I first found out about fasting in a desperate attempt to put an autoimmune condition into remission. I told my physician husband, “Find something! I don’t care what it is, I am so sick of being sick.”
What he found surprised me. It was an ancient practice, a treatment with only short-term and non-dangerous side effects, and it didn’t require a prescription. The demonstrated effectiveness of fasting on everything from hypertension, to diabetes, to neurodegenerative protection, to lessening side effects from chemotherapy, to even reducing symptoms of autoimmune conditions is astounding when you begin digging.
Dr. Mattson of John’s Hopkins School of Medicine says, “Fasting is a challenge to your brain, and we think that your brain reacts by activating adaptive stress responses that help it cope with disease.” The theory is that fasting throws cells into a protective or regenerative state that most of us on a Western diet never achieve.
According to my husband, “Three square meals a day isn’t natural. On a first world diet our cells never get a rest from breaking down sugar.” Based on his research he argued that periodic or intermittent fasting is a natural, even healthier state for humans and animals. Most cultures and religions in the world practiced fasting, and throughout most of history, people could afford or find food 24/7. He says, “Our bodies are amazingly designed to cope with limited food. What they are not designed to cope with is unlimited sugar, which is why we are seeing the heart and diabetes epidemics we see today.”
Who Does Fasting Help
In the last century, research in animals and humans has uncovered several potentially health-promoting physiologic responses to fasting including ketogenesis, hormone modulation, reduced oxidative stress and inflammation, and increased stress resistance, lipolysis, and autophagy [1–4]. Clinical research in humans also indicates that fasting improves hypertension, [5, 6] rheumatoid arthritis,  cardiovascular disease, [8, 9] metabolic syndrome, [10, 11] osteoarthritis,  fibromyalgia,  chronic pain,  and quality of life . (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819235/)
Is it safe?
A comprehensive study on Adverse Effects of Extended Water Fasting is linked here. Below is a summary.
AEs (Adverse Experiences) that were commonly experienced during visits, including nausea, headache, insomnia, back pain, dyspepsia, and fatigue, were predominately mild, grade 1 events and are reactions that are known to occur during fasting [1, 2]. The exceptions were presyncope and hypertension. Presyncope is also known to occur in response to fasting,  and is always defined as a moderate, grade 2 event . Hypertension was the largest category of grade 3 events – both for visits in which the highest event was grade 3 and for the total number of individual grade 3 events – and is not reported to occur in response to fasting. Conversely, water-only fasting has been shown to reduce blood pressure in hypertensive patients [5, 6]. Indeed, we found that in 97% of visits with a grade 3 hypertension AE, the patient had hypertension as a chief complaint. This suggests that the grade 3 hypertensive events occurred in patients being treated for hypertension and that water-only fasting is unlikely a causal factor in the participant’s high blood pressure. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819235/)