Fasting:
the Science, the Myths, and What Actually Works

Fasting has become a buzzword in health and wellness, with many swearing by time-restricted feeding (also called intermittent fasting) for weight loss, better health, and even longer lives. But is it all it’s cracked up to be? Let's break it down.

Fasting and lifespan: what does the science say?

The idea that fasting extends lifespan began with a 1982 study in rats, which showed an 80% increase in lifespan with alternate-day feeding. (read study) However, later studies on rats, mice, and monkeys produced mixed results. For example, monkeys on a high-sugar diet lived longer when fasting, while monkeys eating healthy whole foods showed little to no lifespan increase with fasting. (read studies here and here)

In humans, there hasn't been a good study on the impact of caloric restriction or fasting on lifespan to date.

This suggests that if you’re eating a diet full of starch and sugar, cutting back on how much you eat and how often can make a big difference. But if you’re already eating healthy, fasting doesn’t seem to do much—and you might not even need it.

While we lack clear evidence that fasting extends human lifespan, exercise consistently shows stronger, proven benefits—adding up to 7–10 years to life. (read study)

The benefits of fasting

Even if lifespan extension isn’t guaranteed, fasting has clear health benefits. A big review study from 2019 reviewed the impact of fasting (read study). Here are some of the benefits:

• Lower insulin levels: Insulin increases in the body when we give our body too much glucose to eat (starches and sugars). For example, in the graph below, eating 20 cookies will increase glucose more than eating 1 cookie; more glucose leads to more insulin being released. When we reduce the amount of glucose we consume, insulin goes down. Fasting can help reverse insulin resistance and conditions like Type 2 diabetes.

• Metabolic flexibility: Spending more time fasting allows your body to switch from relying only on glucose for fuel, to also relying on our fat reserves, by burning fat for energy. This is super healthy, and has been shown to reduce cognitive decline. (read study).
• Autophagy: Fasting increases a cellular cleanup process, which recycles old or damaged cells. "Auto" means "self", and "phagy" means "eating", so literally self eating. This is excellent because a lot of problems start from old damaged cells - for instance cancer. We have cancer cells within us all the time, and our body is constantly working on cleaning them up. The more it can do this and eat the damaged cells, the less likely they will develop into cancer. A great 2023 study explains this well: "It is now widely accepted that autophagy suppresses tumour initiation”. (read studies here and here)
  Autophagy also happens in the brain, and has been shown to improve working memory and prevent cognitive decline. (read studies here and here)

The drawbacks of fasting

Fasting isn’t a magic bullet, and there are some concerns to keep in mind:

• Muscle loss: Without enough protein, fasting can lead to muscle breakdown. Your muscles are essential for staying strong, healthy, and even living longer. Research shows that people with low muscle mass have a 40–50% higher risk of early death. (read study) A study found that those on an alternate-day fasting diet lost more lean muscle than those who simply ate fewer calories daily. (read study)
• Stress on the body: Excessive fasting can stress the body, particularly for females, affecting the reproductive system and overall hormonal balance. (read study)
• Breaking your fast matters: Eating high-sugar or processed foods after fasting can lead to a bigger glucose spike, undoing many benefits.

Tips if you fast

Fasting can be powerful, especially for improving insulin resistance and metabolic flexibility. However, it’s not a one-size-fits-all solution. Whether or not you fast, what you eat matters more than when you eat. Focus on improving your overall diet, and if you choose to fast, get the most out of it by focusing on these tips:

• Avoid snacking: Even outside of fasting, reducing snacks lowers insulin levels and improves metabolic health.
• Prioritize protein: Aim for 1 gram of protein per pound of body weight to maintain muscle mass while fasting.
• Start with savoury: When breaking your fast, begin with a savoury, protein-rich meal to reduce your glucose spikes.
• Exercise regularly: Exercise increases autophagy and improves longevity—even more than fasting alone.
• Focus on diet quality: A high-protein glucose-steady diet offers many of the same benefits as fasting, without the stress on the body.

The scientific studies mentioned in this episode

Bredesen D E, "Reversal of cognitive decline: a novel therapeutic program," Aging (Albany NY) 6, no. 9 (2014): 707-717, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4221920/

Colman R J et al., “Caloric restriction delays disease onset and mortality in rhesus monkeys.” Science (New York, N.Y.) 325, no. 5937 (2009): 201-4. https://pubmed.ncbi.nlm.nih.gov/19590001/

Cook F et al., “Compliance of participants undergoing a '5-2' intermittent fasting diet and impact on body weight.” Clinical nutrition ESPEN 52 (2022): 257-261, https://pubmed.ncbi.nlm.nih.gov/36513463/

Debnath J et al., “Autophagy and autophagy-related pathways in cancer.” Nature reviews. Molecular cell biology 24, no. 8 (2023): 560-575. https://pubmed.ncbi.nlm.nih.gov/36864290/

Goodrick C L et al., “Effects of intermittent feeding upon growth and life span in rats.” Gerontology 28, no. 4 (1982): 233-41. https://pubmed.ncbi.nlm.nih.gov/7117847/

Kapogiannis D et al., “Brain responses to intermittent fasting and the healthy living diet in older adults.” Cell metabolism 36, no. 8 (2024): 1668-1678.e5. https://pubmed.ncbi.nlm.nih.gov/38901423/

Kim J et al., “Early Time-Restricted Eating Reduces Weight and Improves Glycemic Response in Young Adults: A Pre-Post Single-Arm Intervention Study.” Obesity facts 16, no. 1 (2023): 69-81, https://pubmed.ncbi.nlm.nih.gov/36318892/ 

Leclerc E et al., “The effect of caloric restriction on working memory in healthy non-obese adults.” CNS spectrums 25, no. 1 (2020): 2-8. https://pubmed.ncbi.nlm.nih.gov/30968820/

Liao C-Y et al., “Genetic variation in the murine lifespan response to dietary restriction: from life extension to life shortening.”Aging cell 9, no. 1 (2010): 92-5. https://pubmed.ncbi.nlm.nih.gov/19878144/

Mattison J A et al., “Impact of caloric restriction on health and survival in rhesus monkeys from the NIA study.” Nature 489, no. 7415 (2012): 318-21. https://pubmed.ncbi.nlm.nih.gov/22932268/

Mattson M P et al., “Impact of intermittent fasting on health and disease processes.” Ageing research reviews 39 (2017): 46-58. https://pubmed.ncbi.nlm.nih.gov/27810402/

Patikorn C et al., “Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials.” JAMA network open 4, no. 12 (2021): e2139558. https://pubmed.ncbi.nlm.nih.gov/34919135/

Ran L et al., “Associations of Muscle Mass and Strength with All-Cause Mortality among US Older Adults.” Medicine and science in sports and exercise 50, no. 3 (2018): 458-467. https://pubmed.ncbi.nlm.nih.gov/28991040/

Reimers C D et al., “Does physical activity increase life expectancy? A review of the literature.” Journal of aging research 2012 (2012): 243958. https://pubmed.ncbi.nlm.nih.gov/22811911/

Solianik R et al., “Two-day fasting evokes stress, but does not affect mood, brain activity, cognitive, psychomotor, and motor performance in overweight women.” Behavioural brain research 338 (2018): 166-172. https://pubmed.ncbi.nlm.nih.gov/29097329/ 

Swindell W R. “Dietary restriction in rats and mice: a meta-analysis and review of the evidence for genotype-dependent effects on lifespan.” Ageing research reviews 11, no. 2 (2012): 254-70. https://pubmed.ncbi.nlm.nih.gov/22210149/

Templeman I et al., “A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults.” Science translational medicine 13, no. 598 (2021): eabd8034. https://pubmed.ncbi.nlm.nih.gov/34135111/ 

Vernieri C et al., “Cyclic fasting-mimicking diet in cancer treatment: Preclinical and clinical evidence.” Cell metabolism 36, no. 8 (2024): 1644-1667. https://pubmed.ncbi.nlm.nih.gov/39059383/

Zhao L et al., “Time-restricted eating alters the 24-hour profile of adipose tissue transcriptome in men with obesity.” Obesity (Silver Spring, Md.) 31, Suppl 1 (2023): 63-74, https://pubmed.ncbi.nlm.nih.gov/35912794/