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Intermittent fasting (IF)¹ is a diet regime characterized by eating within a specific window of time during the day and fasting for the rest. There are many ways to go about it, but typically it involves abstaining from consuming any calories for a certain period of time, followed by a specific amount of hours where you are allowed to eat before fasting again.
One example of this pattern is 16/8 — 16 hours fasting and 8 hours eating. To follow this schedule, you may choose to eat between 12 PM–8 PM and not eat at any other point during the 24-hour period, meaning you will be fasting from 8 PM–12 PM the next day.
Typically, during fasting periods, you can have water, coffee, and tea (without milk, creamers, sugar, or other sweeteners). Another way of doing IF is to choose certain days to eat an average caloric intake and fast the other days of the week.
For instance, people may choose to restrict their diet on alternative days, consuming only 400–600kcal/day during their fasting period, followed by a day of normal eating. Or they may do the 5:2 IF method, fasting two days a week and eating normally for the other five days. Fasting for longer than 24 hours is not recommended.
Intermittent fasting has become very popular in recent years. This popularity could be due to the idea that IF is easier to maintain than other diets that limit the number of calories or the type of foods you eat.
You would not normally eat during your hours of sleep, so adding a few extra hours to each end of this sleeping time can easily fulfill the diet’s requirements. Hence, some may see it as less restrictive than constant eating restrictions.
However, that doesn’t mean you can binge-eat unhealthy, high-calorie foods during the non-fasting hours, as you will still need to eat a well-balanced diet to achieve the best results.
Overall, there is a range of health benefits linked to IF. Some of the potential health benefits that IF may offer include:
Improved physical functioning of cells and organs
Increased life-span or reduced aging
Improved cognitive function
Improved sensitivity to insulin, thus reducing risks of insulin resistance and type 2 diabetes
Improved indicators of cardiovascular disease
Reduction in oxidative stress and enhanced antioxidant effects
A slowing down of the progression of neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease through alternate fasting
Additionally, the flow-on effects from IF regarding weight loss could improve other areas of your health. For example, it could improve asthma or joint problems like arthritis.
Mark Mattson, Professor of Neuroscience at John Hopkins University, who has extensively studied this diet pattern for 25 years, believes that the IF diet works because it aligns with our prehistoric patterns.
Compared to how we live in the world in modern times, the timing of our next meal in prehistoric times was always unpredictable, so our bodies built mechanisms to remain alive during these periods of starvation.
Due to the abundance of food around us and increasing periods of awake hours due to artificial light, we are now eating more than ever and for a longer period of time.
When you don’t eat for some time, your body is forced to use its internal sources for energy (known as metabolic switching). Initially, your body prefers glycogen in the liver (the stored form of glucose), but we only have a small amount that lasts 2–12 hours, depending on your level of activity. We will usually eat again before the stores are depleted.
If your glycogen stores do run out, your body will start to burn fat. With help from enzymes known as lipases, triglycerides (small fat molecules) are released by adipose tissue into the bloodstream.
They travel to the liver and are further broken down to produce ketone bodies which can be used as an energy source.
The underlying mechanism of IF is the glycogen-ketone switch. Although it is not fully understood, changing from glycogen metabolism to ketones creates marked changes in the body.
One reported effect is reduced levels of total cholesterol, triglycerides, and low-density lipoproteins.
Cholesterol is a lipid produced by the liver. Despite its bad press, it has many functions that are essential to the body. They include hormone and vitamin production and maintaining the structure of cell walls.
The majority of cholesterol (about 80%) comes from the liver and other organs. We get the rest through our diet, mainly by eating animal-based products, including beef, lamb, poultry, and dairy products such as eggs, milk, cheese, and butter.
Cholesterol can’t travel on its own through the bloodstream as it is fat, so it requires other molecules to carry it around the body — these are called lipoproteins. There are various types of these molecules, but the most important two are high-density lipoproteins (HDL) and low-density lipoproteins (LDL):
HDL is the ‘good’ form because it removes cholesterol from the blood and takes it to the liver, where it will be processed and removed from the body.
LDL is the ‘bad’ form because it carries cholesterol through the blood to the cells and can accumulate inside the blood vessel walls.
Too much LDL can cause a plaque-like formation in the blood vessels, causing them to narrow and stiffen, a condition called atherosclerosis.
This can create ‘roadblocks’ for blood flow, leading to a reduced blood supply to the tissues that need it. Eventually, this could result in blood clots, heart attack, or stroke.
Hyperlipidemia or dyslipidemia are the medical terms for high cholesterol levels or imbalances in the lipid/cholesterol blood levels. There are many options to reduce cholesterol levels, from lifestyle changes to medications.
Intermittent fasting affects cholesterol levels due to the change in metabolism from glucose to ketones.
When this occurs, the body begins using lipids rather than storing them. Lipids move out of the cells, travel in the bloodstream, and go into the liver to be made into ketones.
The overall direction of the lipids changes the type and amount of cholesterol needed to facilitate the transfer. It, in turn, increases the amount of HDL and decreases the amount of LDL. This favorably changes your cholesterol profile and reduces the risk of dyslipidemia, a significant risk factor for cardiovascular disease.
During fasting, triglycerides break down into fatty acids and glycerol, and those fatty acids are converted to ketone bodies in the liver, providing energy to cells and tissues.
Studies² have suggested that this process in the liver can lead to the expression of certain genes that increases fatty acid oxidation and apolipoprotein type A production (the primary protein component of HDL), leading to higher HDL levels. Also, apolipoprotein B (the primary protein component of LDL) decreases, which causes reductions in liver triglycerides and LDL levels.
Another way that IF can affect cholesterol levels is because of weight loss. Due to the time-restricting nature of the diet, people often consume less food overall, reducing energy consumption. Ketones also require more energy to be made, increasing your energy expenditure.
Being overweight is a modifiable risk factor for dyslipidemia. By reducing your weight, your cholesterol levels are likely to improve.
Most studies³ investigating the link between IF and cholesterol levels show positive results. On average, they report a decrease in total cholesterol that ranges between 6-21%, reductions in LDL between 7-32%, and triglycerides between 16-42%.
Many researchers are asking to what extent weight loss contributes to the results.
One study⁴ investigated how certain diets and exercise affected cholesterol profiles over a 12-week period. They separated the participants into four groups: alternate-day fasting (ADF), constant calorie restriction, exercise, and a control group. They found that ADF decreased LDL levels by approximately 10% and triglycerides by approximately 17%.
A similar effect on LDL was found in the constant calorie restriction group. The researchers questioned whether this effect could be attributed to weight loss. However, the exercise group had similar weight loss percentages and did not influence LDL levels.
Most studies so far have been conducted on a population that was overweight or obese. However, one study⁵ published in the Nutrition Journal investigated the impact of alternate-day fasting in both normal-weight and overweight individuals, with their BMI ranging from 20-30 over 12 weeks.
Participants in the ADF group were fed 25% of their normal energy intake on the fasting days, while they were permitted to eat normally on the non-fasting one. The control group was consuming their normal energy intake every day. They used the ADF schedule and found an average weight loss of 5kg compared to the control.
Furthermore, they found a statistical reduction in triglyceride by about 20% — however, total cholesterol, LDL, and HDL levels remained unchanged.
A recent high-quality systematic review and meta-analysis⁶ of 33 studies concluded that IF and energy-restricted diets can significantly improve total cholesterol, LDL, and triglyceride levels but have no meaningful effect on HDL levels.
IF promotes weight loss, as you are reducing the window of time that you eat, creating a metabolic shift that can aid with weight loss and improve your health. However, it is important that you eat enough to maintain daily functioning, making sure you follow, on average, the recommended daily intake for yourself on the feeding days.
If not, you may suffer from fast, unhealthy weight loss, muscle loss, and deficiencies.
Making sure that you consume all the nutrients and vitamins is important to keep your body running properly. This involves eating a wide variety of foods that include your dietary requirements of fats, proteins, and carbohydrates, including fibers.
If you want to try IF, the same foods that lower cholesterol can be applied to this diet.
Foods to include:
Fruits and vegetables, including beans
Unsaturated fats such as avocado, nuts, and olive oil
Foods to exclude :
Foods high in saturated fats or cholesterol
Foods and drinks high in simple carbs or sugar
Highly processed foods
You will likely see some results just a few days after starting this diet. However, you must stick to your chosen schedule and maintain it for a sustained period of time.
Several factors need to be considered before you decide to try out this diet. Often, people will have a bad time at the beginning of this diet while their body is getting used to the glucose-ketone switch.
You might feel grumpy and dizzy and have headaches in the first two to four weeks, which should resolve by itself.
Because this diet causes your blood glucose levels to drop during fasting periods, it is typically not recommended for those with diabetes. If you have diabetes or are concerned about what you might do, you must discuss the possibility of starting this diet with your physician.
Despite some studies⁷ reporting positive results in individuals with type 2 diabetes who followed this diet, it can be extremely dangerous if not done under close monitoring in individuals with reactive hypoglycemia or who are on antidiabetic medications or insulin.
There are other situations or medical conditions where IF is not recommended. They include:
BMI under 18.5
Children under 18
This article is not a substitute for medical advice given by your doctor. Please consult with your doctor if you are considering following this diet. They will be able to fully investigate whether this diet is right for you or whether there are any other required changes before you start.
It also allows you to get any further questions you might have answered, such as which schedule will work for you.
Furthermore, they may be able to direct you to a dietician to discuss what you should be eating and how much to prevent malnutrition or an unhealthy amount of weight loss.
Intermittent fasting is a diet regime that restricts the period you are allowed to consume calories within the day. There are no restrictions to what you eat during the eating phase, but a healthy diet with a wide variety of foods is encouraged.
IF has been found to have great health benefits, including positive influences on your lipid profile, such as decreasing triglyceride and LDL cholesterol levels. Because IF changes our primary energy source from glucose to ketones, the body begins to burn fat, which can lead to significant weight loss. This, in turn, can assist in preventing or treating dyslipidemia.
IF has many benefits but disadvantages, so be sure to do your research and talk to your doctor.
Intermittent fasting: What is it, and how does it work? | Johns Hopkins Medicine