In recent years, there has been a surge of interest in the potential health benefits of fasting, with individuals seeking effective strategies for weight loss and metabolic improvement. Intermittent fasting, which involves short periods of food abstention for 16 to 36 hours, has become a popular approach. However, "prolonged fasting," characterized by consciously restricting food or caloric beverages for several days to several weeks, has also garnered attention. The main types of prolonged fasting are water-only fasting and Buchinger fasting.
This article aims to provide a comprehensive overview of 14-day fast weight loss plans, exploring their potential effects on metabolic health, including body weight, blood pressure, plasma lipids, and glycemic control. It will also discuss the importance of refeeding after the fast, potential adverse events, and guidelines for safe and effective implementation.
Understanding Prolonged Fasting
Prolonged fasting involves abstaining from food or caloric beverages for extended periods, typically ranging from several days to weeks. There are two primary types of prolonged fasting:
- Water-only fasting: This involves consuming only mineral water or distilled water daily (∼2-3 L) without any food intake.
- Buchinger fasting: This is a medically supervised program that allows small amounts of food daily. Developed by Dr. Otto Buchinger in the 1920s, this protocol typically includes 250 mL of fruit or vegetable juice for lunch and 250 mL of vegetable soup for dinner, resulting in an average total calorie intake of 200-300 kcal and 25-35 g of carbohydrates per day.
The mechanism behind the beneficial effects of prolonged fasting is thought to involve flipping the metabolic switch. After 12 to 36 hours of fasting, the body transitions from utilizing glucose (from glycogen stores) to fatty acids and fatty-acid-derived ketones for energy. Adipocytes release free fatty acids, which are transported to the liver and converted into ketones (ie, beta-hydroxybutyrate and acetoacetate). Ketone levels generally plateau after 5 to 10 days of prolonged fasting and serve as the primary energy source for the body for up to 90 days. In addition to acting as a primary energy source, ketones also have direct physiological benefits.
Effects of Prolonged Fasting on Metabolic Health
Prolonged fasting can have several effects on metabolic health measures:
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Body Weight and Composition
Studies have shown that prolonged fasting for 5-20 days can lead to mild to moderate weight loss of 2-10%. Longer fasting durations generally produce more pronounced weight loss than shorter fasts, but findings were variable. For instance, 5 days of water fasting or Buchinger fasting lowered weight by 4-6% from baseline. Moderate durations of fasting (7-10 d) typically produced greater weight loss (2-10%), while very long durations of fasting (15-20 d) produced the greatest weight loss (7-10%).
However, it is important to note that approximately two-thirds of the weight lost during prolonged fasting is lean mass, and one-third is fat mass. This excessive lean mass loss raises concerns about the breakdown of muscle proteins. Reductions in lean mass can translate into lower resting metabolic rate after fasting, which can put individuals at risk for future weight regain.
Blood Pressure
Consistent reductions in systolic (ranging from 9 to 14 mm Hg) and diastolic (ranging from 6 to 13 mm Hg) blood pressure have been observed in trials that achieved greater than 4% weight loss with prolonged fasting. The mechanism by which prolonged fasting improves blood pressure most likely involves decreased dietary sodium intake, enhanced production of natriuretic peptides, and the activation of the parasympathetic nervous system after 2-3 days of fasting. In addition, increased circulating levels of ketones may play a role.
Plasma Lipids
The impact of prolonged fasting on plasma lipids is less clear. While some trials demonstrate decreases in LDL cholesterol and triglycerides, others show no benefit. High-density lipoprotein (HDL)-cholesterol levels either decreased by 7-18%, or remained unchanged. Reductions in HDL cholesterol were related to the duration of fasting, with longer fasts (15-20 d) producing greater decreases and shorter fasts (5-10 d) producing smaller decreases. Gender-based differences were also noted. At baseline, women tended to have higher HDL-cholesterol levels than men.
Glycemic Control
Reductions in fasting glucose, fasting insulin, insulin resistance, and glycated hemoglobin (HbA1c) have been noted in adults with normoglycemia after prolonged fasting. In contrast, these glucoregulatory factors remained unchanged in patients with type 1 or type 2 diabetes. In patients with obesity, fasting insulin decreased in all studies that measured this parameter. Specifically, insulin levels were reduced by 65-80% after 5 to 17 days of prolonged fasting. Insulin reached a nadir in all studies by day 3-5, and these low levels of insulin were maintained throughout the duration of the study.
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Refeeding After Prolonged Fasting
The effects of refeeding after prolonged fasting have been examined in a few trials. It was shown that 3-4 months after the fast was completed, all metabolic benefits were no longer observed, even when weight loss was maintained. In one study, normal-weight adults lost 6% of their body weight after 5 days of water-only fasting but then gained it all back after 3 months of eating regularly. In contrast, other trials demonstrated only small weight regains (1-2%) 2-4 months after the finishing the fast.
Adverse Events
Adverse events associated with prolonged fasting may include metabolic acidosis, headaches, insomnia, and hunger. It's important to note that longer periods without food, such as 24-, 36-, 48- and 72-hour fasting periods, are not necessarily better for you, and may be dangerous.
Considerations for Different Populations
Prolonged fasting appears to be effective for weight reduction in a variety of population groups, including individuals with normal weight, overweight, obesity, type 1 diabetes, and type 2 diabetes. The degree of weight loss did not appear to be age-dependent, as younger (40 y), middle-aged (55 y), and older (65 y) participants achieved similar reductions in body weight. However, since most studies enrolled middle-aged adults (50-60 y old), more research will be needed to explore how these effects vary in younger and older adults.
It’s important to check with your doctor before starting intermittent fasting. People with type 1 diabetes who take insulin should proceed with caution. While an increasing number of clinical trials have shown that intermittent fasting is safe in people with type 2 diabetes, there have been no studies in people with type I diabetes.
Comparing Prolonged Fasting with Other Diet Strategies
Intermittent Fasting vs. Prolonged Fasting
Intermittent fasting, which involves shorter periods of food abstention, is a more popular form of fasting used today. While both intermittent and prolonged fasting can lead to weight loss and metabolic improvements, prolonged fasting may carry a higher risk of adverse events and lean mass loss.
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Alternate-Day Fasting vs. Daily Calorie Restriction
Alternate-day fasting involves consuming 25% of usual intake on fast days and 125% of baseline energy intake on alternating "feast days." Studies have compared alternate-day fasting with daily calorie restriction (75% of energy needs every day) and found that mean weight loss was similar for participants in both groups. However, the dropout rate was highest in the alternate-day fasting group, suggesting that it may be more difficult to adhere to this type of diet.
Rapid Weight Loss Diets
Rapid weight loss diets, in general, are those in which you lose more than 2 pounds (lb) or 1 kilogram (kg) a week over several weeks. To lose weight this quickly you must eat very few calories. Rapid weight loss may not be safe for some people to do on their own. These diets are only to be used for a short time and are usually not recommended for more than several weeks. People who lose weight very quickly are much more likely to regain the weight over time than people who lose weight slowly through less drastic diet changes and physical activity. Rapid weight loss can also cause some side effects including: Gallstones, Gout, Fatigue, Constipation, Diarrhea, Nausea.
Practical Considerations for a 14-Day Fast Weight Loss Plan
If you are considering a 14-day fast weight loss plan, it is essential to consult with a healthcare professional to determine if it is appropriate for you. Here are some practical considerations:
- Medical Supervision: Prolonged fasting should be done under medical supervision, especially for individuals with underlying health conditions.
- Type of Fast: Choose between water-only fasting and Buchinger fasting based on your individual needs and preferences.
- Nutrient Intake: If following Buchinger fasting, ensure that you consume the recommended amounts of fruit juice and vegetable soup to meet your basic nutritional needs.
- Hydration: Drink plenty of water (2-3 L per day) to stay hydrated during the fast.
- Monitoring: Monitor your blood pressure, blood glucose, and other relevant metabolic markers regularly.
- Refeeding: Plan a gradual refeeding strategy to avoid complications and maintain weight loss.
- Exercise: Incorporate light physical activity, such as walking, to help preserve lean mass and improve metabolic health.
- Listen to Your Body: Pay attention to your body's signals and discontinue the fast if you experience any adverse events.
Potential Benefits of Intermittent Fasting
Intermittent fasting is an eating plan that switches between fasting and eating on a regular schedule. Research shows that intermittent fasting is a way to manage your weight and prevent - or even reverse - some forms of disease. But how do you do it? With intermittent fasting, you only eat during a specific time. There are several different ways to do intermittent fasting, but they are all based on choosing regular time periods to eat and fast. For instance, you might try eating only during an eight-hour period each day and fast for the remainder. Or you might choose to eat only one meal a day two days a week. Most nutrition experts regard the Mediterranean diet as a good blueprint of what to eat, whether you’re trying intermittent fasting or not. Research shows that the intermittent fasting periods do more than burn fat. One of Mattson’s studies published in the New England Journal of Medicine revealed data about a range of health benefits associated with the practice, such as improvements in thinking and memory, heart health, physical performance, type 2 diabetes and obesity, and tissue health.
The Role of Physical Activity
The regulation of carbohydrate and lipid metabolism is critically influenced by hormones, cytokines, and myokines. Moreover, physical activity plays a pivotal role in enhancing metabolic processes, leading to substantial health benefits. Physical exercise, just like fasting, can be a very powerful agent that changes the body’s metabolism in many directions, depending on its intensity and duration.