Is Intermittent Fasting Dangerous?

Recent news stories have highlighted the dangerous downsides with intermittent fasting over the benefits. But what should you believe? Here's dietitian Ro Huntriss with a comprehensive deep dive into the science.

Intermittent fasting (IF) is talked about a lot in the media, by healthcare professionals and in the health and wellness industry, with some people and articles (Can intermittent fasting double your risk of dying from heart problems?) expressing caution about the diet and the effects it can have on our health.

Intermittent fasting is becoming very popular as a tool for weight loss as well as some health conditions so it’s important to understand the science behind intermittent fasting and uncover what we know and what we don’t. Read on to find out more.

What is intermittent fasting?

Intermittent fasting is a flexible eating pattern characterized by cycles of eating and fasting. Rather than a rigid diet, it can offer a sustainable approach to weight management for some people.

One of the most popular intermittent fasting methods is time-restricted feeding, where individuals compress their eating window into a specintermittent fastingic period of the day, typically 8 hours, and abstain from consuming calorie-containing food and drinks for the remaining 16 hours. This approach offers a structured yet adaptable framework for integrating fasting into daily life.

Intermittent fasting’s major benefit is its versatility. Whether you prefer a shorter or longer eating window, the flexibility of intermittent fasting allows you to tailor the approach to suit your lifestyle and preferences. From the 16:8 method to the 20:4 protocol or even the 12:12 protocol, there are many options.

Beyond time-restricted feeding, the 5:2 diet has also garnered attention for its unique approach. With this method, individuals typically restrict calorie intake to less than 500-600 on two days of the week, while maintaining a regular diet on the remaining days. This intermittent calorie restriction has shown promise in promoting weight loss and improving metabolic health markers.

What sets intermittent fasting apart from traditional diets is its lack of restrictions on food choices during eating windows. Unlike many diets that dictate what you can and can't eat, intermittent fasting doesn’t have rules around what foods can be consumed and what foods can’t.

One of the reasons for the growing popularity of intermittent fasting is that research suggests it can be a powerful tool for weight loss and the management of weight and obesity. Moreover, it shows promise in improving insulin sensitivity and blood glucose control, making it a valuable ally in the fight against type 2 diabetes.

What are the risks of intermittent fasting?

Intermittent fasting has gained popularity for its potential health benefits however, like any dietary strategy, it's important to be aware of any potential side effects and risks associated with intermittent fasting, as well as whom it may not be suitable for.

Short-term side effects can include hunger, cravings, and lightheadedness

During fasting periods, especially in the initial stages of adopting intermittent fasting, people may experience hunger, cravings, and lightheadedness. These sensations are a natural response to the body's adjustment to a new eating pattern and the symptoms will most likely improve over time. To minimize discomfort, staying hydrated and consuming balanced meals during the eating window helps to maintain stable blood sugar levels.

Changes in mood

Fluctuations in blood sugar levels and changes to your eating schedule can impact mood. In the short term, blood sugar fluctuations, hunger and cravings may lead to irritability which can improve over time. Some research suggests that in the longer term intermittent fasting can support people who suffer with depression.

Disordered eating risk

Intermittent fasting involves periods of food restriction, which may not be suitable for individuals with a history of eating disorders or a difficult relationship with food. Engaging in intermittent fasting without proper guidance and support can potentially exacerbate disordered eating patterns. It's crucial to assess your relationship with food and consult with a healthcare professional before embarking on an intermittent fasting regimen.

Risk of nutritional deficiencies

Regular fasting for extended periods can increase the risk of nutrient deficiencies if not balanced with adequate nutrient intake during eating windows. To mitigate this risk, focus on consuming a variety of nutrient-dense foods during your feeding periods and consider incorporating supplements intermittent fasting necessary.

This risk can be more prevalent in longer fasting protocols such as “OMAD” which stands for One Meal A Day, whereas within a protocol such as 16:8, it is possible to consume a nutritionally complete diet.


Dehydration can be a risk when fasting if people are not mindful to drink an adequate amount of fluids. Symptoms to watch out for are dizziness, fatigue, or headaches. These may indicate the need for increased hydration. Aim to consume water regularly during your fasting window and aim for at least 2 liters of water per day.

Intermittent fasting is not necessarily the right diet for everyone. People who should avoid intermittent fasting include people who are underweight, pregnant or breastfeeding, young children and teenagers, older adults who experience weakness and people with a history of eating disorders. It’s particularly important for those with medical conditions and on medication to discuss the suitability of fasting with their doctor.

Is intermittent fasting dangerous? 

If intermittent fasting is undertaken with proper planning, for an individual for whom it is appropriate, the current research and practice suggests that it can be a safe and effective way to manage weight and support health and wellbeing.

Numerous systematic reviews have highlighted the potential benefits of intermittent fasting on various health outcomes. Studies suggest that intermittent fasting may positively impact conditions such as metabolic dysfunction-associated steatotic liver disease, non-alcoholic fatty liver disease, excess body weight and fat mass. It may also improve insulin sensitivity, and reduce blood pressure, and can also have benefits on symptoms of depression. As research on intermittent fasting is in its relative infancy, longer-term studies are required to understand its wider-reaching effects.

Although intermittent fasting doesn't place rules on what foods can be eaten during the eating windows, considering what you are eating can play a role in supporting your health and helping you reach the goals you have set out to hit through intermittent fasting.

Focusing on a balanced diet rich in fruits, vegetables, whole grains, lean proteins, and healthy fats alongside a fasting regime can provide all the essential nutrients we require for overall health. Ensure that you choose a sustainable fasting regimen that allows for adequate nutritional intake, the 16:8 protocol is one of the most widely studied protocols that can lead to health benefits as well as allowing time to consume adequate nutrition.

While intermittent fasting can be a safe and effective tool for managing health, it is important to note that intermittent fasting isn't suitable for everyone and if you have a medical condition it is important to discuss this with your healthcare provider first.

How to be safe when intermittent fasting

If you are considering embarking upon intermittent fasting it is important to do so in a way that is safe and that supports your health. Here are our top tips for intermittent fasting:

  • If you have a health condition or take medication, consult with a healthcare professional before you begin an intermittent fasting regime as your medications may need changing.
  • Although there are no diet rules for intermittent fasting, consider what you are eating during your eating windows and choose nutrient-dense foods during your eating windows. This will help you to reach your weight loss goals faster and support your overall health and wellbeing.
  • Start intermittent fasting gradually, when you start, you may wish to consider a longer eating window initially, and gradually build up to a longer fasting window so that your body gets used to the change. This will also help you to manage any side effects that you may experience in the short term. Many people find that the 16:8 protocol is the easiest and most sustainable protocol to stick to.
  • Be aware of the common side effects of intermittent fasting. These can include hunger, cravings, lightheadedness, changes in mood, and sleep, as well as dizziness. These are usually short-lived and often go away once you have an established intermittent fasting routine.
  • Stay hydrated. It’s important to drink adequate amounts when you are fasting; water doesn't break your fast. Aim for 2 liters per day.
  • Be flexible. Allow yourself days off for events and make it work for your lifestyle for maximal benefits.

Words: Ro Huntriss

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