Intermittent Fasting: Do What's Right for You
- Dec 21, 2020
- Kelly Harrington, MS, RDN
An Introduction to Intermittent Fasting
Intermittent fasting, or IF, is a common dietary approach made popular in recent years due to its many health benefits. Fasting involves abstaining from calorie-containing foods and beverages for at least 12 consecutive hours, inducing a number of hormonal and metabolic changes in the body. Research demonstrates IF can promote certain health benefits including weight loss, reduced inflammation, and improved blood glucose and insulin levels.
Variations of IF are endless. Some proponents skip breakfast; others dinner. Some fast from after dinner to breakfast (my preferred choice), some fast all day, every other day, every third day, once per week, or once per month.
While fasting, only water, unsweetened tea, and black coffee should be consumed.
In this blog, I will discuss the different types of IF, how it works, and its benefits. Additionally, I'll tell you about safety considerations and resources for planning.
How it works: examining the science behind fasting
When we eat, food is broken down in the digestive tract into nutrients and absorbed into the body. Carbohydrates in food are broken down to glucose (simple sugar), which can then be absorbed into the bloodstream and distributed into body tissues, where it is used as the body’s primary source of energy. When carbohydrates, fats, and proteins are broken down during digestion, adenosine triphosphate (ATP) is generated. ATP is the primary carrier of energy in cells. When a cell needs energy, ATP is broken down and the energy released is utilized for a variety of cellular processes, such as cell division, protein synthesis, muscle contraction, and transmission of nerve impulses.
The hormone insulin regulates glucose levels in the blood by signaling for your cells to uptake glucose from the blood into cells, where it
provides fuel for body functions. Excess glucose is stored in the liver and skeletal muscles as glycogen and as fat for long-term energy
storage. To meet energy needs, the body taps into these storage reserves when blood glucose levels are low. When glucose levels are
low, the body can undergo gluconeogenesis, a process by which the liver produces glucose from non-carbohydrate sources.
After approximately 18 hours of fasting, insulin levels are low and a process called lipolysis (“lipo” for lipid or fat, “lysis” for breaking down) begins. During lipolysis, the body breaks down adipose tissue (fat) into free fatty acids. Glycogen stores are typically depleted within 24 hours of fasting. When there is insufficient glucose or glycogen available to meet the body’s energy demands, the body will
transition to using fatty acids and fatty acid-derived ketone bodies (ketones) for energy, a metabolic state known as ketosis. Liver cells
are responsible for ketogenesis, the production of ketone bodies. During ketogenesis, fatty acids are broken down in the mitochondria of
cells by a process called beta-oxidation and converted to the ketones acetoacetate and beta-hydroxybutyrate. These ketones are used
by muscle cells and neurons (brain cells) to generate ATP used to fuel cellular processes.
Why fast? Health benefits of fasting
Research has demonstrated intermittent fasting provides a variety of health benefits.
During periods of fasting, the body shifts to using stored body fat as an energy source. This can result in fat loss and a lower body
mass index (BMI). Fasting may also increase levels of human growth hormone (HGH), a hormone produced by the pituitary gland that
improves fat metabolism and helps maintain a healthy body composition. Furthermore, IF reduces total caloric intake on fasted and nonfasted days, resulting in weight loss.
Improved metabolic health
Insulin resistance, a hallmark trait of type 2 diabetes, has been shown to improve in individuals adhering to IF regimens.
Following a fasting period, insulin sensitivity rises, resulting in lower insulin levels – this has long-term positive effects on insulin
resistance, which occurs when the body’s cells don’t respond normally to insulin. Metabolic syndrome, characterized by high
blood pressure, elevated blood sugar, large waist circumference, and abnormal blood cholesterol levels, may be prevented or
reversed by regular periods of fasting.
Reduced cardiovascular disease risk
Research suggests that IF improves blood pressure, triglyceride, LDL, and total cholesterol levels. Improved cardiovascular
risk factors associated with IF appear to be related to weight loss and lowered insulin levels.
The physiological changes associated with IF, such as reduced inflammation, may result in a reduced risk of various inflammatory conditions, including diabetes, cancer, and heart disease. IF may also reduce pro-inflammatory factors, including homocysteine and C-reactive protein (CRP), that can lead to the build-up of plaque in the arteries, a condition known as atherosclerosis.
Research suggests IF supports healthy aging of the nervous system by protecting neurons and nerve cells against genetic and
environmental factors. IF protects neurons by reducing inflammation in the brain and aiding in a process called autophagy, which is the
body’s way of eliminating damaged cells and generating healthier cells. Additionally, IF reduces markers of oxidative stress and improves the body’s stress response, thus helping cells better manage stress and resist disease.
Studies show a relationship between late-night eating and poor quality sleep. Chronic insufficient sleep is associated with an
increased risk of chronic health issues, such as obesity, cancer, and cardiovascular disease. Following a consistent intermittent fasting
routine and avoiding late-night eating may positively influence circadian rhythm, which is the body’s internal clock that regulates the
Improved gut health
IF increases the population of healthy gut microbiota, which play an important role in promoting healthy digestion and immune function. IF also reduces intestinal permeability (compromised gut lining) and improves gut motility by enhancing the parasympathetic nervous system, the system responsible for the body’s rest and digest response.
Side effects and safety considerations
While some side effects have been reported, IF is generally recognized as safe, and side effects are mild for most individuals.
Patients with diabetes should take special precautions while fasting as they are more susceptible to hypoglycemia (low blood sugar). Other populations at increased risk for complications include pregnant and lactating women, young children, certain older adults, and individuals with immunodeficiencies.
Some side effects may be temporary as your body adjusts to periods of fasting and using fat stores as a primary energy source. You can mitigate some of these side effects by easing into an IF routine and slowly increase the duration and frequency of your fasts.
Potential side effects include:
- feeling cold
- low energy
- mood and behavior changes
Planning a fast: choosing your preferred fasting method
There are several types of IF, determined by the duration of the “feeding window”, the timeframe in which food is consumed, and the “fasting window”, the timeframe in which food is avoided. The common types of IF are summarized in the table below. For me, I prefer the time-restricted fasting method where I fast from 7pm to 8am.
|Fasting Method||Description||Feeding window||Fasting window|
|Alternate day fasting (ADF)||Abstain from all calories during the fasting window; consume food without restrictions during the feeding window.||Every other day||Every other day|
|Restrict energy intake to 20-25% of daily caloric requirement during the fasting window; consume food without restrictions during the feeding window.||Every other day||Every other day|
|Time-restricted feeding (TRF)||Abstain from all calories during the fasting window; consume food without restrictions during the feeding window.||4-12 hours per day||12-20 hours per day|
|Early time-restricted feeding (eTRF)||Abstain from all calories during the fasting window; consume food without restrictions during the feeding window.||6 hours per day, early
(ie: 8am - 2pm)
|The remainder of the day|
|5:2 Periodic or cyclic fasting||Restrict calories to 20-25% of daily caloric requirements during the fasting window. Consume food without restrictions during the feeding window.||5 days per week||2 non-consecutive
days per week
|6:1 periodic or cyclic fasting||Abstain from all calories during the fasting window; consume food without restrictions during the feeding window.||6 days per week||1 day per week|
Breaking your Fast
Following a period of fasting, it’s best to slowly reintroduce foods as opposed to consuming a large meal. Stick with whole, unprocessed foods that contain all three macronutrients, including carbohydrates, protein, and fat. Avoid refined carbohydrates and sugary beverages when breaking a fast as they can raise insulin levels.
Below are some examples of healthy foods to include in your first post-fast meal:
- Fruits and vegetables
- Healthy fats (e.g., avocado, olive oil, nuts, seeds)
- Lean proteins (e.g., chicken, eggs, fish, turkey)
- Whole grains (e.g., barley, oats, whole wheat)
- Beans and legume
Consider pairing macronutrients together, such as oatmeal with nuts or seeds, chicken with broccoli and brown rice, or a garden salad with garbanzo beans and an olive oil-based vinaigrette. Eating meals with a combination of all three macronutrients can improve satiety,
helping you stay fuller for a longer period of time.
What if I get hungry while Intermittent Fasting?
Hunger sensations are normal while fasting and are typically short-lasting. Water consumption can ease hunger pangs and increase feelings of satiety. Participate in activities to distract yourself from sensations of hunger, including low-intensity exercise, reading, or meditating. During feeding windows, focus on eating foods rich in fiber and lean protein. Fiber and protein increase satiety, leaving you
feeling fuller for longer.
Meeting your nutritional needs while fasting:
The restricted feeding windows of IF may lead to fewer calories consumed and challenges with meeting nutritional needs. Focus on eating an abundance of nutrient-dense, whole foods during feeding periods for optimal results.
I recommend a multi-vitamin and a high-quality protein powder, at minimum. Adding superfoods is also a convenient way to improve the quality of your diet.
Can I take supplements while Intermittent Fasting?
Any calorie-containing dietary supplement will break a fast, and the body will revert back to using glucose for energy when it becomes readily available. Additionally, supplements that alter insulin levels, such as protein powders, fatty acid supplements, chewable or gummy vitamins, and meal replacement supplements, may impact the desired effects of fasting.
Five tips for taking supplements while Intermittent Fasting?
- Always check the “Supplement Facts” label on the product bottle for the number of calories.
- Look for supplements that are unsweetened or contain calorie-free sweeteners, such as monk fruit (Luo han guo) extract and stevia (Stevia rebaudiana) leaf extract.
- If appropriate, consume your supplements within your feeding window. Note that certain dietary supplements may be recommended on an empty stomach or at a certain time of day.
- Be cautious with supplement ingredients that may further lower your blood glucose levels. Ingredients that have been shown to reduce blood sugar include chromium picolinate, berberine, and psyllium husk.
- Prior to making any changes, consult your integrative healthcare provider regarding intermittent fasting and your supplement regime.
Hydrating during Intermittent Fasting
It is estimated that about 20 to 30% of our total water intake comes directly from the foods we consume. During periods of fasting, proper hydration is especially important. Fluid needs differ based on body size and activity level but aim to consume approximately two to three liters (60 to 100 fl oz) of filtered water per day. Pay close attention to any signs of dehydration you may experience including dark urine, dry mouth, headache, lethargy, and thirst. If you experience any of these signs, drink some water right away.
A quick tip: Create your own electrolyte drink by adding a pinch of sea salt to your water bottle for added minerals. For extra flavor, squeeze in some lemon juice, lime juice, or add fresh fruit.
Can I Exercise while fasting?
Brief, low-impact exercise while in a fasted state is safe for most individuals. Exercising while fasting can improve your body’s ability
to use fat as energy, which may result in weight loss. Fat oxidation (breakdown of fat cells) decreases during feeding windows due
to increased insulin levels post-meal. Pay close attention to signs of dehydration or low blood sugar if you exercise while fasting. If you find that you feel lightheaded or dizzy, take a break and consider eating something before exercising.
A competitive athlete, endurance athlete, college-level athlete, teenagers, are in a growth phase. These athletes should error on the side of extra caution and refrain from participating in high-intensity training while fasting since performance may be negatively impacted. These athletes need more food than the average person who isn't exercising as significantly. It is usually not a good idea to fast during the season because your body needs more fuel to repair, recovery, and rebuild.
Mobile apps for Intermittent Fasting
|Life Fasting Tracker||
Does Intermittent Fasting have different effects on men and women?
Intermittent fasting is NOT recommended for pregnant or breastfeeding women. Also, if amenorrheic (not menstruating, but not in menopause), IF might not be the right strategy either. Not having your period is an indicator of stress, and if you are not getting yours, don't add IF as another stressor.
There is some evidence to suggest intermittent fasting affects men and women differently. After three weeks of alternate day
fasting, women in one study had decreased glucose tolerance, while men’s glucose tolerance remained the same.
Furthermore, early research suggests intermittent fasting may negatively affect reproductive hormones, disrupting a woman’s menstrual cycle. If you are having regular menstrual cycles every month, Nurse Practitioner Cynthia Thurlow suggests the 5-7 days preceding your cycle, you may not want to fast for as long (no more than 12-13 hours). Also, consider changing your nutritional choices to more high-quality carbs and modify the way you exercise. Why? Insulin changes during this time. During your classic PMS time, all your hormones are shifting (decreasing), and as they drop, that affects the way your body handles insulin, glucose, what you crave, and how you crave, and there's typically less energy for workouts. Cravings are really in response to the body needing something. It is critical to intrinsically tune into your body! Your brain and ovaries are connected and guide the body biologically. Once you start bleeding, it is OK to return to the normal fasting window.
The best time of your monthly cycle to do a fast is from day 1 of bleeding until day 14 – you will have more flexibility in the fasting window. After that, shorten the fasting window and change up the type of exercise.
With guidance from an integrative healthcare practitioner, some women may need to consider taking a less rigid approach to intermittent fasting and focus on participating in shorter fasting periods and fewer fasting days overall.
Reference: Holistic Primary Care Interfittent Fasting Guide.