Fasting or Calorie Restriction
Calorie restriction and fasting, whether for 12 or more hours every night or occasionally for longer periods, can contribute to lower body weight, blood sugar, and insulin resistance, plus fewer unpleasant chemotherapy-related side effects.
Fasting or calorie restriction at a glance
Calorie restriction simply means eating fewer calories and is a common approach for weight loss. Fasting, also called time-restricted eating, is an extreme form of calorie restriction in which no calories are eaten for a certain time period. The time period may be a few hours—such as fasting 12 hours every night—or fasting may be for longer periods—usually 24 hours to a few days. Some calorie-restricted diets can mimic the metabolic effects of fasting and are called “fasting-mimicking diets.” We consider these diets to be calorie restriction rather than true fasting.
In cancer care, calorie restriction—especially fasting—can alter the metabolism of cancer cells and may improve the effectiveness of chemotherapy and reduce some side effects. Fasting and calorie restriction are effective for managing excess body weight, high blood sugar, and insulin resistancea condition in which cells in your muscles, fat, and liver don’t respond well to insulin and can’t efficiently take up glucose from your blood for energy. They may also help manage imbalances in metabolic hormones linked to cancer risk and outcomes.
CancerChoices ratings for fasting or calorie restriction
We rate fasting or calorie restriction on seven attributes, with 0 the lowest rating and 5 the highest. We rate the strength of the evidence supporting the use of fasting or calorie restriction for a medical benefit, such as improving treatment outcomes or managing side effects.
See how we evaluate and rate complementary therapies ›
Improving treatment outcomes
See More- Preliminary evidencesignificant effects in small or poorly designed clinical studies OR conflicting results in adequate studies but a preponderance of evidence of an effect (this is the CancerChoices definition; other researchers and studies may define this differently) of higher tumor response rates among nondiabetic women with stage 2 or 3 HER-2 negative breast cancer eating a fasting-mimicking diet for 3 days before and during neoadjuvanttherapy used before a main treatment, such as chemotherapy, radiation therapy, and hormone therapy before surgery chemotherapy
- Preliminary evidence of fewer reported dose reductions or delays in chemotherapy among people with gynecologic cancer following a water-only fast 24 hours before and 24 hours after chemotherapy
- No evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on cancer-specific or all-cause mortality among nondiabetic women with early stage breast cancer fasting 13 or more hours per night in a large study
See How can fasting or calorie restriction help you? What the research says ›
Optimizing your body terrain
See MoreStrong or good evidence of benefit
- Strong evidenceconsistent, significant effects in several large (or at least one very large) well designed clinical studies or at least two meta-analyses of clinical studies of moderate or better quality (or one large meta-analysis) finding similar results (this is the CancerChoices definition; other researchers and studies may define this differently) of more weight loss and less body fat among people fasting, usually for 24 hours at a time
- Strong evidence of more weight loss and less body fat among people restricting calorie intake
- Good evidencesignificant effects in one large or several mid-sized and well-designed clinical studies (randomized controlled trials (RCTs) with an appropriate placebo or other strong comparison control or observational studies that control for confounds) (this is the CancerChoices definition; other researchers and studies may define this differently) of lower levels of insulin-like growth factor-1 (IGF-1)—higher levels are linked to higher risk of cancer and worse outcomes—among people fasting or restricting calories by more than 30% but not among people restricting calories by 30% or less
Modest evidence of benefit
- Modest (and somewhat conflicting) evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of lower blood sugar (glucose) or insulin levels or lower insulin resistancea condition in which cells in your muscles, fat, and liver don’t respond well to insulin and can’t efficiently take up glucose from your blood for energy among predominantly overweight or obese people fasting for several hours to 24 hours
- Modest evidence of lower levels of leptin, a hormone that helps regulate energy balance by inhibiting hunger, among overweight or obese people fasting or restricting calories
Preliminary evidence of benefit
- Preliminary (and somewhat conflicting) evidencesignificant effects in small or poorly designed clinical studies OR conflicting results in adequate studies but a preponderance of evidence of an effect (this is the CancerChoices definition; other researchers and studies may define this differently) of lower blood sugar, insulin, and insulin resistance among people restricting calorie intake
- Preliminary evidence of better insulin sensitivity and glucose tolerance among normal weight or overweight, nondiabetic people exercising to create a 16% energy deficit
- Preliminary evidence of more insulin sensitivity among nondiabetic, overweight people creating energy deficits by both restricting calories and exercising
- Preliminary evidence of more weight loss and less body fat among overweight people exercising to create an energy deficit of about 20%
- Preliminary evidence of more weight loss and less body fat among overweight people exercising to create an energy deficit of about 12.5% and exercising to increase total energy expenditure by 12.5%
- Preliminary evidence of lower total and free IGF-1 among people restricting protein intake
- Preliminary evidence of lower serum T3 levels and energy intake but no evidence of an effect on serum total T4, free T4, reverse T3, and TSH levels among healthy adults participating in long-term calorie restriction
- Preliminary evidence of comparable reductions in free androgen index among generally healthy, overweight, or obese women achieving overall 25% weekly calorie restriction either by fasting or restricting daily calories
- Preliminary and conflicting evidence of lower markers of inflammation among people restricting calories
- Preliminary evidence of less oxidative stress among predominantly overweight or obese people fasting or restricting eating
- Preliminary evidence of favorable changes in the gut microbiota among adults with metabolic syndromea cluster of conditions that occur together, including increased blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol or triglyceride levels restricting calories
- Preliminary evidence of a higher marker of adult tissue and organ regeneration among healthy adults restricting calories periodically
No evidence of benefit
- No evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on levels of the “hunger hormone” ghrelin among nonobese, nondiabetic adults fasting every other day in a small study
- No evidence of an effect on markers of inflammation among people fasting in several small studies
See How can fasting or calorie restriction help you? What the research says ›
Managing side effects and promoting wellness
See MoreModest or preliminary evidence
- Modest evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of lower blood pressure among people (not specific to cancer) fasting periodically or restricting calories
- Preliminary evidencesignificant effects in small or poorly designed clinical studies OR conflicting results in adequate studies but a preponderance of evidence of an effect (this is the CancerChoices definition; other researchers and studies may define this differently) of fewer chemotherapy-related side effects among people with gynecologic cancer fasting before and after chemotherapy
- Preliminary evidence of less blood-related toxicity among people with breast cancer fasting before and after chemotherapy
- Preliminary evidence of less chemotherapy-induced DNA damage in T-lymphocytes among women with breast cancer eating a fasting-mimicking diet for 3 days before or during neoadjuvant chemotherapy
- Preliminary evidence of less fatigue among people with cancer fasting or restricting calories before and/or after chemotherapy treatments
- Preliminary evidence of less nausea and other gastrointestinal side effects among people with cancer fasting or restricting calories before and/or after chemotherapy treatments
- Preliminary and somewhat conflicting evidence of better quality of life scores among people with gynecologic or breast cancer fasting before and/or after chemotherapy treatments
- Preliminary evidence of better sleep among people with breast cancer fasting before chemotherapy treatment
- Preliminary evidence of longer sleep among people with breast cancer fasting more hours every night
- Preliminary evidence of lower blood pressure among generally healthy people (not specific to cancer) fasting at least 14 hours a day
Insufficient evidence or no evidence of an effect
- No evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on chemotherapy-related side effects not related to blood (non-hematological toxicity) among women with breast cancer fasting before and after chemotherapy
- No evidence of an effect on hospital admission among people with gynecologic cancer fasting before and after chemotherapy treatment
- No evidence of an effect on blood pressure among people (not specific to cancer) severely restricting calories on alternate days for 4 weeks and then resuming unrestricted eating for 8 weeks in a small study
- Insufficient (conflicting) evidencepreclinical evidence only OR clinical studies with such poor or unclear methodology that no conclusion can be drawn OR conflicting findings across clinical studies with no preponderance of evidence in one direction; conflicting evidence occurs when studies find conflicting effects (positive effect vs no effect or negative effect) with the same treatment and the same general study population (same cancer type, for example) (this is the CancerChoices definition; other researchers and studies may define this differently) of an effect on sleep quality among people (not specific to cancer) fasting 14 to 16 hours every night
See How can fasting or calorie restriction help you? What the research says ›
Reducing cancer risk
See More- Modest evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of higher risk of recurrence among nondiabetic women with early stage breast cancer fasting less than 13 hours every night
See How can fasting or calorie restriction help you? What the research says ›
Use by integrative oncology experts
See More- Not mentioned in integrative cancer clinical practice guidelines
- Used in several (at least 4) of our program sources
Safety
See More- Generally safe among people without malnutrition or low body mass index (below 18.5)
- Some moderate side effects are possible
- Caution and supervision are needed for people with specific conditions, such as diabetes or muscle loss (sarcopenia)
Affordability and access
See More- Fasting and calorie restriction have no cost
Keep reading about fasting or calorie restriction
Author
Ms. Hepp is a researcher and communicator who has been writing and editing educational content on varied health topics for more than 20 years. She serves as lead researcher and writer for CancerChoices and also served as the first program manager. Her graduate work in research and cognitive psychology, her master’s degree in instructional design, and her certificate in web design have all guided her in writing and presenting information for a wide variety of audiences and uses. Nancy’s service as faculty development coordinator in the Department of Family Medicine at Wright State University also provided experience in medical research, plus insights into medical education and medical care from the professional’s perspective.
Reviewer
Susan Yaguda, MSN, RN
Susan Yaguda, MSN, RN, has been a nurse for nearly 40 years, working in a variety of healthcare settings. She currently works in Charlotte, North Carolina, at Atrium Health’s Levine Cancer Institute as the manager for Integrative Oncology and Cancer Survivorship. She works with a multidisciplinary team to deliver holistic, evidence-based support and education for patients and care partners at any point along the trajectory of cancer care. She completed the Integrative Oncology Scholars Program through the University of Michigan in 2020, is certified as an Integrative Health Coach through Duke Integrative Medicine and has a post-graduate certificate in Nursing Education from the University of North Carolina, Charlotte. Susan also was awarded the Planetree International Scholar’s Award in 2018 and was recognized by the Daisy Foundation for Nurse Leadership in 2021. Susan has a particular interest in empowering patients and care partners with knowledge to help drive informed decision making and educating nurses on the benefits of integrative care for patients and self-care. She has presented nationally and internationally on integrative oncology and nursing education.
She and her husband, Mark, have two adult children and a very spoiled foxhound. She enjoys hiking, knitting, cooking, and pickleball.
“As a frequent consumer of Beyond Conventional Cancer Therapies, and now CancerChoices, for both professional education and patient support, it is an honor to have the opportunity to engage with the dedicated team at CancerChoices to serve those impacted by this disease.”
Last update: December 11, 2023
Last full literature review: April 2023
We are grateful for research support from Dr. Irfan Siddique and Adriana Gutierrez Galvis.
CancerChoices provides information about integrativein cancer care, a patient-centered approach combining the best of conventional care, self care, and evidence-informed complementary care in an integrated plan cancer care. We review complementaryin cancer care, complementary care involves the use of therapies intended to enhance or add to standard conventional treatments; examples include supplements, mind-body approaches such as yoga or psychosocialtherapy, and acupuncture therapies and self carelifestyle actions and behaviors that may impact cancer outcomes; examples include eating health-promoting foods, limiting alcohol, increasing physical activity, and managing stress practices to help patients and professionals explore and integrate the best combination of conventionalthe cancer care offered by conventionally trained physicians and most hospitals; examples are chemotherapy, surgery, and radiotherapy and complementary therapies and practices for each person.
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