Fasting for Health

By Dónal P. O’Mathúna, PhD

Fasting has long been associated with various religious traditions. With recent interest in spirituality and medicine, and the importance of diet being stressed for good health, the role of fasting in promoting health has been receiving attention. While designing controlled studies of any nutritional therapy in disease raises challenges, fasting has been examined in a small number of scientific studies. Although fasting technically means the withholding of all solid food, studies vary in their duration and in what people may consume while fasting. Some fasts require that people consume only water, while others allow limited calorie consumption in the form of juices, usually fresh, natural juices.


Fasting has been given some credence in health promotion within the naturopathic tradition. Fasting has there retained a role in nutritional therapy to reverse the impact of what is viewed as unhealthy eating patterns. The high-fat, high-sodium diets of developed countries are believed to leave people’s palates intolerant to the low-fat, low-sodium foods found in most plant-rich diets.1 This is believed to underlie the difficulty many people have in sustaining healthier eating patterns. Fasting is then viewed as a form of sensory deprivation, which resensitizes taste nerves, making healthy plant foods more palatable.1

However, a completely different proposal recently has been offered for the health effects of fasting. Traditional advice from grandma to "feed a cold" may reflect folk knowledge that immunity and diet are connected.2 While chronic malnutrition generally impairs the immune system, observations during the 1970s in African refugee camps noted that the incidence of certain infectious diseases increased dramatically when starving people were refed.3 Fasting has been shown to have a complicated impact on various components of the immune system.4 This connection has regularly been noted in the context of autoimmune diseases, such as rheumatoid arthritis and multiple sclerosis (MS). However, claims that fasting can ease the symptoms of autoimmune diseases generally have been greeted with skepticism—until recent research on leptin.5

Mechanism of Action

Leptin is a hormone produced in human fat cells (and other tissues). It has come to public prominence because of its role in obesity and regulating food intake. Although early research focused on leptin’s role in satiety, more recent studies have demonstrated a broader impact on insulin metabolism, reproductive function, response to stress, and the immune system.6 Leptin has been shown to be a strong regulator of T cells in the immune system.2 Leptin enhances the activity of T cells that produce cytokines of the type that promote inflammatory reactions.5 These same T cells are involved in many autoimmune diseases.

In early 2003, research with mice confirmed the proposed role of leptin in autoimmune disease, and gave support to the idea that fasting could ameliorate symptoms of some of these diseases. Mice were given an experimental condition recognized as a model for human MS (abbreviated as EAE).7 Immediately before the mice developed EAE symptoms, their leptin levels increased. These mice also showed increased levels of pro-inflammatory T cells and cytokines. When similar mice fasted for 48 hours (something known to reduce leptin levels), EAE symptoms did not develop. Also, in mice that genetically can’t produce leptin, EAE did not develop. This elegant set of experiments supported the proposed mechanism of action that fasting reduces leptin production (see Figure), which promotes the secretion of anti-inflammatory cytokines and at the same time lowers the production of pro-inflammatory cytokines.5

Figure: Fasting and leptin production

Clinical Studies

Controlled clinical trials directly examining the role of leptin and fasting in autoimmune diseases have not yet been conducted. The incidence of MS is higher in populations with a higher intake of saturated fat, but the role of nutrition in causing or influencing the disease is unclear.8 The role of leptin in MS is supported by autopsy findings that genes for leptin and pro-inflammatory cytokines are overly expressed in the brains of MS patients.9

Three observational studies in the 1970s and 1980s found that fasting brought relief to rheumatoid arthritis patients.10 However, as soon as subjects started eating again, symptoms returned to their baseline levels. A subsequent controlled clinical trial was then conducted in which fasting was followed by gradual, controlled reintroduction of various foods. Fifty-three subjects were randomly allocated to either the intervention or control group. The control group stayed at a convalescent home for four weeks, ate normally, and recorded their food intake. The intervention group stayed at a health farm for four weeks, fasting for the first 7-10 days. Natural juices were allowed with subjects ingesting 800-1,260 kJ/d. After fasting, subjects introduced a new food every second day and carefully monitored their arthritis symptoms. If symptoms were exacerbated, the item was excluded from their diet. For 3.5 months, food items were consistent with a gluten-free vegan diet. From 3.5 to 12 months, a lactovegetarian diet was followed.

After four weeks, patients in the intervention group had statistically significant improvements in nine arthritis symptom measurements, grip strength, and a subjective health assessment questionnaire (P < 0.05). Patients in the control group had significant improvement in pain scores only. At the end of the study, the intervention group scored significantly better than the control group in all measurements except platelet count and hemoglobin. The researchers originally attributed these findings to food allergies or intolerance, but later proposed that the dietary changes might have caused changes in fecal flora leading to the absorption of different bacterial substances from the intestine.11 More recent studies have suggested that these clinical results might be related to leptin levels.5

A somewhat different area of research has examined the impact of fasting on hypertension. An inpatient clinic enrolled 174 self-referred adults with blood pressure (BP) of at least 140/90 mm Hg (average, 159/89 mm Hg).12 The treatment protocol involved prefasting (average of 2-3 days consuming only fresh fruit and vegetables), fasting (average of 10-11 days consuming only distilled water), and refeeding (average of 6-7 days introducing a low-fat, low-sodium, vegan diet). During the study, patients also agreed not to smoke or consume alcohol, and to restrict physical activities to avoid side effects. Upon completing the study (ranging from four to 28 days), 89% of the subjects had BP lower than 140/90 mm Hg, averaging 37/13 mm Hg lower than baseline (P < 0.05). Follow-up data were collected six months later on 42 of the original 174 subjects. Their average BP was 123/77 mm Hg, although these data are preliminary and the sample was not randomly selected.

The same investigators enrolled another 68 self-referred adults with baseline systolic BP of 120-140 mm Hg and diastolic BP less than 91 mm Hg (average BP, 129/78 mm Hg).1 The same fasting protocol was used as before, except the group averaged two days of prefasting, 13.6 days water-only fasting, and six days refeeding. At the end of the study, 82% of the patients had BP at or below 120/80 mm Hg (average BP, 109/71 mm Hg), which was statistically significant (P < 0.05).

Adverse Effects

Mild nausea and orthostatic hypotension were reported occasionally in the inpatient studies of hypertension.1,12 These researchers cautioned that their prolonged water-only fasting regimen should not be undertaken without medical supervision. Physical activities were restricted to minimize the occurrence of orthostatic hypotension, arrhythmia, dehydration, and electrolyte disturbances, which have all been reported during fasting accompanied by unrestricted activity.12 Patients on the strict gluten-free vegan diet lost upper arm muscle area, a situation not reversed while on the diet, even with consultations from dietitians.10 However, when these subjects moved to the lactovegetarian phase of the study, this problem was eliminated.


While fasting is an ancient practice, its health impact has not been subjected to substantial scientific study. Studies examining the role of fasting in treating hypertension used multiple interventions and were not controlled. However, improvements were noted in a substantial majority of the patients. Where fasting may prove to have a more substantial impact involves its interaction with leptin and the immune system. Although this is a relatively new area of research, it is rapidly expanding. Laboratory and animal studies offer a mechanism of action that suggests how fasting may provide relief for those with a variety of autoimmune diseases. Given that many of these diseases can only be ameliorated with powerful pharmaceuticals, any effective complementary strategies would be welcome.


For those with hypertension, fasting for a few days may be useful to help kick-start a more healthy diet. However, this should not be done without adequate medical supervision so that the patient’s overall health is closely monitored. Those with autoimmune diseases could explore fasting while monitoring the symptoms of their disease. Again, medical supervision would be very useful, both for objective measurement of symptoms and in checking for adverse effects. Although the research evidence is in its very early days, the relative safety of fasting makes it something worth suggesting to select patients.

Dr. O’Mathúna teaches courses in bioethics and alternative medicine in Dublin, Ireland, and is Visiting Professor of Bioethics at the University of Ulster, Coleraine, Northern Ireland.


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