Nutritionist Sarah Di Lorenzo: Fasting and time restricted eating, what is the difference and what works?

Fasting and time restricted eating: what is the difference and what actually works to avoid losing muscle ?

If you’ve spent any time in the health space recently, you’ve definitely heard both terms — intermittent fasting and time-restricted eating.

They are often used as though they mean the same thing and they actually don’t.

Understanding the difference matters enormously if muscle preservation is on your radar and if you are over 35, it absolutely should be. I am going to share the difference between them both.

Intermittent fasting is an umbrella term that describes any eating pattern that cycles between defined periods of fasting and eating.

This includes popular protocols such as the 5:2 diet (eating normally five days, then restricting to around 500 calories two non-consecutive days per week), alternate day fasting, or extended multi-day fasts.

On fasting days, your body is working with significantly fewer calories, which sets off a distinct set of metabolic responses compared to simply shortening the time you spend eating.

These metabolic responses include glycogen depletion, heightened autophagy, ketone production and elevated cortisol levels.

Time-restricted eating is a specific form of intermittent fasting where you eat all your daily calories within a defined window.

The typical windows are 8, 10 or 12 hours and then you fast for the remainder. There is no intentional caloric restriction with TRE, you eat your normal amount, just within a narrower timeframe.

The most common people do is the 16:8 protocol (16 hours fasting, 8-hour eating window) and most choose to fast until 12 and eat between 12pm-8pm.

So the key difference is time restricted eating is about when you eat and not how much and intermittent fasting is about eating less.

Both approaches do activate many of the same metabolic benefits such as improved insulin sensitivity, enhanced fat oxidation, reduced inflammatory markers, better glucose regulation, and metabolic flexibility (which means your body’s ability to switch between using carbohydrates or fat for fuel). There is a lot of research supporting benefits for both.

But what is reality compared to the research is what works best for real people in real life.

I find that TRE — the 16:8 or even the 10:14 approach is the most sustainable and clinically practical because it fits within a family meal, social eating and does not have a calorie count.

But what I have found with this in my clinical practice is that the majority of people can do the time frame but don’t really have weight loss as they eat whatever they like in the eating time frame. I have seen hundreds of patients still come to me for weight loss after years of skipping breakfast on the 16:8.

So where does this fit in when it comes to muscle as many people on these diets forget about the impact on their muscle. Without the right nutritional strategy, both fasting and TRE can cause muscle loss alongside fat loss.

This is particularly relevant for women over 40, where declining estrogen already accelerates muscle loss (sarcopenia), and for anyone doing regular resistance training.

When you fast for extended periods, the body can break down muscle protein for gluconeogenesis, basically using amino acids to create glucose for the brain and organs. The longer the fast, the more pronounced this risk.

If you are choosing to do IF or TRE to protect yourself against muscle loss be sure to hit your protein targets at meals and make sure the protein is around 1.2-1.6 grams per kg of your own body weight to maintain your muscle and reach the daily target. This is non-negotiable.

Be sure to be resistance training and always break any fast with protein first and after each training session to stimulate muscle protein synthesis.

I never recommend fasting too long, I feel that 16 hours is the maximum to support muscle, anything beyond 20 hours raises cortisol and increases the breakdown of protein working against you.

This is important, keep leucine intake high, leucine is a branched chain amino acid that is a key trigger for muscle protein synthesis. You can find it in eggs, red meat, chicken, fish, whey and dairy products, around 3g per meal is your target.

After years of working with so many different dietary methods what I feel is the best is 12:12. This is eating your last meal around 6pm and fasting until 6am.

I also recommend a combination of both in my weight loss programs, this being the 12:12 and for weight loss 600 calorie days with huge success. This way you are eating aligned with your circadian rhythms and hormones.

The bottom line really is that TRE can work when you combine it with adequate protein and resistance training.