Intermittent Fasting: What Does the Science Really Say?

Intermittent fasting is a topic I hear about almost every week in my clinic. People often ask:

“What do you think of intermittent fasting?”

The information online can be confusing - and sometimes misleading - so it’s worth stepping back and looking at what the evidence actually tells us.

This article explains what intermittent fasting is, how it works, and what research says about its benefits and limitations.

What Is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and fasting. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat.

Its popularity has grown because of claims about weight loss, improved metabolism, and even longevity. Many people are drawn to it because it feels simpler than thinking about what to eat.

There are several common approaches.

16/8 Plan (Time-Restricted Eating)

• Eat within an 8-hour window

• Fast for 16 hours

  
  

For example, if you finish dinner at 8 p.m., you skip breakfast and eat again at noon the next day.

5:2 Diet (Whole-Day Fasting)

• Eat normally for five days per week

• Consume approximately 2000–3000 kJ (500–700 kcal) on two non-consecutive days

  
  

Eat-Stop-Eat

• Fast for a full 24 hours once or twice per week. For example, finish dinner at 7 p.m. and do not eat again until 7 p.m. the next day.

  
  

Intermittent Fasting and Weight Loss

Intermittent fasting may help with short-term weight loss, but we don't have any long term data.

When you fast, you typically reduce your overall calorie intake. This can create an energy deficit, and sustained energy deficits lead to weight loss.

However, research consistently shows:

Intermittent fasting produces similar weight loss to traditional daily calorie restriction.

It works for some people as they eat less overall, not because it triggers unique fat-burning pathways.

Some animal studies suggest fasting may shift the body toward using stored fat and alter lipid metabolism. However, robust long-term human evidence demonstrating superior metabolic effects is lacking.

During fasting periods, levels of ghrelin - often referred to as the “hunger hormone” increase. Ghrelin stimulates appetite and signals your body to seek food.

For some individuals, this increase in hunger can lead to:

• Overeating during eating windows

• Larger portion sizes

• Choosing higher-calorie foods

• Feeling “out of control” when breaking a fast

  
  

In these cases, total calorie intake may not decrease and will probably increase.

When intermittent fasting is compared with continuous calorie restriction (eating slightly less every day), weight loss outcomes are generally similar. This suggests that the total calories consumed over time matter more than the timing of those calories.

Glycaemic Control and Blood Sugars

Intermittent fasting may:

• Improve insulin sensitivity

• Lower fasting insulin

• Modestly reduce HbA1c in some people with type 2 diabetes

  
  

However, these improvements are largely explained by weight loss itself.

There is currently no strong evidence that intermittent fasting improves blood sugar control independent of weight reduction.

People with diabetes - especially those using insulin or sulfonylureas must approach fasting cautiously due to the risk of hypoglycaemia. Medication adjustments and medical supervision are essential.

Heart Health and Lipids

Some studies report modest improvements in:

• Triglycerides

• HDL (“good”) cholesterol

• Blood pressure

  
  

But these changes are generally comparable to those seen with continuous calorie restriction.

Again, benefits appear to be driven primarily by weight loss rather than by fasting itself.

Insights from a 2025 Review in the British Medical Journal

A major 2025 systematic review and meta-analysis published in the British Medical Journal (BMJ) provides some of the strongest evidence to date.

The review included:

• 99 randomised controlled trials

• Approximately 6,500 adults with overweight or obesity

• Study durations ranging from 3 to 52 weeks

  
  

Researchers compared:

• Whole-day fasting (e.g., 5:2)

• Alternative-day fasting

• Time-restricted eating

• Continuous low-calorie diets

• Eating without restriction

  
  

Key Findings

• All forms of intermittent fasting led to modest weight loss compared with eating normally.

• Alternative-day fasting resulted in about 1.3 kg more weight loss than continuous calorie restriction but only in studies lasting less than 24 weeks.

• Whole-day fasting (5:2) and time-restricted eating showed no clear advantage over a standard low-calorie diet.

  
  

The Important Limitation

None of the trials lasted longer than one year.

This means we do not yet know whether intermittent fasting provides sustained long-term benefits or whether weight regain occurs similarly to other dieting approaches.

In short: We have reasonable short-term data. We lack strong long-term evidence.

Disadvantages and Risks of Intermittent Fasting

While intermittent fasting works well for some individuals, it is not suitable for everyone.

1. Sustainability

Some people experience:

• Intense hunger

• Irritability

• Low energy

• Difficulty concentrating

• Overeating during eating windows

  
  

Social events, shift work, family meals, and cultural eating patterns can make strict fasting windows difficult to maintain.

If an eating plan leads to cycles of restriction and overeating, it is unlikely to be sustainable and can lead to or trigger binge eating disorder.

2. Not Appropriate for Certain Groups

Intermittent fasting is not recommended for:

• Pregnant or breastfeeding women

• Children and adolescents

• Individuals with a history of eating disorders

• Underweight individuals

• Frail older adults

• People with certain chronic medical conditions

• Individuals taking medications that require food

  
  

In these situations, risks outweigh potential benefits.

The Bigger Picture: Quality of Diet Matters

While meal timing can influence energy intake, overall dietary quality remains critical.

Dietary patterns with strong long-term evidence - such as the Mediterranean diet - consistently show benefits for:

• Sustainable weight management

• Cardiovascular health

• Metabolic health

• Reduced mortality

  
  

Mediterranean diet

This way of eating emphasises vegetables, fruits, legumes, wholegrains, fish, nuts, and olive oil — and has decades of supportive research behind it.

The Bottom Line

Intermittent fasting is not harmful for most healthy adults and can support weight loss.

However:

• It is not superior to standard calorie restriction.

• Its benefits largely reflect reduced calorie intake.

• Long-term evidence beyond 12 months is limited.

• It is not appropriate for everyone.

  
  

If intermittent fasting fits your routine and helps you maintain healthy habits, it can be a reasonable option.

But there is no magic eating window.

Sustainable weight loss and long-term health depend on:

• Consistent dietary quality

• Adequate protein and fibre

• Physical activity

• Sleep

• Behavioural support

The best nutrition plan is one that supports your health, your lifestyle, and your long-term wellbeing, not just the latest trend.

Are you looking for guidance from a registered dietitian to help with meeting your health/fitness goals or manage a health concern like type 2 diabetes or other dietary needs? Get in touch for expert support tailored to your goals! Book your appointment here.

Joanna Tsintaris, Specialist Dietitian & Biomedical Scientist

Joanna Tsintaris is the Founder of Nourish Dietitian—an online health and nutrition clinic offering personalised 1:1 dietetic advice. She is a registered Dietitian in the UK and Biomedical Science graduate with a first-class MSc in Nutrition and Dietetics. Specialising in weight loss and Type 2 Diabetes with an interest in sports nutrition, Joanna empowers clients to make sustainable lifestyle changes, addressing conditions like pre-diabetes, PCOS, and more. With a holistic approach, she fosters a healthier relationship with food, challenges weight stigma, and rejects diet culture. Her clinical expertise, coupled with a commitment to evidence-based practice, ensures clients receive compassionate and person-centred care. Beyond her professional pursuits, Joanna embodies the principles of holistic well-being in her own life, whether practising yoga, enjoying brunch, or savouring moments with coffee in hand.