By Carla Gerryts, Registered Dietitian
Fasting is hardly a new concept. Many individuals have practised fasting for religious, cultural, or personal reasons for generations. However, in recent years, fasting has re-emerged in mainstream nutrition conversations. It comes up in discussions with my patients without fail, often promoted as a powerful tool for weight loss, metabolic health, and longevity.
I also realised that people define fasting differently.
Fasting refers to voluntary abstinence from food for a defined period. Unlike traditional calorie restriction, which consistently reduces daily intake, fasting focuses on when food is eaten rather than how much (chrono nutrition). Intermittent fasting (IF) is an umbrella term that encompasses several structured eating patterns that alternate between fasting and eating periods.
Common Fasting Approaches
- Time-restricted eating (TRE), such as the popular 16:8 method. Food intake occurs within an 8-hour window and fasting occurs for 16 hours.
- Alternate day fasting (ADF), requiring fasting every second day, often allowing very low-calorie intake (approximately 25% of the requirements) on fast days.
- The 5:2 method. This consists of normal eating five days per week and significantly restricting calories on two non-consecutive days.
- OMAD – One Meal A Day, at the timing of your choice.
- Religious fasting, such as Ramadan, where fasting occurs daily from sunrise to sunset.
Each of these methods differs in intensity and practicality, and not all approaches are equally suitable for every individual.
Fasting and Weight Management
These days, one of the primary reasons people adopt fasting is weight management. Research consistently shows that fasting can result in modest weight loss. A large randomised clinical trial published in JAMA Internal Medicine found that alternate-day fasting produced approximately 6% weight loss over one year, comparable to traditional daily calorie restriction.
It may come as a surprise that fasting is not necessarily superior to conventional calorie reduction, but may serve as an alternative strategy that suits the lifestyle of some individuals better. For individuals navigating food noise and disordered eating patterns, it can be easier said than done to just reduce portion size. Some people find fasting easier to adhere to than daily calorie restriction because it simplifies decision making — instead of constantly monitoring portion sizes, they follow structured eating windows.
Metabolic Benefits
Fasting may positively influence metabolic health. Some research suggests that intermittent fasting can improve insulin sensitivity, glucose regulation, and lipid profiles — critical for the management of conditions such as type 2 diabetes and metabolic syndrome. Furthermore, fasting has been associated with reductions in fasting insulin levels and improvements in cardiovascular risk markers.
Emerging evidence also suggests that fasting can enhance metabolic flexibility — the body’s ability to switch between using glucose and fat as fuel. This shift promotes fat utilisation and may contribute to improvements in body composition and cardiometabolic health.
Limitations and Cautions
Despite its potential benefits, fasting is not without limitations. Maintaining muscle mass through the weight loss process can be challenging, especially for females. Adherence is another challenge — in the JAMA trial, the alternate-day fasting group had the highest dropout rate. For many, fasting may be difficult to maintain long term.
Individuals who should avoid fasting, or at least discuss it comprehensively with their healthcare provider, include:
- People with diabetes using insulin or glucose-lowering medication
- Individuals with a history of eating disorders
- Pregnant or breastfeeding women
- Children and adolescents
- Individuals with high energy demands from their occupation or sport/exercise programme
I often see individuals who unintentionally under-consume essential nutrients while fasting. Skipping meals can reduce the overall intake of protein, fibre, vitamins, and minerals — particularly if eating windows consist of convenience foods rather than balanced meals. The importance of quality cannot be understated. Consuming highly processed foods within a restricted eating window will not confer the same benefits as eating balanced, nutrient-dense meals.
A Tool, Not a Miracle
Fasting is a tool and not a miracle solution. Research shows that its benefits stem largely from calorie reduction and improved dietary structure, rather than unique metabolic effects alone. For some, fasting may provide a structured approach that supports weight management and metabolic health. For others, it may be unsustainable, inappropriate, or even harmful. The key is that the route you choose should be sustainable, nutritionally adequate, enjoyable, and tailored to individual needs and context.
From practical experience with patients: males tend to benefit more in all areas when fasting. Females in menopause have the poorest body composition outcomes when fasting — suggesting a hormonal implication that should be considered.
In the end, your body is your best messenger. Listen to it.