​Intermittent Fasting: How to Do It Safely

While other diets focus on what to eat, intermittent fasting (or IF for short) focuses on when you eat. It doesn’t specify the foods you should eat but rather when you should eat them. In this respect, it’s not a diet in the conventional sense but rather a useful tool to adjust eating patterns.

“The idea behind intermittent fasting (IF) is that by providing structure and limiting the amount of time the person spends eating, total calorie intake will be reduced and weight loss can be achieved,” explained Dr Lee Phong Ching, Senior Consultant from the Department of Endocrinology at Singapore General Hospital (SGH), a member of the SingHealth group.

“Even while following IF, it is crucial to eat a healthy, well-balanced diet. Staying hydrated is also important – a person should be drinking about two litres of water daily. If you have a medical condition, you should consult with your doctor before trying intermittent fasting,” Dr Lee added.

“IF can be one approach to support weight management, but it is not a substitute for healthy eating habits.

Current evidence suggests that IF is not superior to conventional calorie restriction for weight loss, and the effectiveness of IF is influenced by factors such as overall calorie intake, dietary quality, and adherence to the eating pattern.

During eating periods, focus should be placed on nutrient-dense meals with sufficient protein, fibre, and essential nutrients to support health and maintain muscle mass. The most suitable approach is one that is safe, sustainable, and suited to an individual’s lifestyle, preferences and health needs,” said Tan Ai Shan, Principal Dietitian from the Department of Dietetics at SGH.

1. 16:8 method

This time-restricted eating method involves fasting for 16 hours a day, leaving an 8-hour window during the day to eat food normally.

Some choose to skip breakfast and eat lunch and dinner; others choose to eat breakfast and lunch and skip dinner. Many may find this pattern easier to incorporate into their lifestyle.

2. 5:2 approach

In 5:2, you eat normally for five days a week, then cut back to 20 percent of your normal daily calorie intake (500 – 600 calories a day) for the other two days.

The two days of caloric restriction can be achieved using specially formulated meal replacements. You can discuss suitable options with your doctor or dietitian.

3. Alternate‑day fasting (ADF)

This method involves alternating between fasting days and days of regular eating.

On fasting days, individuals may either consume no calories or follow a very low-calorie intake (approximately 500 calories).

A modified version of ADF allows a small meal or limited calorie intake on fasting days.

4. One meal a day (OMAD)

This is a more restrictive IF approach involving approximately 23 hours of fasting followed by a short eating window with one main meal.

While OMAD has gained popularity, it has been less extensively studied than other commonly used IF approaches, and its restrictive nature may make it challenging for some individuals to meet their nutritional needs.

At the start, you may feel weak and have difficulty concentrating due to hunger. However, this should only be temporary, as it can take some time for your body to adapt to the new meal schedule.

“Some individuals may also experience symptoms such as headaches, dizziness, or irritability. From a nutrition perspective, it is important to maintain a balanced diet during eating periods, with adequate intake of protein, fibre, vitamins, and minerals to support nutritional needs and overall health,” Ms Tan added.

As for the long-term effects of intermittent fasting, evidence beyond six months remains limited, although research in this area continues to grow.

Longer-term evidence regarding its sustainability, safety, and health outcomes is still evolving, and the suitability of intermittent fasting should be individualised according to a person’s health status, lifestyle, and nutritional needs.

If you want to do intermittent fasting, there are precautions you can take to ensure it does not harm your body.

1. Do not fast for over 24 hours as it can lead to dehydration, irritability, fainting, lack of energy and inability to focus.

2. Maintain a well-balanced and nutritious diet during your eating periods. Include quality carbohydrate sources such as brown rice and wholegrains, lean protein, and a variety of fruits and vegetables to support adequate nutrient intake and promote fullness (avoid overeating or binge-eating).
   

3. Stay hydrated by drinking a minimum of 8 cups of water daily (equivalent to a total of at least two litres), or according to your individual needs. Water is allowed during the fasting period.

4. Do not overexert yourself when fasting (especially during exercise).

5. If you feel unwell for any reason, stop fasting and seek medical help.

Intermittent fasting is not safe for everyone. Those who should not do it are:

• Pregnant or breastfeeding women.

• Patients with kidney problems.

• Patients with liver cirrhosis.

• Patients who have any medical conditions (such as diabetes or high blood pressure) or who are on any medications are advised to seek advice from their doctor before embarking on an intermittent fasting program.

• People with low blood pressure.

• People with a history of eating disorders or mental health conditions affecting eating patterns.

• People with nutritional deficiencies.

• People with diabetes who are on insulin or drugs that can cause hypoglycemia (low blood sugar).

Ref: F26

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