What’s new in intermittent fasting research? More not-so-good news

On Nutrition

If doing the same thing over and over and over again and expecting different results is a fruitless endeavor, what does this say about intermittent fasting research? A year or so ago, I wrote about some of the less-than-sparkly research on this enduringly trendy method of dieting. The 300-plus intermittent fasting studies from 2022 offer more of the same.

A few studies, including one I will talk about shortly, did look promising, but they had concerning flaws. I also stumbled upon some intermittent fasting studies from the 1960s and 1970s, confirming that there really is nothing new under the sun when it comes to dieting — and if any of these diets worked they wouldn’t intermittently fall out of favor. I will give these early studies credit for the truthful audacity of some of their language. One 1970 study even used the term “intermittent starvation.” Indeed.

But back to the present. Overall, this latest round of studies continues to find that intermittent fasting isn’t any better than good old-fashioned calorie restriction for weight loss and improving health markers such as blood sugar and cholesterol levels. Considering that caloric restriction hasn’t proven to be good for those things either — except maybe in the short term — that’s not a ringing endorsement for intermittent fasting.

One study, published last spring in the New England Journal of Medicine, compared time-restricted feeding — limiting eating to an 8-hour window each day — to daily calorie restriction and found no difference. Because participants were only followed for 12 months, we don’t know if any benefits observed in that time frame persisted.

These lackluster results echo two significant studies from the past few years, both published in JAMA Internal Medicine. The first trial enrolled 100 participants for a year and found no differences in weight or health outcomes between alternate-day fasting — eating 25% of calorie needs on one day, then 125% the next — and restricting calories to 75% of calorie needs every day. The second similar-sized study found that time-restricted feeding was also no better than eating throughout the day.

Now I want to highlight one of the flawed 2022 studies. When results of this small randomized clinical trial from China, which concluded that intermittent fasting could reverse Type 2 diabetes, were published in December, it got a lot of good press from writers I suspect did not read the entire study. (It’s behind a paywall.) I did read the entire study, and Houston, we have some problems. First are the ethical issues — the researchers enrolled participants and completed most of the study before they registered it with the clinical trial registry. That’s a big no-no, and the study should never have been published for that reason. It also appears that the researchers may have only enrolled people they were confident would be successful. This means the results cannot be reasonably applied to real-world, everyday people. Of the 36 participants randomly assigned to the fasting protocol, less than half achieved “remission,” and those were more likely to be people who hadn’t had diabetes for very long.

What alarms me about the attention given to this study is that there’s more than one kind of diabetes, and a lot of people proselytizing about intermittent fasting. A 2020 paper published in Journal of the Endocrine Society related the case of a 58-year-old woman with stable Type 1 diabetes — an autoimmune disease in which the body stops making insulin altogether — who started a ketogenic intermittent fasting diet based on advice from a friend. She ended up in the emergency room with diabetic ketoacidosis. That’s when the body doesn’t have enough insulin to allow blood sugar into the cells for energy, so it breaks down fat for fuel, resulting in a dangerous buildup of ketones in the blood. She was lucky to get prompt emergency care, because she could have died.

Another thing that concerns me about the continued popularity of intermittent fasting is what this might mean for older adults. I recently had two relatives in their 70s who started fasting every morning because their doctor recommended it for weight loss. There are so many problems with that, that I barely know where to start.

First, once you’re out of your 30s, your body’s default is to break down muscle rather than repair it. To maintain and possibly build new muscle, you need to be intentional about it by engaging in physical activity and eating enough protein. Not only does research show it takes more protein to kick-start muscle building as we get older, but the timing of that protein matters.

Eat three meals a day with adequate protein — about 25 to 30 grams, or 3.5 to 4 ounces — at each, and you give your body three opportunities to build new muscle cells. Skip a meal, or eat a meal with little protein, and you miss one of those opportunities. It doesn’t matter if you end up eating a lot of protein — say an 8-ounce steak or a huge chicken breast — at another meal. Your body can only break down about 4 ounces of that steak or chicken breast from that meal into the amino acids that are the building blocks of new muscle cells. The rest will just be converted to usable energy or stored as fat. So it’s not about getting enough protein in a 24-hour period, it’s about distributing that protein across your day.

The bottom line is that the accumulating human research does not demonstrate that intermittent fasting is better than calorie restriction for weight loss, and there’s no good evidence that either method produces long-term weight loss. Any type of weight loss from calorie restriction includes muscle loss (yes, even if you’re also exercising), but intermittent fasting also reduces opportunities to eat a protein-rich, muscle-supporting meal. Maintaining muscle is important for healthy aging, and not just because muscle loss increases the risk of falling, which can lead to fractures. Just some food for thought.

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