Fasting Diets

Medically Reviewed on 10/21/2022

Things to know about fasting diets

Fasting Diets
Fasting is distinct from starvation in that starvation describes a state of chronic nutritional deficiency. When people fast for weight loss, they are drawing upon their excess reserves.

Fasting in some form has long been part of many spiritual practices. For example, during the month of Ramadan, Muslims fast from dawn to dusk. In Jewish, Christian, Buddhist, and Mormon religions, followers fast on certain days of the week or month. Fasting for health and to lose weight is also nothing new, but a new crop of trendy diets has brought the practice back into the spotlight.

Diets like the 5:2 Diet, Lemonade Diet, and others claim to help people lose weight fast and detoxify the body. There are different types of fasting diets that vary in intensity and duration. There are water-only fasts (typically done under medical supervision in residential facilities). Intermittent fasting involves fasting for one to two days per week or for 12-14 hours each day. Modified fasts include liquid-only diets (such as before a colonoscopy), caloric restriction (typically 20%-40% less), and ketogenic diets (a high-fat diet that induces the same metabolic changes as fasting).

Research demonstrates that there is a role for fasting for weight loss, longevity, and some specific health conditions. Much of the data comes from animal research, but high-quality human research has demonstrated benefits, including lower rates of cardiovascular disease and diabetes, weight loss, and improvements in lab markers of metabolic health and aging. Caloric restriction and intermittent fasting are the only research-proven strategies to extend the life span.

How does fasting work?

When dietary intake is restricted, the body must draw upon stored energy reserves to continue metabolic activities. Specifically, when carbohydrates are restricted, metabolism changes from the standard use of glucose for fuel to ketosis. Ketosis is the term for burning fatty acids (from stored body fat) for fuel. This is the fat-burning phase that decreases fat mass, inches, and body weight. Also, when fasting, inflammatory lab markers decrease; cholesterol, glucose, insulin, and other markers of potentially are unhealthy metabolism improve. Fasting is distinct from starvation in that starvation describes a state of chronic nutritional deficiency. When people fast for weight loss, they are drawing upon their excess reserves. When people follow modified fasts, they get most of the metabolic benefits of fasting but can still maintain nutritional adequacy by consuming some food.

What are intermittent fasting diets?

The 5:2 diet was recently the topic of a popular BBC documentary and book, and it's based on the principle of intermittent fasting (IF) or alternate-day fasting (ADF). Intermittent fasting means you eat normally at certain times and then fast or dramatically reduce your calorie intake at other times.

The 5:2 fasting plan calls for eating normally on five days of the week and fasting (eating no more than 500 calories for women and 600 for men) on the remaining two days. An alternative pattern of intermittent fasting involves a 12-hour overnight fast between dinner and breakfast. The Bulletproof diet suggests an extended daily fast (14-18 hours), which is achieved by consuming only coffee enriched with grass-fed butter and coconut oil in the morning and consuming a healthy low-carbohydrate meal in the evening. This pattern induces ketosis. Alternate-day fasting has been researched in animals and has variable benefits.

Research on intermittent fasting demonstrates fasting can increase life span, improve how the brain works, and protect against disease; but most of those claims are based on research conducted in animals like rats and mice. Among high-quality human studies, intermittent fasting appears to improve stress resilience at a cellular level, resulting in fewer spikes and troughs in blood sugar, cortisol, and other markers of oxidative stress.

Small studies in humans suggest that an intermittent fasting diet may help promote weight loss and reduce risk factors for heart disease, diabetes, and other weight-related health problems. For example, a small 2012 study in Nutrition & Metabolism showed that 30 obese women who followed a liquid intermittent fasting diet lost 8.8 pounds and 2.3 inches from their waistlines after eight weeks. (Excess body fat around the waist has been associated with a higher risk of heart disease.)

Research published in the Journal of the American Medical Association (JAMA) on caloric restriction (-25%) in humans suggests improvement in biomarkers of longevity. Studies in rodents demonstrate increased telomere length and other biometrics of anti-aging.

Drinking water can help you feel less hungry.

Best Diet Tips for Weight Loss Slideshow

Tip 1: Drink plenty of water.

Drink a glass of water before you dive in to a calorie-laden snack. Sometimes thirst can be confused with hunger, so if you drink water first you may feel less hungry. Herbal tea (unsweetened) and flavored sparking water are good options if you're craving more than plain water.

What are detox diets?

Other types of fasting diets that have been in the headlines are called "detox diets." Detox is a nonspecific term, but it conveys a sense of cleansing or purifying the body of toxins. Singer and actress Beyoncé reportedly used the Master Cleanse (also known as the Lemonade Diet) to lose weight to prepare for her role in Dreamgirls. But detox diets may actually help your body process the chemicals and exposures that are part of our day-to-day urban lives.

"Cleansing" is difficult to measure, medically, but we know the liver is the primary organ of detoxification. Lab metrics that may reflect cleansing include liver enzymes (ALT, AST), GGT (which is both a liver enzyme and a metric of liver glutathione or antioxidant reserves), and lower triglycerides and increased HDL, which are signs that fatty acids are moving out of circulation back to the liver for processing and elimination.

In my integrative medical practice, I run a two-week Clean and Lean detox program several times per year. People substitute one to two meals per day with an organic pea-based protein smoothie and eat a full meal of vegetables, lean protein, and healthy fats. They take extra fiber, lots of water, and an herb called milk thistle (Silymarin) that improves liver function and detoxification. During the program, people abstain from sugar, alcohol, caffeine, red meat, gluten, and dairy products. People generally report that they feel fantastic after the first few days (during which some have an initial headache). Most lose weight and inches off their waists. I use the detox program to jumpstart people's weight loss and healthy habits so they can more easily transition into a lifelong healthy lifestyle.

What health conditions benefit from fasting?

Fasting has been shown to improve

Intermittent fasting has been shown to improve

  • cognitive function and
  • increase neuronal plasticity.

Dr. Alan Goldhamer has been supervising water-only fasting at his Santa Rosa, Calif., clinic for more than 20 years and has published data on the outcomes of fasting among patients with hypertension, metabolic syndrome, and cancer. People interested in trying any form of fasting for their chronic health conditions should do so with the input of a practitioner (such as a certified nutritionist, naturopathic doctor, or integrative medicine provider) knowledgeable about the topic.

Are there diets that mimic the benefits of fasting?

A ketogenic diet (a diet high in fat and very low in carbohydrates) will shift metabolism into burning fatty acids for fuel instead of glucose (ketosis). This is thought to have all of the benefits of fasting without the caloric restriction.

What are the downsides of fasting diets?

The disadvantages of fasting diets are the following.

  • Fasting done inappropriately can lead to losses in muscle mass (rather than fat), headaches, decreased basal metabolic rate, and acidosis.
  • Perhaps the biggest downside to fasting is that people aren't prepared to transition into a healthy diet afterward, and the benefits will be quickly lost.

Modified fasting can be a good way to jumpstart weight loss (such as the South Beach diet) as long as there is a good plan to transition into a healthy, low-to-moderate carbohydrate diet afterward.


According to the USDA, there is no difference between a “portion” and a “serving.” See Answer

What are the health risks of fasting diets?

Although more research is needed to fully understand the long-term benefits and risks of fasting diets, the short-term risks to consider include headaches, irritability, and fatigue. Electrolytes can be disrupted by fasting and can be monitored with blood tests.

  • Anyone considering a fasting or detox diet should first consult with a healthcare professional, and certain people should never try one of these diets.
  • People who are on medications for blood pressure or diabetes may have to reduce their medications while fasting; in addition, some diabetics may have dangerous high and/or lows in glucose levels if they attempt certain diets.
  • They should be monitored and work in partnership with a healthcare provider (for example, their primary care provider and/or specialist).
  • Also, fasting or any other type of diet that severely restricts calories is not recommended for pregnant or women who are breastfeeding.

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Medically Reviewed on 10/21/2022
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Goldhamer, A.C., et al. "Water-only fasting and an exclusively plant foods diet in the management of stage IIIa, low-grade follicular lymphoma." BMJ Case Rep Dec. 10, 2015.

Heilbronn, L.K., et al. "Effect of 6-month calorie restriction on biomarkers of longevity, metabolic adaptation, and oxidative stress in overweight individuals: a randomized controlled trial." JAMA 295.13 Apr. 5, 2006: 1539-1548.

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