I got a ton of questions from patients and readers about the recent UCSF study on a form of fasting called time-restricted feeding (TRF), being ineffective for weight loss.  This is a perfect opportunity for me to dig deeper into how fasting can either be a powerful tool that potentially promotes weight loss and enhances overall health, or potentially does the very opposite.

I also recommend you listen to my podcast titled “Have We Taken Fasting Too Far?” which you can listen to on my site here or on your favorite podcast player (Meta Health Podcast episode #7).

The Study

This was a randomized control study (the highest quality study design) looking at 116 overweight or obese adults that were split into the following 2 groups:

  1. CMT (Consistent Meal Timing) Group: This group was told to eat 3 structured meals per day, but they could eat ad libitum, meaning anything they want at whatever quantity they want.  Participants were also allowed to snack between meals.
  2. TRE (Time-restricted Eating) Group: This group was also allowed to eat ad libitum, but only between the hours of 12p to 8p.  This is the popular 16/8 fast where you eat for 8 hours and fast for 16 hours.

The study lasted 12 weeks and the primary outcome was weight loss and secondary outcomes were fat mass, lean mass, fasting insulin, fasting glucose, and A1C levels and a few others.  Some of the findings were as follows:

  1. Weight Loss: Both groups lost a little weight, but there was NO difference between the 2 groups.  In other words, TRE was no better than eating 3 meals a day.
  2. Body Composition: No significant change in whole body fat mass between both groups, but there was a significant decrease in lean body mass (muscle) in the TRE group but not the CMT group.  In other words, the fasters lost significantly more muscle.
  3. Cardiac Risk Markers: There were no significant overall differences in lipids, glucose, fasting insulin and other cardiometabolic markers.  Interestingly there was a significant drop in the systolic (top blood pressure number) blood pressure for the CMT group but not the TRE, while the opposite was noted for the diastolic (bottom blood pressure number) where the TRE group had a significant drop but not the CMT group.
  4. Oura Ring Data: The Oura ring measures sleep, activity, and other metrics like heart rate variability and temperature.  There was a significant reduction in daily movement in the TRE group but not the CMT group.

So in summary, when compared to CMT, TRE showed no improved weight loss, caused more muscle loss, had no appreciable impact on heart disease and metabolic risk markers and the Oura ring data showed a reduction in daily movement which might imply a lower energy state.  Is this a nail in the coffin for intermittent fasters?

News Headlines

I applaud the UCSF team for performing the highest quality study design to investigate whether TRE is effective.  However, as a reminder from my previous writings, recall how nutrition studies, even when done under “rigorous conditions,” are the bottom of the barrel when it comes to research.  Unfortunately these studies deteriorate from study to news headlines to lay reader interpretations.  I discussed this destructive process in detail in this post where I dissect the anatomy of nutrition headlines while analyzing a study on low carbohydrate diets.

The degenerative process from study to lay interpretations further led to e-mails and questions to me asking “I heard fasting doesn’t work” or “Did you read that fasting makes you lose muscle?”  and so on.  In other words, the practice of fasting, which has been around for thousands of years has been not only questioned but deemed ineffective (by many lay readers and even researchers) through a single 3 month study involving just over 100 participants.  Read the headline below published in UCSF’s own newsletter

The main title says TRE doesn’t work and the subtitle connects TRE to intermittent fasting (IF), so the conclusion from just skimming this UCSF-published newsletter is that TRE and IF are ineffective for weight loss.  I was especially surprised by the bold and definitive nature of this headline (“Doesn’t Work”) given the intrinsically flawed and uncertain nature of nutrition studies, including this one.

Now let’s back off these headline generalizations and get specific.  This study did not cover all forms of fasting, deeming fasting an altogether ineffective practice.  It looked at a very specific form of fasting called time-restricted eating (TRE).  The study was also insufficient in saying TRE doesn’t work, because it further used a very specific eating window from 12p-8p.

This study looked at TRE as a singular intervention in the absence of other lifestyle practices (prescribed exercise, specific nutrition guidelines during eating window, sleep and stress management, etc.). Is this sufficient to put a nail in the coffin for fasting?  Let’s discuss further.

Muscle Loss Is An Issue

When I look at research headlines on interventions that I use in my clinical practice, I try to connect study findings with my anecdotal clinical findings, in addition to looking at a larger body of research and time-tested ancestral traditions (if available), to make more definitive assumptions, but not conclusions.

Researchers and clinicians need to keep an open mind as they explore lifestyle interventions like fasting, and for physicians like myself and others who have used it successfully in practice for years, reading definitive headlines and conclusions from researchers who have little to no clinical experience in using fasting therapeutically leaves me a little frustrated.

The findings from this study that do resonate for me are the loss of muscle.  This is a real finding which I see over and over in my patients who fast improperly.

Fasting is a double-edged sword where one edge of the blade can cut body fat, but the other edge can also cut essential muscle if fasting is not prescribed in conjunction with other lifestyle changes like optimal protein intake and strength training.

This is why I’ve written extensively on the importance of protein intake here and the importance of muscle mass here.  For those of you who are my patients, you know how much I emphasize protein intake and strength training and have highlighted these in my book as well.

One of the greatest benefits of fasting done the right way is reduced hunger.  My patients lose their craving for excessive carbs, sweets, and processed foods.  That’s a really good thing that individuals eating 3 meals and 2 or more snacks (the CMT group in the study was allowed to snack) rarely experience.  The down side of reduced hunger, and this is occasionally an issue for me, is you have to literally remind yourself to eat sometimes and when you do eat, you might actually undereat.

I see this often in my patients who have adopted fasting.  Their caloric intake is often low and their protein intake is often abysmal.

Individuals who combine fasting with a sedentary lifestyle or an exercise program that neglects adequate strength training will inevitably lose muscle and eventually gain back body fat very quickly.

The UCSF study findings correlate with this same observation I’ve seen in the clinic for years, and they even cite another study that highlights this point when they mention “the extent of lean mass loss during weight loss has been positively correlated with weight regain.”  Again, refer to my protein and muscle posts mentioned earlier for context and guidance.

The most powerful trigger for muscle preservation and growth is sufficient strength training.

You can’t guzzle protein shakes and eat protein bars while sitting all day, expecting that dietary protein will spontaneously grow muscle without you doing any meaningful physical work.

This will likely contribute to caloric overload and weight gain, or go to the liver to be converted to glucose.  Strength training must be combined with adequate protein intake.

Finally, most of my patients can’t easily tell that they are losing muscle.  Muscle deterioration (aka sarcopenia) can happen in a subtle manner if you are not tracking it, and looking at yourself naked in a mirror won’t tell the story.

This deterioration progresses more rapidly as we age, and I have many patients who are 50 and older who don’t realize that the majority of their weight loss is coming from muscle loss, not fat loss.  Yes their belly might be flatter, but their muscles are also deflating which to me is an ominous sign.

Preservation of muscle is one of the most significant markers of longevity and overall quality of life, so it must be tracked.

If you are familiar with my work, you should know about how important muscle is as a glucose disposal organ.  After meals, around 80% of the glucose goes to your muscle parking lot to be stored or burned for energy.  Progressive muscle loss will reduce your ability to clear glucose from your blood stream, a major contributing factor to insulin resistant conditions (prediabetes, diabetes, heart disease, etc.).  This is a situation where fasting improperly can make insulin resistance and blood glucose control worse.

Doing specific, measurable weight training exercises also serve as a key barometer of your muscle strength.  Are you making slow but steady progress on your lifts and/or body weight exercises over time?  I know my pull-up max, my push-up max, my squat max without weights.  I also know my max for weighted deadlifts, lunges, squats, bench press, etc.

I’ve been using resistance bands recently and know how many reps I can do with different exercises using different bands.  I know my 1 mile run time, my 5K time, my 400m sprint time, and my vertical leap.  You get the picture and no, you don’t need this many metrics, but do choose a few.

Objective measurements of strength and endurance are at least as important and I would argue more important than regular measurements of lipids, glucose, and other common biometrics.

These are your performance vital signs.  For endurance measurements, please read my VO2 max post here if you haven’t.  Measuring limb circumferences and chest span may be a rough tool for measuring muscle mass, but it doesn’t always reflect strength.  For example, my legs are getting progressively stronger from exercise, but they are not necessarily getting much visibly larger.  Functional strength is far more important than visible size increases.  Arms and legs can grow larger from fat and bloating from excess glycogen and water, which doesn’t translate into significant strength improvements.

When it comes to body composition, there are 3 categories of body composition to keep in mind and I’m going to expand on the 2nd one because it is the one that is most easily missed, with potentially devastating consequences on long term health and quality of life.

1. A gain in body/belly fat with a loss of muscle strength: This is the absolute worst outcome.  Increase in inflammatory belly fat with a reduction in metabolically active, longevity-promoting muscle.  Avoid this at all costs through proper nutrition and exercise.

2 A loss in body/belly fat with a loss of muscle strength:  This is a common scenario in my fasters or patients that are undereating.  I see this so often in my seniors and/or vegetarians who are undereating calories, especially protein.

The problem is they think they are healthier because their waistline is slimmer, but their loss in muscle strength, especially in their legs, contributes to future disability, dementia, impaired immunity, and chronic disease.

I recently saw a 68 y/o Indian male in the clinic who I’ve known for years.  He was proud to report a weight loss of 15+ pounds during the quarantine due to intermittent fasting and deliberately cutting back his overall animal protein intake.  I congratulated him on his visibly smaller belly, but I noticed since he always wears shorts and a t-shirt, that his limbs had become much slimmer.  He had loose skin on his arms and legs and no appreciable muscle mass.

This was highly concerning to me.  He stopped weight training since his gym shut down.  I prescribed a higher protein diet (mostly plants and some animal) and gave him a guide with free weights and resistance bands that he could start using at home.  He e-mailed a few weeks later telling me he was absolutely shocked at how much strength he had lost from stopping weight training and just walking exclusively for exercise.

I love walking and recommend it to patients, but it is NOT strength training and it barely builds any muscle to counteract our mostly sedentary lives.

I estimate that the majority of this patient’s weight loss was from muscle.  Beware of this phenomenon in yourself and in any elderly family members, especially those who have taken on a more restrictive diet (fasting, vegetarian/vegan, etc.).  If you need more vegetarian sources of protein, read my post here.  There are plenty of plant-based bodybuilders and athletes who are able to gain strength without meat.

Don’t use your vegetarian status to under consume protein. Just remember if you aren’t regularly assessing strength and training, you can rapidly lose muscle mass without noticing it.  Using a personal trainer is another way to help you train and track muscle if you need a guide and further motivation.  It is one of the best investments you can make in your health and the health of those you care about.

3. A loss in body/belly fat with a gain in muscle strength:  This is the ideal combination we all need to strive for.

So bottom line is If you are fasting and getting simultaneously weaker and more winded despite losing weight, that’s a key sign that you are losing significant amounts of muscle mass and mitochondrial density.  Not only are you at a high risk for regaining body fat, you are shortening lifespan instead of extending it.

Timing and Frequency

The other pitfall in this study was the time interval used.  I think 8p is a late ending to an eating interval, especially in overweight/obese low metabolic individuals who are told they can eat whatever they want in unlimited quantities.  Unlike metabolically health individuals who can clear glucose rapidly after meals, these individuals take up to several hours to clear glucose.  They are likely going to bed with elevated sugars.

I have played around with the eating end times in different patients and although the optimal end time varies, most benefit by finishing all their eating by 6 or 7p and others have done even better when  finishing earlier (4 or 5p).  The point is you need to follow your own metrics and experiment with different eating windows.  This study just used one and made a comprehensive and conclusive statement.

This would be analogous to me doing a similar study on patients with high blood pressure and giving the study group a microdose of a blood pressure pill and comparing them to a placebo group who took no medication.  By the end of the study I would likely find no difference in benefit due to such a small dose, but this would be due to an insufficient dose of the blood pressure pill.  Would my overall conclusion be blood pressure pills are therefore ineffective at lowering blood pressure, despite there being multiple classes and doses of different blood pressure pills?  You get the picture.

How often TRE is dosed during the week can be variable.  I have patients who thrive by doing TRE every single day.  Others just need it a few times a week.  I personally do a version of fasting called IF, but instead of standing for intermittent fasting, it’s Intuitive Fasting.  I can tell when I need to give my body a longer break from food with a fasted exercise session, while on other mornings I wake up and know I need to eat breakfast.  Again, self-experimentation is key here.  Apart from intuition, I might occasionally check quantitative measures to guide me (weight, glucose, ketone levels, etc.).

Fasting Must Be “Prescribed”

The big message from this post is that fasting is a drug that comes in different formulations and doses and must be personalized and prescribed just like a drug, if we are using it therapeutically to achieve a specific goal (weight loss, reduce cholesterol, etc.).  This is similar to exercise.  Exercise at the wrong dose and frequency can promote injury, excessive hunger, and other complications.  Done properly, exercise can burn body fat and have endless other health benefits.

If you want to enhance the effect of fasting, you must do the following:

  • Self-experiment to find the right interval and dosing regimen
  • Pay attention to what you are eating during the eating window.  Sufficient protein, plants and other nutrient dense foods are essential.  Schedule meals if you tend to under eat and stock up on protein sources or use a healthy meal delivery service to keep you on track.
  • You must combine fasting with an exercise program that includes strength training
  • Track yourself periodically by monitoring body metrics (weight, waistline, body fat possibly), metabolic numbers (lipids, glucose, etc.), and performance (strength and aerobic).  If you are getting weaker and/or endurance is taking a hit during exercise, you may need to refeed and put a pause on fasting while you refuel.
  • Consider working with a dietitian or doctor who is experienced with prescribing and personalizing fasting.

One other key point I want to make is to pay attention to hunger after you become healthier.  When we chronically overeat and eat the wrong foods, our hunger signal is aberrant and driven by the addictive nature of foods and other miscues like chronic stress and sleep deprivation, rather than authentic biological hunger.

When you stop overeating and eat clean, then hunger becomes a natural signal you must pay attention to.

I worry about my overdriven patients who fast every single day for 18+ hours despite their intensive work and workout schedules.  It’s very difficult to squeeze in all the necessary protein and nutrients into a 6 hour window on a daily basis, and some days your body might need more, especially if you are exercising hard.  Don’t try to discipline your way out of hunger.  I’ve done that before and paid the price.

I hope this post helps you understand the finer nuances of fasting and also serves as another of my recurrent reminders of being highly skeptical of conclusive headline studies, especially involving the field of nutrition.


Be sure to listen to my podcast episode “Have We Taken Fasting Too Far?” which you can access on my site here or on your favorite podcast player (Meta Health podcast, episode #7).