Intermittent fasting (IF) is a general term used to describe a variety of approaches that change the normal timing of eating throughout a day, with short-term fasts used to improve overall health. In other words, the one consistent theme of intermittent fasting is that individuals periodically fast for a longer duration than the typical overnight fast. So unlike other diets, the focus is not on how to cut calories from each meal, but rather cutting out the entire meal itself.

The main premise of fasting is that it simply allows the body to burn excess body fat. When we eat, we ingest more food energy than our body can immediately use. As a result, this energy is stored away for later use. Enter the hormone, insulin. Insulin is the key hormone involved in the storage of food energy. To put it simply, insulin rises when we eat, helping to store the excess energy, known as glycogen, in the liver. There is, however, limited storage space; and once that is reached, the liver starts to turn the excess glucose into fat.

Most of this newly created fat is shifted to other fat deposits around the body, which have unlimited storage space. When we fast, this process happens in reverse. Insulin levels fall, signalling the body to start burning stored energy since no more is coming through food. Blood glucose also falls, so the body must now pull glucose out of (fat) storage to burn for energy.

Glycogen is broken down into glucose molecules to provide energy for other cells in the body. This can provide enough energy to power the body for 24-36 hours. After that, the body will start breaking down fat for energy.

The different types of IF

There are a few different approaches to intermittent fasting, which range from skipping one meal of the day and extending the duration of the overnight fast (anywhere from 12 to 20 hours) to whole-day fasts that usually involve fasting for 24 to 30 hours, performed anywhere from once to twice per week to just once or twice per month. Some of the different types of intermittent fasting protocols include:

  • The 5:2 Diet – is one of the most popular intermittent fasting plans. The diet allows you to follow a ‘normal’ healthy pattern of eating for five days (feasting) followed by two ‘fasting’ days. On fasting days, energy intake is restricted to around 500 calories for women and 600 calories for men, per day.

    Fasting days can be consecutive or split over the week. The daily calorie allowance can be eaten over various combinations of one, two, or three meals plus snacks, depending on personal and practical preferences.

    There are also variations of the 5:2 Diet, including 6:1 (restricting only 1 day per week), 4:3 (restricting 3 days per week) or ADF (alternate day fasting).

  • 16/8 Fasting –This method recommends you fast for 16 hours a day and limit your eating to an eight-hour window. Most often, this simply involves skipping breakfast and eat between the hours of 12-8pm.
  • Eat-Stop-Eat:Pick one or two days out of the week and fast for 24 hours, eating nothing from dinner one day until dinner the next day. On the other days, you should aim to eat normally. Lots of fluid is encouraged during these fasts.

What are the benefits of IF?

Fasting’s most obvious benefit is fat loss. However, research has shown that there are numerous benefits beyond this. These include:

  • Fasting promotes the secretion of human growth hormone (HGH) – increased HGH production helps build lean muscle mass, which is important for burning fat and building strength.
  • Fasting helps normalise insulin sensitivity – lowering insulin production means you decrease your risk of insulin resistance, which can be a precursor of type 2 diabetes.
  • Fasting lowers blood glucose levels – as a result of the body becoming more insulin sensitive, your body will also improve/lower your blood glucose levels reducing your risk of type 2 diabetes, metabolic syndrome and cardiovascular disease.
  • Fasting improves cardiovascular markers – IF has shown to lower your ‘unhealthy’ cholesterol markers, decreasing triglycerides in the process, thereby reducing your risk of cardiovascular disease.
  • Fasting helps normalise ghrelin levels – ghrelin is a hormone responsible for telling our body it’s hungry. By improving ghrelin levels, your body will become more in tune to when it actually needs food.
  • Improved immune function – research suggests starving your body kick-starts stem cells into producing new white blood cells, which fight off infection.
  • Decrease in inflammation – fasting decreases the production of inflammatory cytokines, and reduces inflammatory markers (CRP and homocysteine) and oxidative stress, reducing your risk of chronic diseases.
  • May improve brain health and function – IF studies in rats has shown improvements in memory, coordination, learning and neurogenesis (production of neurons in the brain).
  • May decrease neuroinflammation – chronic neuroinflammation is increasingly associated with neurodegenerative diseases like Alzheimer’s and mood disorders such as depression.
  • Longevity – there may be beneficial changes in several genes and molecules related to longevity and protection against disease.

Are there any negatives associated with IF?

Although IF has benefits on many different aspects of health, it may not be ideal for everyone and some people should avoid it.I would strongly recommend that it be pursued with the guidance of a qualified healthcare professional or dietitian who understands the risks and benefits and can help determine if it’s right for you. For example, IF should always be avoided during pregnancy and breastfeeding and should generally be avoided during times of increased stress. IF is also not recommended if you are a child or teenager as it may interfere with growth and development. People with uncontrolled diabetes should also proceed with caution.

Other health risks that are generally associated with diets that are too low in calories, include risk of nutritional deficiencies, electrolyte abnormalities, and potentially more serious risks if extreme diets are undertaken without appropriate supervision.

Other things to be wary about IF:

  • It may create or exacerbate eating disorders – IF’s binge-and-purge mentality could encourage unhealthy eating behaviours and trigger symptoms associated with bulimia and other eating disorders.
  • If you’re an athlete or heavy gym goer, IF has the potential to compromise performance if training on a fasting day due to low consumption of fueling carbohydrate.
  • Potential negative impacts on recovery (failure to adequately replace glycogen stores or promote muscle repair).
  • Over-reliance on coffee – since coffee is allowed, when fasting you might find yourself gravitating towards caffeine to keep you going without food. This may lead to increased anxiety and sleep disturbance or insomnia.