Quick answer: Intermittent fasting can create an energy deficit, where you burn more than you eat, which leads to weight loss. During a fast, the body first uses stored glucose (glycogen) and then shifts toward burning fat. Some muscle can be lost with any weight loss, so adequate protein and strength training are important to keep the weight you lose mostly fat.
Key takeaways
Weight loss depends on energy balance, and fasting is one way some people naturally eat less.
During a fast the body uses stored glycogen first and then relies more on stored fat, which the liver turns into ketones.
Because weight loss often includes some muscle, enough protein and strength training two to three times a week help protect it.
Energy balance
Reducing energy intake creates a negative energy balance, which leads to weight loss. Background: what is intermittent fasting.
What fuels you during a fast
First, your body uses stored glucose (glycogen) in the liver and muscles. Then, increasingly, stored fat, which the liver turns into ketones. The body tries to spare protein, but some muscle breakdown can occur.
Body composition
Body composition is the balance of fat, muscle, bone and water. During weight loss, it’s common to lose both fat and muscle. Preserving muscle matters for metabolism and overall health.
How to protect muscle
- Eat enough protein in your eating window (see nutritional considerations)
- Strength train two to three times a week (see strength training)
- Avoid very long or very frequent fasts without guidance
Healthy aging lens
Muscle naturally declines with age. Protecting it while losing fat supports strength, blood sugar control and independence later in life.
When to see a naturopathic doctor
It may be time to see a naturopathic doctor if you have diabetes, take blood sugar or blood pressure medication, or have a chronic condition, or if you are pregnant, breastfeeding, underweight, or have a history of disordered eating. The same is true if fasting leaves you dizzy, shaky or exhausted, or if you want a fasting approach tailored to your health goals.
See a medical provider promptly for fainting, confusion, chest pain, a racing heartbeat, or very low blood sugar symptoms such as shaking, sweating and confusion.
What the research shows
Time-restricted eating may not protect muscle on its own. In the TREAT trial by Lowe and colleagues in 2020, 116 adults with overweight or obesity ate only between noon and 8 p.m. or ate three structured meals a day for 12 weeks. Weight loss did not differ between groups, and in the 50 people measured in person, the time-restricted group lost more lean mass from the arms and legs. There was no specific protein or exercise plan, which may matter.
With strength training, results look different. In a 2016 study by Moro and colleagues, 34 resistance-trained men ate the same calories within 8 hours or spread over the day for 8 weeks while following a standard lifting program. The 8-hour group lost more fat while keeping lean mass and strength, although testosterone and IGF-1 fell. This was a small study of trained young men, so results may differ for other people.
Want a fasting plan that fits your health?
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Questions to bring to your clinician
These questions can help you get the most out of your next visit.
How can I protect my muscle mass if I try fasting to lose weight?
How could fasting fit with my goals for body composition?
Should I change how much protein I eat when I fast?
How could we track my progress beyond the scale?
Frequently asked questions
Does intermittent fasting burn fat?
It can, by creating an energy deficit and shifting fuel use toward fat.
Will I lose muscle with intermittent fasting?
Some muscle loss can occur with any weight loss; protein and strength training help.
When does the body start burning fat during a fast?
Gradually, as glycogen stores fall, often after many hours.
Is the scale the best measure?
Waist measurements and strength are useful too.
Sources
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med 2020. PMID: 32986097. https://pubmed.ncbi.nlm.nih.gov/32986097/
- Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med 2016. PMID: 27737674. https://pubmed.ncbi.nlm.nih.gov/27737674/
- Hall KD, et al. Energy balance and its components: implications for body weight regulation. Am J Clin Nutr 2012. PMID: 22434603. https://pmc.ncbi.nlm.nih.gov/articles/PMC3302369/
- Anton SD, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring) 2018. PMID: 29086496. https://pmc.ncbi.nlm.nih.gov/articles/PMC5783752/
- Cava E, Yeat NC, Mittendorfer B. Preserving Healthy Muscle during Weight Loss. Adv Nutr 2017. PMID: 28507015. https://pmc.ncbi.nlm.nih.gov/articles/PMC5421125/
- Leidy HJ, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr 2015. PMID: 25926512. https://pubmed.ncbi.nlm.nih.gov/25926512/
- Cruz-Jentoft AJ, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing 2019. PMID: 31081853. https://pmc.ncbi.nlm.nih.gov/articles/PMC6593317/
- MedlinePlus: Weight Control
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before starting supplements or changing medications, especially if you are pregnant, nursing, or managing a health condition.
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