Intermittent Fasting and Metabolic Health: Impact on Body Weight

Intermittent fasting (IF) is a form of time-restricted eating in which specific periods of little to no calorie intake are alternated with designated eating windows. Several approaches exist, with common methods including the 18:6 model, where food intake is restricted to a 6-hour window followed by 18 hours of fasting, and the 5:2 approach, where calorie intake is significantly reduced on two days of the week while eating patterns remain relatively normal on the remaining days. (1)

The rationale behind intermittent fasting is that prolonged fasting periods may shift the body’s energy utilisation away from glycogen stores and toward greater fat oxidation. Glycogen, the body’s primary short-term energy reserve derived from carbohydrates, is gradually depleted during fasting, leading to an increased reliance on stored fat as a fuel source. (2) This metabolic shift leads to the assumption that intermittent fasting may be an effective strategy for reducing body fat and promoting weight loss.

While this mechanism is physiologically plausible, an important question remains: does current scientific evidence actually support intermittent fasting as an effective method for long-term fat loss?

What does the evidence say?

• A systematic review and meta-analysis from 2025 included 15 randomised controlled trials examining the effects of intermittent fasting on body composition in obese adults and found that both time-restricted eating and alternate-day fasting led to a reduction in body weight of a pooled average of 3.73kg. This 3.73 kg is the additional loss compared to a control diet over ~12 weeks. However, the authors noted no significant reduction in waist circumference. Additionally, most studies did not go beyond 12 weeks, with no follow-up reporting, therefore making it difficult to determine whether these effects are sustainable long-term. (1)

• Another systematic review and meta-analysis from 2024 looked specifically at fat loss in overweight and obese adults undergoing time-restricted eating across 20 studies and reported that 12 studies observed a reduction in fat mass. Subgroup analyses suggested that an eating window of 6-8 hours reduced fat mass compared to 10-12 hours. A subgroup analysis also showed no lean mass reduction when time-restricted eating was combined with exercise, highlighting the importance of muscle maintenance during fasting. (3)

• A systematic review from 2025 included 13 randomised controlled trials with female participants only. The meta-analysis showed a modest weight loss in the time-restricted eating group but no significant effects on fat mass and visceral fat. (4)

• A systematic review and network meta-analysis from 2025 compared different types of intermittent fasting approaches and their efficacy for weight loss. The authors reported that overall intermittent fasting reduced body weight, with alternate-day fasting showing the largest short-term reduction in body weight. Ninety-nine papers were included in this analysis, involving both healthy and unhealthy participants. However, the interventions varied substantially between studies, and the long-term sustainability of the observed effects remains unclear. (5)

What does that mean in practice?

• Intermittent fasting can be an effective tool for reducing body weight and fat mass, particularly in overweight and obese individuals.

• However, long-term effects remain difficult to determine as most studies only include intervention periods of 12 weeks. Additionally, rapid dietary changes often produce stronger short-term results, which may not necessarily reflect long-term sustainability. (5)

• The importance of exercise is consistently highlighted, not only to potentially enhance weight loss during intermittent fasting, but also to help preserve lean body mass.

• Greater attention should also be given to the quality of meals consumed during eating windows, ensuring that they are nutritionally adequate and provide sufficient overall protein intake.

Key takeaway

Intermittent fasting can be a useful tool for weight loss and fat mass reduction. However, rather than being a unique fat-burning solution, it should be viewed as one possible dietary structure that may help some individuals to maintain a sustainable energy deficit. Ultimately, long-term weight loss is often more likely to be driven by sustainable habits, adequate nutrition, and regular physical activity.

References:

1. Wang B, Wang C, Li H. The impact of intermittent fasting on body composition and cardiometabolic outcomes in overweight and obese adults: a systematic review and meta-analysis of randomized controlled trials. Nutr J. 2025;24(1):120. doi:10.1186/s12937-025-01178-6.

2. Anton SD, Moehl K, Donahoo WT, Marosi K, Lee SA, Mainous AG 3rd, et al. Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity (Silver Spring). 2018;26(2):254-68. doi:10.1002/oby.22065.

3. Xie Y, Zhou K, Shang Z, Bao D, Zhou J. The effects of time-restricted eating on fat loss in adults with overweight and obese depend upon the eating window and intervention strategies: a systematic review and meta-analysis. Nutrients. 2024;16(19):3390. doi:10.3390/nu16193390.

4. Chen S, Zhang X, Kortas J, Liu H. Effects of time-restricted eating on body composition and metabolic parameters in overweight and obese women: a systematic review and meta-analysis. Front Nutr. 2025;12:1664412. doi:10.3389/fnut.2025.1664412.

5. Semnani-Azad Z, Khan TA, Chiavaroli L, Chen V, Bhatt HA, Chen A, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. doi:10.1136/bmj-2024-082007.

Disclaimer: This article is for educational purposes only and does not replace medical advice. Individual needs vary, and personalised guidance from a qualified healthcare professional is recommended for tailored support.