360° smarthealth – Scott Levine MD

Fasting mimicking vs. water fasting: a doctor's view on doing it safely

By Scott Levine MD. Last reviewed by Scott Levine MD, November 2026.

Patients ask me about water fasting all the time. They have read that going without food can help, and they want to know if they should do it. I answer the way I would sitting across from a patient. Fasting can be beneficial to your health. If it is not done properly, it can be detrimental.

A water fast means you drink water and you do not eat. In my clinical experience, a fast that is not done carefully can release a large amount of stored toxins into the bloodstream. It can also cause a significant loss of muscle. That muscle is very hard to replace, and building muscle is critical as you age.

In 2015, Brandhorst and colleagues described a periodic diet that mimics fasting. They developed it to minimize the burden of prolonged fasting. In mice, cycles of that diet were followed by regeneration, a longer lifespan, lower visceral fat, and, in old mice, better cognitive performance. A pilot clinical trial in people found that three cycles decreased risk factors and biomarkers for aging, diabetes, cardiovascular disease, and cancer without major adverse effects.1

A 2024 paper describes periodic water-only fasting as several days without food, with water still allowed. Preclinical studies have tied that kind of fast to autophagy, and also to weight loss, improved insulin sensitivity, cell renewal, and anti-cancer effects. The same paper says water-only fasting carries safety and compliance concerns, which may explain its limited place in ordinary medical practice. When I talk with patients about autophagy, I describe it as the body's cleanup. It clears old, damaged, senescent cells that can reduce metabolic efficiency. That description is my clinical view of the process.3

Prolon is the fasting plan I recommend when someone wants a fast and still needs to eat. It is a 5-day, plant-based meal program from L-Nutra, developed from research led by Dr. Valter Longo at the University of Southern California. You still eat. The meals are low in calories and protein: vegetable-based soups, energy bars, drinks, snacks, tea, and a micronutrient supplement, for five days in a row rather than water only. The 2017 trial described the fasting-mimicking diet as low in calories, sugars, and protein and high in unsaturated fats. The amount of food is small enough to keep your body thinking it is fasting, and it is still food. Prolon is designed to make your body think it is fasting without actually fasting.1,2

The same 2017 study randomized 100 generally healthy adults in the United States. One group kept their usual diet. The other ate the fasting-mimicking diet for five consecutive days each month for three months. Three cycles reduced body weight, trunk fat, and total body fat, lowered blood pressure, and decreased IGF-1. No serious adverse effects were reported. With the crossover included, 71 people completed three cycles. The authors called the cycles safe, feasible, and effective for markers and risk factors of aging and age-related disease in that generally healthy group, and they said larger studies are still needed in people with a diagnosed disease.2

In 2024, Brandhorst and colleagues analyzed fasting-mimicking diet trial samples from the L-Nutra program boxed as Prolon. Three cycles were associated with a 2.5-year decrease in median biological age, independent of weight loss. They also reported lower insulin resistance and other pre-diabetes markers, and lower liver fat on MRI in the subset who were scanned. A second study found nearly the same biological-age result. The authors call this initial support, which is how I describe it to patients.3

In my view, mitochondrial health is the key to health and longevity, and this diet helps create mitochondrial health. That is my clinical view, not a measurement reported in those papers. The papers speak to weight, fat, blood pressure, IGF-1, risk markers, liver fat, and a biological-age estimate.

Prolon's own pages are clear about who should not do this. Do not use the 5-day plan if you are pregnant or breastfeeding, under 18 or over 70, have a BMI under 18.5, have an eating disorder, have an active infection or a risk of repeated infection, are malnourished or protein deficient, or have specific dietary restrictions. People with diabetes, especially anyone using insulin or another glucose-lowering medicine, are at higher risk of low blood sugar and should not fast on their own. Prolon says medical oversight is critical for them. If you take medication or you have a medical condition, consult your physician first. I am double boarded in Internal Medicine and in anti-aging and regenerative medicine, and I tell my patients the same thing.4,5

On timing, follow Prolon rather than a schedule of my own. The published guidance is three cycles over three consecutive months to start, then three to four cycles a year. Prolon also says the plan is only intended once a month. More frequent use has not been tested for safety or efficacy.6,7,8

The green button below is my provider link. Order through it and the 5-20% discount is applied. For the study list and the longer note on this plan, read my Prolon Longevity page, and decide with your physician.

Claim your 5-20% discount on ALL Prolon products

Read the Prolon Longevity page

References

  1. Brandhorst S, Choi IY, Wei M, Cheng CW, Sedrakyan S, Navarrete G, Dubeau L, Yap LP, Park R, Vinciguerra M, Di Biase S, Mirzaei H, Mirisola MG, Childress P, Ji L, Groshen S, Penna F, Odetti P, Perin L, Conti PS, Ikeno Y, Kennedy BK, Cohen P, Morgan TE, Dorff TB, Longo VD. A periodic diet that mimics fasting promotes multi-system regeneration, enhanced cognitive performance, and healthspan. Cell Metabolism. 2015;22(1):86-99. doi:10.1016/j.cmet.2015.05.012. PMID: 26094889. https://pmc.ncbi.nlm.nih.gov/articles/PMC4509734/
  2. Wei M, Brandhorst S, Shelehchi M, Mirzaei H, Cheng CW, Budniak J, Groshen S, Mack WJ, Guen E, Di Biase S, Cohen P, Morgan TE, Dorff T, Hong K, Michalsen A, Laviano A, Longo VD. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Science Translational Medicine. 2017;9(377):eaai8700. doi:10.1126/scitranslmed.aai8700. PMID: 28202779. https://pmc.ncbi.nlm.nih.gov/articles/PMC6816332/
  3. Brandhorst S, Levine ME, Wei M, Shelehchi M, Morgan TE, Nayak KS, Dorff T, Hong K, Crimmins EM, Cohen P, Longo VD. Fasting-mimicking diet causes hepatic and blood markers changes indicating reduced biological age and disease risk. Nature Communications. 2024;15(1):1309. doi:10.1038/s41467-024-45260-9. PMID: 38378685. https://pmc.ncbi.nlm.nih.gov/articles/PMC10879164/ https://www.nature.com/articles/s41467-024-45260-9
  4. Prolon. Is the FMD for me? Prolon 5-day product page. https://prolonlife.com/products/prolon-offer
  5. Prolon. The Ultimate Guide to Fasting. https://prolonlife.com/blogs/stories/everything-you-ve-always-wanted-to-know-about-fasting-but-didn-t-know-who-to-ask
  6. Prolon. 5-Day Fasting Program, “How often should I do the program?” https://prolonlife.com/pages/five-day
  7. Prolon. How many cycles of Prolon can be recommended to an individual in a one-year period? https://prolonlife.com/blogs/faqs/how-many-cycles-of-prolon-can-be-recommended-to-an-individual-in-a-one-year-period
  8. Prolon. Can I do Prolon more often than once per month? https://prolonlife.com/blogs/faqs/can-i-do-prolon%C2%AE-more-often-than-once-per-month