast?
For most healthy individuals, fasting for 12-16 hours is considered safe. This eating window is quite common with intermittent fasting approaches like the 16:8 method. With this schedule, you fast for 16 hours per day and restrict eating to an 8-hour window.
Studies show that fasting less than 24 hours effectively lowers insulin levels without causing blood sugar spikes in non-diabetics (6). However, fasting for longer intervals can start increasing glucose. One study found that a 36-hour fast significantly raised blood sugar levels in healthy subjects (7).
Fasting for longer than 24 hours on a regular basis may not be recommended. Very low-calorie diets with less than 800 calories per day tend to result in higher post-meal and fasting blood sugars (8). Extreme calorie restriction can increase gluconeogenesis. Your body senses starvation and starts pumping out more glucose for energy.
Therefore, most experts advise against extended fasting periods for healthy individuals. Time-restricted feeding with 8-12 hour eating windows is generally considered safe for regulating blood sugar (9).
Tips to prevent blood sugar spikes when fasting
Here are some tips to help prevent increases in blood sugar levels during fasting periods:
- Avoid fasting for longer than 16 hours if you have diabetes or blood sugar regulation issues.
- Include protein and healthy fats when you break your fast. This helps slow the digestion of carbohydrates and minimize glucose spikes.
- Choose low-glycemic index foods like vegetables, legumes, nuts and healthy whole grains. These foods do not cause rapid rises in blood sugar.
- Stay hydrated by drinking water, herbal tea and broth during fasting periods.
- Monitor your blood sugar frequently. This helps you assess if your fasting habits are raising levels.
- Supplement with branched-chain amino acids (BCAAs). BCAAs help reduce protein breakdown during fasting.
- Follow a regular meal schedule instead of sporadic fasting. This helps your body acclimate and lowers metabolic fluctuations.
- Ask your doctor before starting intermittent fasting if you take diabetes medications. You may need medication adjustments to prevent low blood sugar.
Bottom line
In most cases, fasting for 12-16 hours is unlikely to cause spikes in blood sugar levels. The body is designed to use stored glucose and increase gluconeogenesis to prevent energy crashes. However, fasting for longer than 24 hours on a consistent basis can start increasing blood sugar levels in some people.
Those with diabetes need to be especially mindful of prolonged fasting. Nutrient depletion signals the liver to release more glucose through gluconeogenesis, which can spike blood sugar levels higher if you have insulin resistance or deficiency. Monitoring your levels and following a regular meal schedule with a balanced diet can help prevent hypoglycemia or hyperglycemia when fasting.
What causes blood sugar to rise without eating?
There are a few key reasons why blood sugar levels may start to climb even if you haven’t eaten anything:
- Gluconeogenesis – As mentioned earlier, fasting triggers the process of gluconeogenesis. The liver converts non-carbohydrate molecules like amino acids into glucose when glycogen stores run low (10).
- Dawn phenomenon – In some people, blood sugar rises in the early morning hours even before eating. This is due to changes in growth and stress hormones.
- Medications – Certain medications like steroids, antipsychotics, and hypertension drugs can increase blood sugar levels.
- Illness – Inflammation and infections raise counter-regulatory hormones that can elevate blood glucose.
- Dehydration – Not drinking enough fluids thickens the blood, forcing the body to pull water from cells. This concentrates circulating glucose.
- Inactivity – Lack of exercise causes insulin resistance, which makes it harder to transport glucose into cells from the bloodstream.
- Stress – Stress hormones like cortisol and epinephrine raise blood sugar levels.
Therefore, many factors beyond diet can cause fluctuations in blood glucose. Getting adequate sleep, hydration, exercise and stress management are key for maintaining steady blood sugar levels.
How fasting impacts insulin
Insulin is the hormone responsible for allowing glucose from the bloodstream to enter cells for energy. It is secreted by the beta cells within the pancreas in response to rises in blood sugar, such as after a meal (11).
Insulin has two primary functions:
- Allow glucose to enter cells – Insulin attaches to receptors on cell membranes. This activates a signaling pathway that causes glucose transporters (GLUT proteins) to translocate to the cell surface. This allows glucose to move from the blood into cells, lowering circulating blood sugar levels (12).
- Store excess glucose – Insulin signals cells like muscle, fat and liver cells to take up glucose from blood and convert it into glycogen. This stored form of glucose provides future energy reserves.
When you fast, insulin secretion decreases due to lower blood sugar levels. This drop in insulin triggers the breakdown of glycogen into glucose. The reduction in insulin also stimulates lipolysis – the breakdown of triglycerides in fat cells into fatty acids for fuel (13).
However, insulin levels won’t stay low indefinitely. As liver glycogen gets used up, gluconeogenesis kicks in after 12-16 hours to maintain glucose levels. This release of glucose from the liver causes circulating insulin to start rising again (14).
In healthy people, insulin is able to keep up with the increased gluconeogenesis prompted by fasting. But those with insulin deficiencies may experience sharper spikes in blood sugar as insulin fails to keep pace with the greater glucose production and release.
How does fasting affect blood sugar in diabetics?
People with diabetes need to take extra precautions with fasting to prevent either severe hypoglycemia or hyperglycemia. This is because those with diabetes have impaired insulin secretion or insulin resistance, which can make it harder to adapt to changes in glucose homeostasis prompted by fasting.
Studies show that intermittent fasting may improve insulin sensitivity and blood sugar regulation in those with type 2 diabetes (15). However, diabetes medications need to be adjusted to prevent hypoglycemia which can be dangerous.
Longer fasts can also cause severe hyperglycemia in diabetics. One study found that fasting for more than 15 hours significantly increased glucose levels in type 2 diabetics (16). Fasting for 24 hours caused an even more dramatic spike in blood sugar.
Working with a doctor is crucial before starting fasting with diabetes. Your medications, activity, and glucose levels will need to be closely monitored to prevent blood sugar extremes. Shorter fasts of 12-14 hours with regular mealtimes may be safer for diabetics than sporadic prolonged fasting.
Tips for diabetics fasting safely
Here are some tips for safer fasting with diabetes:
- Speak with your doctor before changing your meal schedule or fasting duration.
- Check blood sugar frequently when starting intermittent fasting.
- Adjust medications as needed to prevent hypoglycemia.
- Stick to shorter fasts no longer than 16 hours.
- Include protein, fat and low glycemic index foods when breaking fast.
- Supplement with branched chain amino acids (BCAAs) during fasting periods to reduce glucose spikes.
- Stay hydrated by drinking unsweetened beverages.
- Monitor ketone levels if doing longer fasts.
- Break the fast if you experience signs of hyperglycemia like excess thirst, headache, blurry vision.
Diabetics can practice intermittent fasting safely under medical supervision. But prolonged fasting is generally not recommended due to risks of blood sugar fluctuations. Monitoring your glucose levels regularly is key to ensuring your fasting habits are not causing spikes or crashes.
How can you stabilize blood sugar when fasting?
Here are some effective tips for stabilizing blood sugar levels during fasting periods:
Eat fiber-rich foods before fasting
Consuming high fiber foods like non-starchy vegetables, nuts, seeds and legumes before fasting can help slow glucose absorption and maintain steady blood sugar levels (17). Soluble fiber forms a gel that traps carbohydrates and prevents sharp insulin spikes.
Include protein and fat when breaking the fast
Eating protein and fat with carbohydrates helps buffer their impact on blood sugar. Protein and fat digest more slowly and elicit a lower insulin response compared to carbs alone (18).
Exercise during fasting
Doing physical activity while fasting helps enhance insulin sensitivity and facilitates glucose uptake into muscles. This can effectively lower blood sugar levels (19).
Drink apple cider vinegar
Some research indicates apple cider vinegar may improve insulin sensitivity and reduce fasting blood sugars, though more studies are needed (20).
Try intermittent fasting
Alternating fasting with regular eating periods allows your body to adapt and reach a metabolic equilibrium. This helps minimize glucose spikes.
Choose low glycemic foods
Opt for foods like stone fruits, nuts, seeds, lentils, greens and eggs that do not drastically impact blood sugar.
Stay hydrated
Drink plenty of water and herbal tea to avoid dehydration. Dehydration can increase blood glucose concentrations.
Sample meal schedule to stabilize blood sugar when fasting
Here is a sample meal plan to help maintain steady blood sugar levels when fasting:
Day 1
Morning before fast:
Oatmeal with nuts and berries
Green tea
Afternoon:
Sparkling water with lemon
Green tea or broth
Early evening meal to break fast:
Grilled salmon over greens with olive oil and vinegar
Avocado
Herbal tea
Day 2
Morning before fast:
Scrambled eggs with vegetables
Apple with peanut butter
Afternoon:
Sparkling water with lemon
Iced hibiscus tea
Early evening meal to break fast:
Turkey burger on portobello mushroom bun
Kale and beet salad with walnuts
Herbal tea
Day 3
Morning before fast:
Greek yogurt with nuts, seeds and berries
Coffee with coconut milk
Afternoon:
Diluted apple cider vinegar drink
Sparkling water
Early evening meal to break fast:
Tofu veggie stir fry with cauliflower rice
Miso soup with seaweed
Green tea
Conclusion
In summary, intermittent fasting does not inherently lead to spikes in blood sugar if kept at 16 hours or less for most people. However, fasting for longer periods can start increasing glucose through gluconeogenesis. This effect tends to be exaggerated in diabetics.
Eating fiber-rich low glycemic foods before fasting, exercising, staying hydrated and breaking fasts with protein and fat can help stabilize blood sugar levels. Diabetics should consult a doctor before fasting and monitor glucose levels diligently to prevent hyperglycemia. With proper precautions, intermittent fasting can be sustained without causing blood sugar fluctuations. But prolonging fasting beyond 24 hours may come with greater risks of dysregulated blood glucose.