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10-01-2019 | Diet | Case study | Article

A physician’s introduction to therapeutic fasting

Jay Shubrook, Jamie Katuna

When a patient asks about intermittent fasting, what information would be helpful for the physician?

An illustrative case study

A 38-year-old female presents to the clinic for a diabetes follow-up. She has had type 2 diabetes for 6 years and would like to lose weight, take less medication, and have better control of her diabetes. 

The patient’s diabetes medications comprise 1000 mg metformin twice daily and 5 mg glipizide twice daily. She reports that she is trying to eat well, but dines out a few times a week, particularly when she is too busy to pack a lunch for work. On a typical dine-out day the patient will have:

  • Morning meal: A latte and pastry from the coffee shop
  • Evening meal: A dish she prepares from a recipe box delivery service (eg, Fresh Choice, Hello Fresh).

She checks her glucose twice per day and does not drop low. Morning readings are between 150 mg/dL and 190 mg/dL (8.3–10.6 mmol/L), evening readings are between 180 mg/dL and 210 mg/dL (10.0–11.7 mmol/L). In addition to her diabetes medications, the patient also takes 40 mg atorvastatin for hyperlipidemia and 20 mg lisinopril daily for hypertension. Her physical exam highlighted the following:

  • BMI: 32 kg/m2 (obese)
  • Blood pressure: 142/88 mmHg 
  • Glycated hemoglobin (HbA1c): 10.5 mmol/mol (8.2%)
  • Estimated glomerular filtration rate (eGFR): 88 mL/min per 1.73 m2

She has been reading about intermittent fasting and would like to try it, but would like the opinion of her physician before going any further. Below are some talking points that would be helpful to address with such a patient.

Is intermittent fasting safe?

Excluding specific conditions (see below), fasting is exceedingly safe. It has been practiced worldwide for millennia and is a facet of every major religious culture, including Judaism, Christianity, Islam, Hinduism, and Buddhism.

Anecdotally, concern has been expressed about the effect of entering “starvation mode,” a catabolic state in which the body ceases basal metabolic function, turning to muscle protein as a source of fuel. This, however, is physiologically inaccurate with intermittent fasts (for definition, see tables below). The opposite occurs: basal metabolic rate increases during a fasted state, and is accompanied by increases in serum levels of norepinephrine (noradrenaline) and growth hormone. This surge in sympathetic hormones might interfere with patients’ sleep, which may be advised.

Hypoglycemia is a risk for patients who take insulin or sulfonylureas. In these cases, careful review of self-monitored blood glucose before fasting is recommended. Diabetes medication may need to be reduced or withheld during fasting periods to minimize the risk of hypoglycemia.

Does intermittent fasting work?

Both intermittent fasting and time-restricted feeding (see below) have demonstrated efficacy in reducing weight, central adiposity, and HbA1c in clinical trials ranging from 8 to 52 weeks. There is also evidence that intermittent fasting and time-restricted feeding improve insulin sensitivity as well as biomarkers relating to cardiovascular disease such as blood pressure and resting heart rate.

A therapeutic fasting regimen in patients with type 2 diabetes is also a known strategy for de-prescription and inducing disease remission.

Ongoing research is exploring the relationship between periods of fasting and improved immune function, increased longevity, treatment for chronic neurological disorders, enhanced cognitive performance, and more.

How does intermittent fasting work?

We have evidence that molecular and biochemical networks are affected by fasting. These findings are beyond the scope of this synopsis.

Type 2 diabetes is a bodily state of hyperinsulinemia. The key to fasting is reducing insulin: the primary anabolic (and anti-catabolic), obesogenic, and glycogenic hormone. Frequent meals – especially those of a “typical” North American diet – maintain high levels of insulin, rendering the body perpetually anabolic and dependent on glucose for energy. Periods of fasting “flip the metabolic switch,” allowing the body to mobilize stored fats to use for energy via fatty acid and ketone oxidization. This counter-regulatory surge is driven by glucagon, glucocorticoid, and catecholamine hormones.

Continuous caloric restriction – that is, traditional dietary advice – fails to enable the release of counter-regulatory hormones. This results in a failure to mobilize energy from fat. Energy restriction without access to adipose storage results in the classic dieter’s scenario: hungry, fatigued, and irritable. The cyclical nature of feasting and fasting allows the body to oscillate between glucose and fatty acids for energy, avoiding the unfavorable effects of a continuous reduced-calorie approach. It is conceivable that evolutionary processes adapted the plasticity of the human body for these cycles of feasting and fasting.

Indications and contraindications of intermittent fasting

As mentioned, fasting is safe. It can be used to improve symptoms related to disease or as primary prevention. Many people use intermittent fasting to optimize performance (and/or aesthetics), as fatty acid and ketone oxidation allow for retention of lean body mass while reducing adiposity.

Those who should not fast include:

  • pregnant or breast-feeding women children
  • people with a history of eating disorders or body dysmorphic disorder 
  • patients with cachexia, advanced liver or kidney insufficiency, hyperthyroidism, or advanced cerebrovascular insufficiency or dementia (Source)

People with chronic disease should seek medical advice before starting a fasting program, and precautions should be taken in patients who are fasting while on hypoglycemic medications.

What’s the best way to start intermittent fasting?

In short, there isn’t one. However, an advantage of fasting is its flexibility: it can be used to complement any diet and can be adjusted to fit an individual’s lifestyle. Popular methods include the following:

Type of fast

Fast regime

12-hour fast

Eat only within a 12-hour window daily

16-hour fast

Eat only within an 8-hour window daily

24-hour fast

Eat one meal per day

5:2 fast

Eat normal meals 5 days of the week, reduce caloric intake to <500 on two days

Alternate day fast

Eat normal meals every other day

Longer fasts

These vary; one option is to fast for 5 consecutive days every 3 months

Finding a method that works for your patient is critical for adherence. Experiencing hunger is common and benign, especially at the beginning, but fatigue, dizziness, and other systemic symptoms are more worrisome and warrant breaking of a fast. 

How long should patients commit to intermittent fasting?

The length of a fasting regimen will depend on multiple factors including patient and physician goals (eg, weight loss, de-prescription, cardiovascular health, performance, general wellness).

A distinct advantage to fasting that can promote long-term adherence is its adaptability. Fasting can accompany any diet and fits into a variety of schedules. It is unique from other diets in its low-maintenance, no-cost character: specific foods are not required to prepare, meal replacements do not mandate purchase, and no additional time is required to adhere to a fasting regimen.

While religious practices have incorporated fasting for millennia, clinical trials on fasting protocols range from 8 to 52 weeks. Case series report de-prescription from insulin within weeks to months, and show fasting regimens to be well-tolerated over several months.


With few exceptions, structured fasting is a safe and effective dietary strategy for patients with type 2 diabetes. Type 2 diabetes is characterized by hyperinsulinemia. Fasting results in a reduction of endogenous insulin, an anabolic hormone that prevents lipolysis. Reduction of insulin enables a “metabolic switch” for the body to undergo lipolysis for energy production, physiology not created by standard dietary advice of continuous caloric restriction. Fasting protocols have been shown to reduce HbA1c, central adiposity, and weight, and to improve insulin sensitivity and cardiovascular disease risk factors.

Back to the case

This patient has an interest in time-restricted eating. Her motivations at this point are simple and straightforward: weight loss, improved glycemic control, and fewer medications. She appears to have good renal function (eGFR = 84 mL/min per 1.73 m2). Based on her current schedule, she will benefit from excluding her high-calorie, high-carbohydrate morning meal.

First, thank the patient for researching ways to be in control of her health. A 16-hour daily fast would work with her current schedule: she could fast during her non-nutritive eating period, eliminating the morning latte and pastry. She is taking glipizide, a sulfonylurea, so careful consideration to avoid hypoglycemia should be taken. At a minimum, she should reduce the dose to once daily in the morning. Over time, there is a possibility that she could discontinue taking glipizide altogether. Metformin can be continued, but dosage timing could change to be taken with food if she experiences gastrointestinal upset when taken on an empty stomach. We would recommend encouraging the patient to return with any questions or concerns.

About the authors

Jay Shubrook

Jay H Shubrook, DO, is a Board-Certified Family Physician and Diabetologist. He is a Professor in the Primary Care Department at the Touro University California College of Osteopathic Medicine. Disclosures 

Full biography

Jamie Katuna

Jamie Katuna is a DO/MPH candidate in her third year at Touro University California College of Osteopathic Medicine. Disclosures 

Read her full bio

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