Doctor Nguyen Hoai Thu, from the Vietnam Institute of Applied Medicine, states that many individuals practice intermittent fasting—an eating pattern that alternates between fasting and eating periods within fixed windows, sometimes fasting for 14-16 hours daily—yet fail to achieve effective fat loss due to three common mistakes:

Starting too quickly and extreme fasting

Many individuals err by immediately adopting an intermittent fasting regimen, such as the popular 16:8 method (fasting for 16 hours and eating only during the remaining window), from day one. This sudden reduction in the eating window can shock the body, leading to headaches, dizziness, irritability, and ultimately, premature abandonment of the diet.

A gradual approach allows your body to adapt, reducing the risk of fatigue, irritability, or giving up prematurely. Crucially, this method fosters consistency, a factor often more critical for long-term success than the duration of the fast itself.

Excessive calorie reduction causes muscle loss

Many mistakenly believe that prolonged fasting means eating as little as possible or skipping meals drastically. In reality, the focus of this method is the timing of meals, not extreme dietary restriction or severe calorie reduction. Without adequate nutrients, individuals may experience issues such as fatigue, nutritional deficiencies, and loss of muscle mass.

According to the Recommended Dietary Allowances for Vietnamese People (2026), to protect muscle mass and maintain resting metabolic rate (RMR), the diet during the eating window must still ensure a balanced macronutrient distribution (AMDR):

Carbohydrates: 50-65% of total energy.

Fats: 20-25% of total energy.

Proteins: 13-20% of total energy (equivalent to a minimum of 0,93 g protein/kg/day for adults).

Overeating during the eating window

Many mistakenly believe that after 16 hours of fasting, they can eat whatever they desire. Consuming excessive amounts of food, especially ultra-processed items, fried foods, or sugary drinks, leads to a calorie surplus, completely negating the energy deficit required for weight loss.

Studies comparing intermittent fasting with traditional calorie-restricted diets show comparable weight loss when the total calorie intake is equal. The eating window merely serves as a supportive tool; total energy intake remains the decisive factor.

Overeating during the eating window is one reason why you may not lose fat even when fasting for 16 hours daily. Illustration: Tu Nguyen

Doctor Thu notes that intermittent fasting is a time-restricted eating method chosen by many for weight management and metabolic health improvement. However, incorrect application not only diminishes weight loss effectiveness but also leads to exhaustion, muscle loss, and hormonal imbalances. The expert does not recommend intermittent fasting for the following groups:

- Children and adolescents undergoing physical development.

- Pregnant or breastfeeding women.

- Individuals with a history of eating disorders (anorexia, bulimia).

- People with certain medical conditions requiring regular medication (such as diabetes, stomach issues).

Many individuals achieve better results by starting with shorter fasting periods. Instead of immediately adopting the 16:8 model, they can begin with an overnight 12-hour fast—for example, eating dinner at 8 p.m. and breakfast at 8 a.m. the next day—and then gradually extend the fasting duration as their body adapts.

One of the most crucial principles when practicing prolonged fasting is to monitor your body's reactions. When the method is suitable, many people experience more stable energy, improved concentration, reduced cravings, better digestion, and gradual weight loss over time. Conversely, if symptoms such as dizziness, headaches, irritability, insomnia, difficulty concentrating, constant thoughts about food, or decreased exercise performance appear, these may signal that your fasting period is too long or that intermittent fasting is not suitable for you.

Thuy Quynh