Intermittent fasting is a dietary approach that focuses on when to eat as much as what to eat. It’s considered by some to be an effective method for supporting health.
A few weeks of intermittent fasting (IF) has been associated with changes in blood pressure, cholesterol, inflammation, body weight, insulin levels, and more.
But how do you do it? And what is the evidence basis for intermittent fasting? This article will review the different types of intermittent fasting, how to do them, lab tests to consider before and after a fasting period, and the potential benefits and considerations of intermittent fasting.
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Different Types of Intermittent Fasting
Four different types of intermittent fasting have been studied as health interventions. These include
- Time Restricted Feeding
- Complete Alternate Day Fasting
- Modified Fasting Regimens
- Religious Fasting
Time Restricted Feeding (TRF)
Time-restricted feeding is a practice that limits the number of hours in the day that someone can eat. The most common forms of TRF allow for an 8 to 12-hour eating window, generally from mid-morning until early evening.
Complete Alternate Day Fasting
Complete alternate day fasting is also called the “Every Other Day Diet.” People who follow this plan can eat freely one day as long as they abstain entirely from food the next day. Then, the cycle repeats.
Modified Fasting Regimens
Modified fasting regimens are similar to complete alternate-day fasting but less extreme. They allow for consuming some calories on alternate days rather than none. The most famous form of this type of intermittent fasting is called 5:2 and allows for the consumption of 20-25% of your specific energy needs during two non-consecutive days of the week. This type of fasting is also sometimes called alternate day calorie restriction and alternate day modified fasting.
Religious Fasting
Many religious practices include periods of fasting that can vary from several hours to several days. Ramadan, for example, is a month of fasting from sunrise to sunset observed by over 1 billion Muslims worldwide each year. In general, fasts last about 12 hours but can be longer, up to 18 hours, in places in the world where the sun shines for most of the day. Researchers have gathered a great deal of information about intermittent fasting by studying the large number of people participating in this type of IF each year.
Another fasting practice in accordance with Sunnah Muslim teaching includes a twice-weekly fast on Mondays and Thursdays throughout the year. Fasting days allow a small breakfast before dawn and a larger meal after sundown. Some studies suggest that this fasting practice may help manage blood pressure and cholesterol.
Non-Religious Types of Intermittent Fasting
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7 Health Areas Where Intermittent Fasting is Commonly Explored
Insulin resistance
Insulin resistance is thought to be one of the root causes of conditions like metabolic syndrome, PCOS, and type 2 diabetes. Insulin resistance is the metabolic state in which insulin receptors in cells can no longer absorb sugar normally, so the sugar stays high in the bloodstream (hyperglycemia) and low in the cells where it is needed. Effectively, this leaves your cells starving for food while your body stores fat.
Intermittent fasting has been associated with improved insulin sensitivity and may help manage insulin resistance, even without weight loss.
Type 2 Diabetes
Type 2 Diabetes (T2D) is a disorder in which cells no longer respond to insulin appropriately, and the pancreas cannot produce insulin at optimal levels. T2D is typically diagnosed by HbA1c levels higher than 6.5 combined with symptoms of blood sugar instability.
Intermittent fasting may support the management of type 2 diabetes and can be part of a Functional Medicine Approach to Type 2 Diabetes management. In one year-long study of modified alternate day fasting (a type of IF), people with type 2 diabetes experienced a reduction in their A1C (0.3 points). For someone with early or mild blood sugar instability in T2D, this could mean the difference between being considered prediabetic and diabetic.
PCOS
PCOS is a metabolic disorder that impacts fertility and hormonal health in people with ovaries. It is characterized by insulin resistance, hyperandrogenemia, and alterations in menstrual cycles. These and other clinical hallmarks of PCOS were improved in five weeks of TRF (time-restricted fasting).
Obesity
Studies have shown that IF may be an effective intervention for fat loss that is similar to calorie restriction in both long-term and short-term efficacy. Time Restricted Feeding (TRF), in particular, may help people who are obese to lose body fat without losing muscle mass.
Cardiovascular Health
Intermittent fasting has been associated with a decreased risk of heart disease. This could be correlated with intermittent fasting's potential to support healthy weight, cholesterol, blood pressure, and insulin sensitivity – all of which are factors in cardiovascular health.
Many studies have suggested that IF may help manage blood pressure in people with hypertension. Five weeks of time-restricted feeding has been suggested to reduce systolic blood pressure by roughly 11 points, and diastolic pressure by 10 points - a change similar to the numbers decreased by many blood pressure medications!
A recent systematic review revealed that intermittent fasting practices could lower total cholesterol and LDL cholesterol (bad cholesterol) by about 6 points over several weeks.
Gastrointestinal Health
Intermittent fasting may support gastrointestinal health by increasing short-chain fatty acid production and reducing lipopolysaccharide (LPS) production in the intestines. High levels of LPS are linked with intestinal permeability / leaky gut and endotoxemia, which is implicated in several metabolic disorders.
Ramadan fasting is associated with increases in beneficial bacteria in the research. One of the bacteria that frequently increases during fasts is Akkermansia muciniphila. This beneficial bacteria is commonly reduced in people with obesity and metabolic diseases.
Neurodegenerative Health
It's theorized that IF may support brain health due to potential effects on mitochondrial metabolism.
Mild Cognitive Decline: In one study, adults who reported regularly participating in intermittent fasting experienced less cognitive decline than their peers over three years.
Multiple sclerosis: Intermittent fasting is well tolerated in people with M.S. and may induce beneficial microbiome and immune system changes, which are currently being studied to improve M.S. outcomes.
Research Backing Intermittent Fasting: How Does It Work?
Researchers think three major mechanisms may be at work when it comes to the potential benefits of intermittent fasting in supporting health.
Intermittent Fasting and Circadian Metabolism
Time-restricted feeding and other alternate fasts take advantage of the circadian rhythm's influence on body processes by timing feeding to set hours. Our biological clocks are designed to consume and use energy during daylight hours - usually between 6 a.m. and 9 p.m. - and to repair tissues and regenerate hormones at night. These processes are regulated by several genes and hormones produced when certain parts of the eyes and brain perceive light.
Eating events have the potential to link all body system rhythms together, provided that eating is kept to daylight hours. When these clocks are disrupted, as in shift work or late-night eating patterns, insulin resistance and other hormonal disorders are usually the result. When all systems are connected, insulin sensitivity stays optimal, inflammation stays low, and hormones are produced appropriately.
Intermittent Fasting and Oxidative Stress
Oxidative stress damages cell membranes, mitochondria, DNA, and more and is thought to be one of the major underlying contributors to chronic disease and aging. Consuming food and digestion of those foods - as well as any pro-oxidant compounds they contain - contributes to oxidative stress. Fasting may promote autophagy and could help protect against free radical damage. For example, alternate day fasting for two months significantly lowered levels of inflammatory compounds like isoprostanes that are associated with oxidative stress and chronic inflammation.
Intermittent Fasting and Insulin Sensitivity
Glucose regulation is determined partly by the sensitivity of cell receptors to insulin - a hormone produced by the pancreas that allows for the influx of sugar into cells after a meal. Insulin resistance occurs when cells have been flooded with insulin in response to large, frequent eating events and chronic inflammation. Intermittent fasting may reduce the instance of large and frequent eating events. It is also associated with decreased inflammation. Combined, these two aspects of IF make it a potential tool for supporting insulin sensitivity.
Labs to Consider for Patients on Intermittent Fasting Type Diet
Fasting Glucose
Fasting glucose measures your blood sugar after an overnight fast (not eating). A fasting blood sugar level of 99 mg/dL or lower is normal, 100 to 125 mg/dL indicates you have prediabetes, and 126 mg/dL or higher indicates you have diabetes.
Insulin levels
Fasting insulin can be used along with fasting glucose to calculate a HOMA-IR calculation, which can be used to assess insulin resistance.
HbA1c
Hemoglobin A1c is a 3-month snapshot of average blood sugar based on the amount of glycosylation present on red blood cells.
Continuous Glucose Monitor
Some people use a continuous glucose monitor to keep regular track of their blood sugar levels.
Comprehensive Female Hormones Panel
Assessing reproductive hormones can give practitioners insight to what is causing hormonal imbalances so that treatment can be adjusted accordingly. Retesting periodically can guide practitioners during treatment to adjust intermittent fasting hours as well as other labs to consider during treatment.
Boston Heart Lifestyle Panel
One of the most popular Boston Heart Labs ordered at Rupa is the Lifestyle Panel which contains a basic lipid panel (LDL, HDL, triglycerides), a hs-CRP, and two unique tests called the HDL Map Test and the Cholesterol Balance test. This panel goes beyond the basics to provide multiple cardiac risk markers and a unique analysis of cholesterol synthesis and HDL particle size. The Lifestyle Panel allows practitioners to stratify their patient's cardiac risk further and provides insight into the best nutritional and medication regimen for each unique individual.
Comprehensive Stool Test
The microbiome comprises bacteria, yeast, and other digestive tract organisms that all play a role in helping metabolize food, regulate the immune system and produce vitamins. A comprehensive stool test can help individualize a nutritional and supplemental approach to the patient's exact needs during intermittent fasting, including the need for digestive enzymes, probiotics, and prebiotics. It is recommended to reorder the test after three months of intermittent fasting to adjust nutritional and supplemental needs accordingly.
Who Should Not Intermittent Fast?
It's important to note here that intermittent fasting is a dietary approach that should be overseen by a medical or nutrition provider. It is not recommended to start intermittent fasting without a conversation with your healthcare team, particularly if you fall into any of the following categories.
- People with type 1 diabetes
- People who use insulin who have not had a conversation with their doctor about how to alter insulin schedules to accommodate intermittent fasts
- People with a dangerously low body weight
- People with low blood pressure
- People with dysautonomias
- People with severe nutrient deficiencies
- People with eating disorders
- People who are pregnant, breastfeeding, or trying to conceive
- People who do not have regular or reliable access to food
- People with low appetite and low energy intake
- People with hormonal issues
Summary
Intermittent Fasting is a dietary approach that may support overall health and could be considered as part of a strategy to manage various health conditions. If you or a loved one have any of these conditions, it's worth conversing with your doctor or nutrition professional to figure out if intermittent fasting could be a helpful tool.
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