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The Scientific Pros and Cons of Fasting – With Water or IF

The Scientific Pros and Cons of Fasting – With Water or IF

Fasting is a bit of a buzzword in the health industry and it's a recurring theme in the news. There’s certainly some truth in the hype – fasting is essential before surgery and diagnostic blood work. Fasting can melt away unwanted visceral fat in some people, but there are some drawbacks that you should be aware of.

There are many health benefits of fasting that have been scientifically proven. Intermittent Fasting (IF) protocols have been found to stabilise blood sugar, improve mood, enhance performance, and slow aging and disease process.

Sounds great, right? But is there a catch? Well, kind of.

Read on to learn more about the pros and cons of fasting in various contexts.

The Adverse Implications of Fasting

Fasting isn’t an option for everyone. People who are at risk and who should avoid fasting protocols include people who have:

• A severe illness

• Hypoglycaemic tendencies

• Adrenal problems

• Type 1 diabetes

• A history of eating disorders

Or who are:

• Pregnant or breastfeeding

• Taking medication

• Underweight

If in doubt, speak with your doctor before beginning a fasting regime.

4 Different Types of Fasts Used to Optimise Health

Fasting comes in many shapes and forms, from religious fasting to time-restricted feeding.

Let’s dive into the top four types of fasts that are reportedly great for your health:

• Intermittent fasting

• Long-term medically supervised water fasts

• Time-restricted feeding

• Fasting-mimicking diet

The Scientific Benefits of Intermittent Fasting

Intermittent fasting is has been popularised due to its many health benefits, especially when combined with a ketogenic diet.

There are two states in IF – the “fed state” and the “starvation state”. These states are manipulated in different ways to give us a variety of IF regimes.

There are four main types of intermittent fasting as follows:

1. 5:2 (5 days eating normally, two days fasting)

2. Alternate day fasting

3. Time-restricted eating (16/8 or 14/10 method)

4. The 24-hour fast (known as the “eat: stop: eat” method)

A quick word on alternate-day fasting: this involves modified fasting every other day, as the name suggests, which might entail limiting calories on fasting days (e.g., eating one small 500 calorie meal on those days) and consuming a balanced healthy diet on non-fasting days.

This form of IF has been proven to lower LDL cholesterol when followed for six months.

There are many metabolic mechanisms that fuel the benefits of intermittent fasting.

Fluctuating between using ketones (fat) for fuel and glucose (sugar) is known as metabolic switching and has been found to make the body more flexible in what fuel source it uses.

What Does Science Say About Alternate-day fasting?

Studies have focused on a variety of markers to measure the efficacy of IF, such as weight loss, metabolic markers, cognitive function and the prevention of diseases like dementia.

In one 2019 study of 24-hour water fasting researchers observed a reduction in TMAO (trimethylamine N-oxide) which impacts microbiome markers for cardiovascular disease.

Another study found therapeutic use of intermittent fasting for people with type-2 diabetes as an alternative to insulin. Participants in the study lost significant amounts of body weight, reduced their waist circumference and also reduced their glycated haemoglobin level.

Long-term Medically Supervised Water Fast

Long-term water fasting is best done under the supervision of a medical team, because they can be harmful if not carried out correctly.

A medically supervised 7-day water fast has been found to help people with type 2 diabetes.

In one study, participants who took part in the water fast lost an additional 1.5kg of weight, much of which was abdominal fat.

Adverse Effects of Long-term Medically Supervised Water Fasting

Long-term medically supervised water fasting sounds daunting, but it may not be as dangerous as you think.

Researchers carried out a systematic review of water fasts that lasted from 4 days to 41 and found that the majority of adverse events were mild and included:

• Fatigue

• Nausea

• Insomnia

• Headaches

• Hypertension

• Back pain

Time-Restricted Eating

Time-restricted eating is often considered a form of IF. Time-restricted eating is where you eat during a designated eating window, for example the 16/8 or 14/10 method.

16/8 is where you fast for 16 hours and feast for 8 hours, 14/10 is where you fast for 14 hours and eat all of your meals inside a ten hour eating-window.

There have been many studies done on the outcomes of time-restricted eating, sometimes referred to as time restricted feeding (TRF). One study on the 16/8 protocol in humans found that there was a reduction in both weight and blood pressure.

A promising animal study that would work well for humans is the study that followed rats who followed a 15/9 regime. The interesting finding was that the reduction in weight gain was observed even when the rats followed a normal eating regime at the weekends.

In other words, you could conceivably follow the 15/9 during the week and eat a normal healthy diet at the weekend and still reap the same rewards.

The Fasting-Mimicking Diet

The fasting-mimicking diet (FMD) is similar to consuming Instant Exogenous Ketones.

Consuming ketones tricks your body into quickly getting into ketosis, and a fasting-mimicking diet is designed to make the body think that it’s fasting, when it is not.

The fasting-mimicking diet was created by Dr. Vlater Longo. To mimic fasting, Dr. Longo uses a low-calorie, high-fat, low-carb diet focused on consuming whole-food and plant-based foods.

Fasting mimicking diets may be a good option for people with type 1 diabetes, as it focuses on changing diet, rather than introducing a more rigorous version of fasting.

Dr. Longo found that he could restore insulin generation in animals with type 1 diabetes. In another animal study focusing on type 2 diabetes, the FMD normalised blood glucose, increased insulin sensitivity and promoted a healthy gut microbiota.

The Downsides of Intermittent Fasting

Some studies carried out with animal models have not reported such beneficial effects of IF.

One study found that restricted feeding IF resulted in higher glycogen storage in the liver and skeletal muscle, plus an increase in insulin resistance.

Another animal study found that IF led to excessive eating on the non-fasting days, which makes sense. The over-eating was reportedly due to a change in hypothalamic function.

There can be some negative outcomes of IF, that are normally temporary. For example, an increase in the hormone cortisol, the main stress hormone. An outcome of which can be stress-related symptoms like migraines.

These negative stress factors have been found to reduce after a few weeks of following an IF regime. This temporary stress response may explain why some people take longer to achieve weight-loss results from IF.

One reason people with disordered eating should refrain from IF is the fact that it can heighten the food reward sensors in the brain. Scientists call this the “relative-reinforcing value of food” (RRV) – which basically means we crave food more if we have a high RRV.

High RRV for snack foods was found in people who had fasted for just 24 hours.


There are other forms of fasting, however there are not many studies on juice fasting or the long-term benefits of IF regimes – so it pays to proceed with caution and listen to your body.

As ever, it’s a good idea to discuss your plans with your primary care physician before taking up new dietary regimes. This will eliminate many of the negative outcomes that we’ve discussed in this article.

If you’re fit and healthy and interested in IF, trying out different feasting and fasting phases to find out what best suits your body is advisable.

Written by best-selling author and integrative nutrition health coach Rowanna Watson, who has a passion for natural health. Rowanna is an expert in all areas of holistic health, plant-based nutrition, detoxification and personal development.