Intermittent Fasting

A patient guide

What Is Intermittent Fasting?

Historically, intermittent fasting (IF) regimens have been used for medicinal purposes by ancient Roman, Greek and Chinese civilizations and is a common practice for many religions. 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 that IF can promote certain health benefits including weight loss, reduced inflammation and improved glucose and insulin levels.

How Your Body Uses Energy

When we eat, food is broken down in the digestive tract into nutrients and absorbed into the body. Carbohydrates are broken down to glucose, which is absorbed into the bloodstream and distributed into body tissues, where it is used as the body’s primary source of energy. When carbs, fats, and proteins are broken down during digestion, adenosine triphosphate (ATP) is generated. ATP is the primary carrier of energy in cells, used for protein synthesis, muscle contraction, and transmission of nerve impulses.

The hormone insulin regulates glucose levels in the blood by signaling for your cells to take up glucose from the blood, 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, and can undergo gluconeogenesis, a process by which the liver produces glucose from non-carb sources.

What Happens During a Fast

After approximately 18 hours of fasting, insulin levels are low and a process called lipolysis begins. During this, 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 ketones for energy. This metabolic state is known as ketosis. Liver cells are responsible for ketogenesis, the production of ketones. 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 to generate ATP used to fuel cellular processes.

Benefits

Weight Loss — 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), which improves fat metabolism and helps maintain a healthy body composition. Furthermore, IF reduces total caloric intake on fasted and non-fasted 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. Metabolic syndrome 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.

Decreased Inflammation — The physiological changes associated with IF may result in a reduced risk of various inflammatory conditions, including diabetes, cancer, and heart disease. IF may also reduce proinflammatory factors, including homocysteine and C-reactive protein (CRP), that can lead to the build-up of plaque in the arteries.

Better Sleep — 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. Following a consistent intermittent fasting routine and avoiding late night eating may positively influence circadian rhythm.

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 and improves gut motility by enhancing the parasympathetic nervous system.

Neuroprotection — Research suggests that 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 autophagy, the body’s way of eliminating damaged cells and generating healthier cells. It also reduces markers of oxidative stress and improves the body’s stress response.

Side Effects & Safety Concerns

While some side effects have been reported, IF is generally recognized as safe, and side effects are mild for most individuals. 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 increasing the duration and frequency of your fasts.

Patients with diabetes should take special precautions while fasting as they are more susceptible to low blood sugar. Other populations at increased risk for complications include pregnant and lactating women, young children, certain older adults and individuals with immunodeficiencies.

The following are normal side effects:

  • Fatigue
  • Feeling cold
  • Hunger
  • Irritability
  • Low energy
  • Mood and behavior changes

Time Restricted Feeding

Abstain from all calorie-containing food and beverages during the fasting window; consume food as needed during the feeding window. A common pattern is an 8 hour feeding window — eating until 80% full — paired with a 16 hour fasting window.

Planning Your Fasting & Feeding

Mobile apps are a useful tool for tracking fasting and feeding windows. Paper trackers on a calendar are also known to be helpful. Recommended apps include Do Fasting, Fastic and Simple Life App.

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 carbs, protein and fat. Avoid refined carbs and sugary beverages when breaking a fast as they can raise insulin levels.

Healthy Foods To Consume Post Fast

  • Fruits and vegetables
  • Healthy fats — avocado, olive oil, nuts, seeds
  • Lean proteins — chicken, eggs, fish, turkey
  • Whole grains — barley, oats, whole wheat
  • Beans and legumes

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 longer.

Meeting Nutritional Needs

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.

Common Questions

How much should I eat while fasting? — Avoid all calorie-containing foods or beverages during fasting periods. Only water, unsweetened tea, and black coffee should be consumed — under 50 calories.

Can I exercise while fasting? — Brief, low-impact exercise while in a fasted state is safe for most individuals, and can improve your body’s ability to use fat as energy. Pay close attention to signs of dehydration or low blood sugar. If you feel lightheaded or dizzy, take a break and consider eating something before exercising. Endurance athletes should refrain from high-intensity training while fasting since performance may be negatively impacted.

Does IF affect men & women differently? — There is some evidence that it does. After 3 weeks of alternating day fasting, women in one study had decreased glucose tolerance, while men’s remained the same. Early research suggests intermittent fasting may negatively affect reproductive hormones, disrupting a woman’s menstrual cycle. With guidance from your provider, some women may need a less rigid approach with shorter fasting periods and fewer fasting days overall.

What if I get hungry? — Hunger is normal while fasting and is typically short-lasting. Water consumption can ease hunger pangs and increase feelings of satiety. Participate in activities to distract yourself, including low-intensity exercise, reading or meditating. During feeding windows, focus on food rich in fiber and lean protein.

Can I take supplements? — Any calorie-containing dietary supplement will break a fast, and the body will revert to using glucose for energy when it becomes readily available. Supplements that alter insulin levels — protein powders, fatty acid supplements, chewable or gummy vitamins, and meal replacement supplements — may impact the desired effects of fasting.

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