What Science Says About Intermittent Fasting and Weight Loss
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Intermittent fasting has become a popular way to manage weight and improve health. From skipping breakfast to fasting for entire days, people use many different approaches. But how well does it actually work? And how does it compare with traditional dieting where you simply eat fewer calories every day?
A recent study published in The BMJ looked at these questions by analyzing 99 clinical trials involving over 6,500 adults. Here's what they found in simple terms.
Different Types of Intermittent Fasting
Intermittent fasting (IF) is not one single method. It includes:
- Time-restricted eating (TRE): Eating all meals within a specific time window each day, like 8 hours, and fasting for the rest.
- Alternate day fasting (ADF): Eating every other day or greatly reducing calories every other day.
- Whole day fasting (WDF): Fasting completely for one or two full days each week, such as the 5:2 diet.
The study also compared these with:
- Continuous energy restriction (CER): A daily calorie-reduction plan, often used in traditional weight loss diets.
- Ad-libitum diet: A normal eating pattern without specific restrictions.
Intermittent Fasting Can Help With Weight Loss
The study found that all types of intermittent fasting and continuous calorie restriction helped people lose weight when compared to an unrestricted diet. Most people lost between 1.5 and 4 kg (3 to 9 pounds), depending on the method and how long the study lasted.
Among the fasting methods, alternate day fasting showed slightly more weight loss than others, especially in shorter studies. On average, it led to about 1.3 kg more weight loss than continuous calorie restriction. However, these differences were small and may not be very noticeable in everyday life.
Effects on Other Health Markers
Besides weight, the researchers looked at other health indicators like cholesterol, blood sugar, and blood pressure.
- Cholesterol and fats in the blood: Alternate day fasting reduced total cholesterol and triglycerides more than time-restricted eating.
- Blood sugar and insulin: All diets led to small improvements in fasting glucose and insulin resistance, but no major differences were seen between the fasting methods and daily calorie restriction.
- HbA1c and HDL (good cholesterol): No significant changes were seen across any of the diets.
This suggests that while fasting can be useful for weight loss, its impact on other health markers may depend on the individual and the specific method used.
What About Long-Term Effects?
The benefits of intermittent fasting appeared stronger in short-term studies (under 24 weeks). In longer studies (over 24 weeks), all diets still helped with weight loss, but the differences between methods became less clear. This might be due to people finding it harder to stick to strict diets over time.
In fact, some of the longest studies showed that adherence to fasting diets dropped significantly after several months. That means the best diet may be the one a person can stick with in the long run.
Are There Any Risks?
Most studies reported few or no serious side effects. Some people experienced mild issues like hunger, dizziness, or constipation. One study reported a fall due to low blood sugar, but such events were rare.
Final Thoughts
This large analysis shows that both intermittent fasting and traditional calorie restriction can support weight loss and improve some health markers. Alternate day fasting may offer slightly more benefits in the short term, but differences between methods are generally small.
If you're considering a fasting approach, the best choice may come down to personal preference and lifestyle. Some people find it easier to skip meals, while others prefer to eat smaller portions every day. The key is choosing a plan you can maintain consistently and safely.
As with any diet, it's best to talk to a healthcare professional before making big changes—especially if you have existing health conditions or take medications.