Downsides of fasting

1. Increased hunger and tiredness – especially at the start

It is common to experience symptoms such as headaches, irritability, tiredness, weakness and reduced concentration when fasting. These effects are linked to the body adjusting to lower energy intake and changed eating patterns.

Symptoms are most common at the beginning, particularly if calorie intake is reduced a lot. For some people, they ease over time. For others, they can persist and affect both wellbeing and the ability to stick with fasting in the long run.

2. Risk of overeating

One challenge with fasting and intermittent fasting is that hunger can build up during the fasting period and lead to compensatory eating when you do eat. This can manifest as:

• larger portion sizes
• eating more quickly
• higher intake of energy-dense foods

This type of overeating can make it harder to tune into hunger and fullness signals. It can also work against weight loss if it leads to a cycle of strict restriction followed by high energy intake, which over time may cancel out the calorie deficit the method aims to create.

3. Impact on hormones and energy levels

Fasting involves changes in energy and nutrient intake, which can affect hormonal signals linked to hunger, fullness and energy balance. This may result in:

• reduced concentration
• mood swings
• sleep problems
• lower energy levels or poorer exercise performance

Effects vary between individuals and depend on factors such as the fasting approach, overall energy intake and individual physiology.

4. Increased risk of nutrient deficiencies

Fasting itself does not automatically cause nutrient deficiencies, but a limited eating window can make it harder to meet the body’s needs. If meals during eating periods are not nutrient-dense, fasting can increase the risk of low intake of:

• essential fats
• fibre
• vitamins and minerals

The risk is higher with very low energy intake, especially if it continues for a long time.

5. Can trigger disordered eating behaviours

Fasting involves deliberate calorie restriction, which for some people can have negative psychological effects. In those who are vulnerable around food, it may lead to:

• relapse into or triggering of disordered eating
• increased fixation on food, body and health
• feelings of guilt or shame linked to eating

For this reason, people with a current or past eating disorder are generally advised to avoid all forms of fasting.

6. Not everyone responds the same – results vary

The effects of fasting and intermittent fasting vary between individuals. Overall, it does not lead to better or faster long-term weight loss compared with other ways of reducing energy intake. Results are mainly influenced by factors such as:

• how easy the method is to stick to
• how hunger and fullness are experienced and managed
• lifestyle factors like work, social life and sleep
• personal preferences and previous experience with restrictive eating

For some people, fasting can be helpful. For others, it may increase stress, worsen their relationship with food or reduce control over eating habits.

7. Some people should avoid fasting altogether

Fasting is generally not harmful for healthy adults, but certain groups should avoid it. This includes people who:

• are pregnant or breastfeeding
• have diabetes treated with insulin
• have or have had an eating disorder

If fasting is still being considered by anyone in these groups, it should only be done after individual medical assessment and in discussion with a doctor or other qualified healthcare professional.

8. Can be hard to combine with everyday life, medication and exercise

Fasting can be difficult to fit around daily life, especially if it:

• reduces energy levels or exercise performance
• makes regular meals harder with family life or social situations
• affects mood, concentration or ability to work
• clashes with medical needs

Even if a fasting plan works in theory, it may not be practical or sustainable for everyone in real life.

Intermittent fasting, 16:8 and the 5:2 diet – specific challenges

Intermittent fasting has become popular for weight loss. Two of the most common approaches are 16:8 and the 5:2 diet. The 16:8 method involves fasting for 16 hours and eating within an 8-hour window each day. The 5:2 diet involves eating normally five days a week and significantly reducing calorie intake on two days.

A key challenge with these approaches is that they can be hard to maintain over time, especially if results do not appear or if daily life is negatively affected. To reduce unwanted effects, it is important to listen to your body and adapt the approach based on how you feel. Careful meal planning is also important to ensure you get enough nutrients from the smaller amount of food you eat.

Intermittent fasting does not suit everyone, and it is completely fine to choose another approach if fasting does not work well in your everyday life or does not feel helpful for you.

Fasting and age

In older adults, fasting can come with additional challenges. As we age, changes in muscle mass, energy needs and nutrient absorption can make people more vulnerable to low energy and protein intake, especially with restrictive eating patterns like fasting.

Older people are also more likely to have long-term health conditions and use medication that can be affected by irregular eating. This can increase the risk of side effects and poor nutritional status. Because of this, fasting in older adults should always be assessed individually, and more restrictive approaches are best avoided.

Fasting during pregnancy and breastfeeding

During pregnancy and breastfeeding, energy and nutrient needs are higher to support both the parent and the baby’s growth and development. Restrictive eating patterns such as fasting can increase the risk of not getting enough essential nutrients, which may negatively affect health.

Fasting is generally not recommended during pregnancy or breastfeeding. If you are pregnant or breastfeeding and are considering fasting, this should always be discussed with a qualified healthcare professional first.

Fasting and social life

Fasting does not just affect the body. It can also have a big impact on social life. Many social activities revolve around food and drink, which can make it challenging to take part in dinners, coffee breaks or other gatherings while fasting. It can also be difficult to explain fasting choices to friends and family, which may lead to misunderstandings or questions.

To reduce stress or feelings of exclusion, it can help to be open about your goals and explain why you are fasting. Finding alternative ways to spend time together, such as going for a walk or exercising together, can also help. Feeling supported and confident in your decision can make social challenges easier to handle.

Fasting and Yazen

At Yazen, we do not recommend any specific diet. Instead, we take a whole-person approach that focuses on:

• healthy, sustainable eating habits
• medical treatment when there is a clear indication
• long-term lifestyle changes
• individual adaptation based on medical and personal needs

Often, a combination of healthier eating habits, behaviour change, physical activity and, when needed, medical treatment leads to more sustainable results than relying on a single method.

Fasting may be an option for some, but it is not a requirement for weight loss. At Yazen, the focus is on finding a way of eating and living that works in everyday life and supports long-term health.

Downsides of fasting – common questions

Is fasting always good for weight loss?
No. Fasting and intermittent fasting are not suitable for everyone. For people who are pregnant or breastfeeding, those with insulin-treated diabetes, and those with a current or past eating disorder, fasting can pose health risks and is not recommended.

Some people also find that fasting increases hunger, leads to overeating and reduces wellbeing, which can work against weight loss.

Can fasting cause nutrient deficiencies?
Fasting does not automatically cause deficiencies, but a restricted eating window can make it harder to get enough healthy fats, protein, fibre, vitamins and minerals.

What are the most common downsides of fasting?
Common downsides include increased hunger and tiredness, risk of overeating after fasting, changes in energy levels and wellbeing, and a higher risk of nutrient deficiencies if meals are not well planned. Fasting can also be hard to combine with daily life, exercise and some medications, and for some people it can increase stress or trigger disordered eating behaviours.

Is fasting suitable for everyone?
No. Certain groups should avoid fasting, including pregnant and breastfeeding women, people with insulin-treated diabetes, those with a history of eating disorders, and people with some other medical conditions. Always speak to a qualified healthcare professional if you belong to one of these groups and are considering fasting.

Can fasting affect my exercise and energy levels?
Yes. Fasting can reduce energy levels and stamina, which may affect exercise performance, especially with intense training or very low energy intake. Effects vary depending on the fasting approach, type of exercise and overall energy balance.

How does fasting affect older adults?
In older adults, changes in muscle mass, energy needs and nutrient absorption can increase the risk of low energy and protein intake when following restrictive eating patterns like fasting.

How can I reduce the downsides of fasting?
If you choose to fast, it is important to tailor the approach to your own needs. This includes ensuring adequate energy and nutrients during eating periods, staying well hydrated and paying attention to how your body feels. If fasting leads to poorer wellbeing, increased hunger or difficulty coping with everyday life, it may be sensible to adjust the approach or choose a different way of eating.

Summary

Fasting and intermittent fasting are often used for weight loss, but they are not suitable for everyone and can come with downsides and health risks for some people. Common challenges include increased hunger and tiredness, risk of overeating, effects on energy levels and exercise performance, and a higher risk of nutrient deficiencies if meals are not planned carefully. For some individuals, fasting can also negatively affect their relationship with food or trigger disordered eating.

Long-term weight loss results vary between individuals, and fasting does not generally lead to better outcomes than other ways of reducing energy intake. It can also be difficult to combine with everyday life, social situations, medication and exercise. Older adults, pregnant and breastfeeding women, people with insulin-treated diabetes, and those with current or past eating disorders should generally avoid fasting or only consider it with medical guidance.

Overall, fasting can work for some people, but long-term weight loss and good health are best supported by an individual, sustainable approach to eating, lifestyle and health.

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