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6 Intermittent Fasting Side Effects That Can Impact You In The Long Term

Intermittent Fasting is a popular weight loss method but not recommended for everyone. Learn what the side effects are and better alternatives.
Foods for Mental Performance
Foods for Mental Performance

By Vision Personal Training

What is intermittent fasting? 

Intermittent fasting is arguably one of the world’s most popular health and fitness trends, most commonly used as a method for weight loss through calorie-restricted eating. So what exactly is it? 

Intermittent fasting is a process where you sustain from eating for certain periods of time. It is referred to as an ‘eating pattern’ where you cycle between periods of fasting and eating. 

There are associated health benefits to intermittent fasting. It is important to note though, with implementing a fasting protocol, often comes a reduction in calories being consumed. Some experts argue the success of intermittent fasting in aiding in weight loss is largely attributed to the fact that the time available to consume food is significantly reduced. Arguing it is the result of being in a calorie deficit and maintaining this overtime

For example, if you normally eat from 8am - 8pm (12 hours) and then move into a fasting protocol such as the 16/8 method where you’re only eating for 8 hours a day, there is a strong likelihood the volume of food and therefore calories you’re consuming will be reduced. 

The three types of intermittent fasting 

There are many ways to approach intermittent fasting. However, the following three are typically the most popular and most commonly used today. 

  1. The 16/8 method. This is also referred to as the Leangains Protocol. This is where you fast for 16 hours and restrict your food consumption to an 8 hour eating window. For example, you might eat between 12pm and 8pm and then fast from 8pm through to 12pm the next day. 
  2. The 5:2 Diet. In this approach, you eat as you normally would for five days of the week and then drastically reduce your calorie intake (usually to around 500-600 calories) on the other 2 non-consecutive days. These 2 fasting days are sometimes referred to as “fasting” days, but it’s important to note that you’re not completely abstaining from food. 
  3. Alternate-Day Fasting. As the name suggests, this method involves alternating between days of regular eating and days of fasting or very low-calorie intake. On fasting days, you might either abstain from food completely or consume a very minimal amount of calories (usually around 500).  

Are there any side effects of intermittent fasting? 

Now, you may be wondering, does intermittent fasting have side effects? Like with anything, there are benefits and risks and it’s important to consider these before starting any new protocol. There may also be an easier (and safer) solution, which we’ll get onto shortly. There are intermittent fasting side effects that are important to note and more importantly the long term intermittent fasting side effects that could impact your health. 

1. Extreme hunger and cravings

The most obvious side effects of intermittent fasting is increased hunger and cravings. This can be quite challenging, particularly in the early stages of adopting a new eating pattern. Now, you may be thinking “I can handle feeling hungry here and there”, and whilst you might be right, it’s important to think about the impact these increased feelings of hunger will have on your day-to-day life such as your long-term performance at work, how you show up for your family, etc. To no surprise, studies have confirmed fasting protocols such as alternate-day fasting often lead to feelings of increased hunger and cravings, especially on the fasting days (Varady et al. 2013). 

2. Headaches

There is also a potential for an increase in headaches, especially in the early stages of changing your eating patterns. This could be due to factors like changes in blood sugar levels, caffeine withdrawal or dehydration. The long term intermittent fasting side effects of ongoing headaches can be detrimental to your every day. 

3. Mood disturbances

Imagine you’re experiencing increased feelings of hunger, cravings and headaches - mood swings are almost inevitable. We know good food creates a good mood - If you’re going 16 hours without food this may be a tough adjustment. We’ve all heard of being “hangry” right? Avoiding processed foods during fasting windows can help mitigate these effects. It’s also not uncommon to experience sleep disturbances, which is one of the common side effects of intermittent fasting which could also impact your mood long-term. 

4. Feeling Sick

Some people may experience feelings of nausea, especially if they are not adequately hydrated or if they consume large meals after fasting periods. It’s important to break fasts with balanced, nutrient dense meals and ensure your hydration levels are maintained. 

5. Dehydration

Fasting can also lead to reduced water intake, which may increase the risk of dehydration. It's crucial to stay hydrated, especially during fasting periods. Water intake should be distributed throughout the non-fasting and fasting periods to maintain proper hydration levels. Long-term dehydration can have serious implications for your health and wellbeing. 

6. Overeating

Now I know you might be thinking “overeating whilst fasting”? How is that possible? Hunger is a normal response to fasting, so some people may then find themselves overcompensating by overeating on non-fasting days, which could potentially negate the benefits of fasting. It’s important to practice mindful eating and make healthy food choices even during non-fasting periods. 

Who shouldn't try intermittent fasting? 

Intermittent fasting isn’t for everyone, there are risk factors, especially for certain individuals with specific health conditions. Always consult with your healthcare professional before starting any fasting protocol. As a general guide, pregnant or breastfeeding individuals, children and adolescents, individuals with eating disorders or a history of eating disorders and individuals with medical conditions that have pre-existing conditions (such as high blood pressure) should avoid intermittent fasting protocols and consult with their healthcare professional for personalised advice. 

Alternatives to intermittent fasting 

Here at Vision, we avoid restricted diets and aim to educate our clients on how to effectively eat for life with eating habits that are sustainable! Intermittent fasting, whilst can potentially help accelerate weight loss goals, is not an effective long term solution and can result in the weight lost during fasting protocols, being gained again. 

It’s important to find a balance and avoid any practices that result in heavy restrictions - this is not sustainable, nor physically or mentally healthy. Life is to be lived and our approach to food is one that empowers you to enjoy what you love, fuel your body to feel great all whilst being able to achieve your health and fitness goals. 

As part of your Vision program, you are prescribed with personalised daily macronutrient goals that have been developed specifically for your current condition, body shape, individual goal and time frame. These macronutrients in combination with a personalised exercise plan work together to help you feel better, stronger, fuller and ready to take on the day without any side effects. 

As you will already know, there is no magic pill. Sustainable weight loss results take time, effort and consistency (the magic 3). But we do understand that sometimes there’s a desire to further accelerate your results. 

Only then if you have successfully followed your normal macronutrient guidelines for the first 9 weeks, will we introduce what is called an accelerator day.  On an accelerator day, your individual macros are adjusted to the following: 

  • Carbohydrates - 30% of normal recommendations
  • Protein - 90% of normal recommendations
  • Fat - 80% of normal recommendations​​

On these days, we recommend that all meals and snacks include only high protein / low carbohydrate foods at least six times throughout the day. Examples include eggs, cheese, beef, lamb, chicken, turkey, pork, tuna, salmon, nuts and low carbohydrate protein bars /drinks. Carbohydrate foods such as starch-based vegetables, fruits, breads, rice, pasta, potatoes and cereals should be reduced on these days. This ensures you remain satisfied and keep your metabolism elevated. 

​As your body begins to utilise fat as a fuel source rather than carbohydrates, it is not uncommon to get headaches and feel a little dizzy. To combat this, we recommend that you drink at least 2 litres of water per day to ensure you stay hydrated

Additionally, planning your accelerator day on a set day each week, such as a Monday, is a good strategy, as it sets the tone for a positive week. Accelerator days are a safe short term alternative to intermittent fasting that offer a great way to further accelerate your results without any of the side effects that come with intermittent fasting, all under the guidance of your dedicated PT. 


If you're wondering about the side effects of intermittent fasting, it's essential to be aware that they can vary from person to person. It's important to consider potential long-term intermittent fasting side effects. Some common concerns include fluctuations in mood, increased hunger, cravings, headaches, and potential disturbances in sleep patterns. 

As with any dietary or lifestyle change, it's advisable to consult with a healthcare professional or nutritionist to ensure it aligns with your individual health and wellness goals. Remember, there is no one-size-fits-all approach, and what works best may differ for each person.


  • Varady, K. A., Bhutani, S., Klempel, M. C., Kroeger, C. M., Trepanowski, J. F., Haus, J. M., & Calvo, Y. (2013). Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal, 12(1), 146. [DOI: 10.1186/1475-2891-12-146]

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