The Side Effects of a 40-Hour Fast
Despite the many benefits a 40-hour fast can have, it may induce some side effects in some individuals. It doesn’t mean that they can definitely happen for everyone, but knowing the side effects help us make more informed decisions.
Here are some of the most common side effects of a 40-hour fast.
- Hunger and Irritability: Restricting caloric intake can cause hunger and irritability in prolonged fasting.
Source: Johnstone, A. (2015). Fasting for weight loss: an effective strategy or latest dieting trend? International Journal of Obesity, 39(5), 727-733.
- Reduced Energy and Fatigue: A 40-hour fast can decrease energy levels and cause fatigue since the body tries to use stored energy.
Source: Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., ... & Varady, K. A. (2018). Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and Healthy Aging, 4(4), 345-353.
- Dehydration and Electrolyte Imbalance: Long hours of fasting without consuming enough water and beverages can cause dehydration and electrolyte imbalances.
Source: Hartman, M. L., Veldhuis, J. D., Johnson, M. L., Lee, M. M., Alberti, K. G., Samojlik, E., & Thorner, M. O. (1992). Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. The Journal of Clinical Endocrinology & Metabolism, 74(4), 757-765.
- Blood Sugar Fluctuations: Long hours of fasting can cause blood sugar fluctuations, especially hypoglycemia (low blood sugar) in some dieters.
Source: Antoni, R., Robertson, T. M., Robertson, M. D., & Johnston, J. D. (2016). A pilot feasibility study exploring the effects of a moderate time-restricted feeding intervention on energy intake, adiposity and metabolic physiology in free-living human subjects. Journal of Nutritional Science, 5, e2.
- Gastrointestinal Distress: Fasting can cause bloating, constipation, diarrhea or other digestive issues.
Source: Chung, H., Chou, W. C., Sears, D. D., & Patterson, R. E. (2016). Webster's new world public speaking handbook (4th ed.). New York, NY: Wiley.
- Dizziness and Weakness: A 40-hour fast can cause dizziness, weakness, or lightheadedness.
Source: Heilbronn, L. K., Smith, S. R., Martin, C. K., Anton, S. D., & Ravussin, E. (2005). Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. The American Journal of Clinical Nutrition, 81(1), 69-73.
- Mood Changes and Mental Health: Consuming no calories can cause mood swings or anxiety or worsen mental health conditions in some individuals.
Source: Hajek, P., Myers, K., Dhanji, A. R., & West, O. (2018). Mood and neuropsychological effects of different doses of nicotine gum in habitual smokers and never-smokers. Psychopharmacology, 235(3), 907-914.
Learn More: [Expert Guide] 20-Hour Fast Results, Side Effects, and How to
Summary: A 40-hour fasting can have side effects like hunger, fatigue, dehydration, blood sugar changes, and mood swings in some individuals.
How to Do the 40-Hour Fast?
To make drastic changes to your eating habits, like fasting for 40 hours, you first need to consult a healthcare professional to make sure it’s safe. After that, you can follow the steps below to start a healthy 40-hour fast.
To prepare for a 40-hour fast:
- Set a Specific Time: Pick the best period you can fast for 40 hours without significant physical or mental stress.
- Hydration: Drink enough water before and during the fast.
Then, to do the actual fast:
- Start with an Early Dinner: Have a balanced and nutritious dinner containing lean proteins, vegetables, and whole grains, early in the evening, around 6-8 hours before your usual bedtime.
- Stay Hydrated: Drink enough water, herbal tea, or other non-caloric beverages during the fast.
- Plan Distractions: Stay busy to keep your mind off the diet and can handle the long hours more easily.
- Focus on Electrolytes: Add some salt to your water to maintain electrolyte balance.
- Listen to Your Body: Monitor your body’s changes and break the fast if you feel too much hunger, dizziness or discomfort.
- Sleep Well: Sleeping enough and well can help manage hunger and support your body during the fasting period.
To Break the Fast:
- Reintroduce Nutrients Gradually: Avoid overeating and break the fast with a small but nutritious meal containing protein, healthy fats, and carbohydrates.
- Include Whole Foods: Avoid processed foods and opt for lean proteins, vegetables, fruits, and whole grains.
- Stay Hydrated: Keep drinking water during the eating period to stay hydrated.
- Listen to Your Body: Monitor your body’s reactions to food after the fast.
After the fast try to
- Reflect and Learn: Keep a record of your body’s reactions and changes to make necessary changes and let your body adapt better.
- Space Out Fasts: Consult a healthcare professional to determine enough time you need to wait before starting the next round of fasting to avoid negative effects on metabolism or energy levels.
What Happens to Your Body on a 40-Hour Fast?
Various physiological changes occur in the body during a 40-hour fast including the following.
- 0-6 Hours: Your body keeps digesting and absorbing nutrients from your last meal. Blood sugar level start to decrease as the body burn glucose to produce energy.
- 6-12 Hours: The body starts to burn glycogen stored in the liver and muscles for energy. Insulin levels drop so that the stored fat can be broken down into fatty acids.
- 12-18 Hours: Fat breakdown leads to ketosis and the body begins to produce ketones. Hunger hormones are suppressed, which may reduce appetite.
- 18-24 Hours: Ketosis continues and ketones become the main energy source, sparing muscle protein. More growth hormone is produced to support fat metabolism and muscle maintenance.
- 24-30 Hours: Cellular autophagy increases as the body breaks down and recycles damaged or dysfunctional cellular components to produce energy. Insulin sensitivity increases, which helps regulate glucose.
- 30-40 Hours: Ketosis continues and more ketones are produced to provide energy for various tissues, including the brain. The body relies mainly on fat for energy, which boosts fat oxidation. Also, the body tries to conserve energy, which may slightly decrease the metabolic rate.
- After 40 Hours: Hunger may reappear, signalling the body's need for nourishment as the body keeps using fat for energy.
Learn More: 24-Hour Fast: How to Do It? Is It Good for Weight Loss? [Expert Guides]
How Often Should I Do a 40-Hour Fast?
The frequency of fasting for 40 hours depends on various personal factors and only two people can work together to find it out, i.e., you and your physician!
The factors involved in deciding the frequency of a 40-hour fast are your individual health status, goals, and lifestyle. You can use the following tips before deciding on the frequency of fasting for 40 hours.
- Consult a healthcare professional.
- Listen to your body.
- Start slowly with shorter fasting periods to see how your body reacts.
- Review your goals as they influence the fasting frequency. Some goals include weight loss, improved metabolic health, improved general health, etc.
- Find a balanced approach to health and combine fasting with a good diet and enough daily exercise to see the best results.
- Give your body enough time between fasting periods to recover and get adequate nutrients.
- Don’t just do what others do as you need to consider your daily routines, work commitments, social events, and lifestyle when planning fasting periods.
40-Hour Fast and Autophagy
A 40-hour fast can initiate autophagy in the body, which is a cellular cleansing process activated when the body is under stress like during fasting, helping the body get rid of damaged or dysfunctional components.
During fasting, when glucose and glycogen are used up, the body starts to use fatty acids and ketones to produce the necessary energy for the body, which triggers autophagy to optimise energy use and ensure cellular survival.
Autophagy means breaking down and recycling cellular components like damaged proteins, organelles, and other cellular debris, i.e., cleaning cellular waste and helping the body reproduce new, healthy cells.
Autophagy leads to various health benefits, including cellular repair, cellular longevity, disease prevention, immune system support, and metabolic health.
40-Hour Fast and Weight Loss
One of the main benefits of a 40-hour fast is weight loss, but the amount of it depends on various individual factors. You can’t lose weight healthily just by avoiding food for long hours. You need to do it step by step as explained earlier in this article.
Anyhow, here's how a 40-hour fast can impact weight loss:
Fasting for 40 hours restricts calorie intake, which leads the body to use stored fat for energy, resulting in fat melt and weight loss over time.
The process of shifting from depleted glycogen to fat for energy is called fat oxidation, which can lead to increased fat loss, boosting weight loss.
Fasting helps insulin sensitivity, which means better regulating blood sugar levels. Improved insulin sensitivity prevents overeating and promotes fat loss.
Fasting impacts appetite and metabolism hormones related to appetite (ghrelin and leptin), which reduces hunger and decreases food intake, boosting weight loss.
- Increased Metabolic Rate:
Intermittent fasting can increase metabolic rate, which involves burning more calories and losing more weight.
Fasting can suppress appetite, making you less prone to overeating.
Learn More: 18-Hour Fast Diet: Benefits and Weight Loss Results
What Should I Break My 40-Hour Fast With?
Start with small meals and don’t put too much pressure on your body, especially the digestive system, by eating a heavy meal. Let your body adapt to the long fasting situation. Here are the foods you need to eat and avoid for a healthy fast.
Foods to Eat
- Protein-Rich Foods: Including lean meats (chicken, turkey), fish (salmon, tuna), eggs, and plant-based proteins (tofu, tempeh)
- Healthy Fats: Including avocado, nuts (almonds, walnuts, etc.), seeds (chia, flax, sunflower), and olive oil
- Fibre-Rich Vegetables: Including leafy greens (spinach, kale), cruciferous vegetables (broccoli, cauliflower), bell peppers, and zucchini
- Complex Carbohydrates: Including whole grains (brown rice, quinoa, oats), sweet potatoes, and legumes (beans, lentils)
- Fruits: Including berries (blueberries, strawberries), citrus fruits (oranges, grapefruits), apples and pears
- Herbal Tea
- Coconut Water
Foods to Avoid
- Processed Foods: Including fast food, sugary snacks, packaged and processed foods
- Highly Sugary Foods: Including candies, pastries, soda and sugary beverages
- Greasy and Fried Foods: including fried chicken, french fries, deep-fried foods
- Excessive Dairy: Including heavy cheeses, cream-based dishes
- Large Portions
What Can I Drink on a 40-Hour Fast?
- Herbal Tea
- Black Coffee
- Green Tea
- Plain Tea
- Sparkling Water
- Electrolyte Drinks
- Lemon Water
- Apple Cider Vinegar (Diluted)
- Bone Broth (Limited)
A 40-hour fast can have positive impacts on health and fitness if done correctly. Simply put, it can be beneficial for cell repair, weight loss, and blood sugar levels, but the side effects include hunger, dizziness, and fatigue.
You need to stay hydrated during the fasting period and eat nutrient-dense foods out of the fasting period to provide the necessary vitamins and minerals and let your body adapt to the strict fasting situation.
Also, if you’re new to fasting, it’s better to start with less restrictive versions of intermittent fasting, like 12-hour fasting, to see how your body reacts.
It’s no secret that consulting a healthcare professional helps you avoid unwanted health issues and ensure you make the most of fasting while achieving your health and fitness goals.
Remember that health always comes first!
Here are the answers to the most frequently asked questions about a 40-hour fast.
What Are The Stages of a 40-Hour Fast?
- Fed state (0-3 hours)
- Early fasting state (3-18 hours)
- Fasting state (18-40 hours)
What Happens If You Don't Eat for 40 Hours?
In addition to the side effects mentioned above, a 40-hour fast can cause dehydration if you don't drink enough fluids.
Can Fasting Reduce Belly Fat?
Yes, fasting can help reduce belly fat and, generally, body fat.