In late April 2022, headlines were plastered across the internet calling for the death of intermittent fasting. This was in relation to a New England Journal study out that week titled “Calorie Restriction with or without Time-Restricted Eating in Weight Loss.” I would have named it, “If you ask the wrong question, then you don’t get a helpful answer.”
The researchers asked if people lose more weight while on a calorie-restricted diet in an 8-hour eating window versus the same calorie restriction, but no special eating window. The rigorous study showed conclusive results that the narrow eating window wasn’t any better for weight loss than eating all day. Women ate 1,200 to 1,500 calories and men 1,500 to 1,800 calories. Both groups lost 14 to 18 pounds. This was the question they wanted to answer, but it’s not the right question for most of us.
The problem with the study was the calorie restriction itself. This eating style is a diet and has long been blamed for yo-yo dieting. People restrict calories for a set amount of time, then when they get tired of restricting, they eat more and regain weight. The number on the scale goes up and down, like a yo-yo.
One of the most unhealthy issues with calorie-restricted diets is that they promote the avoidance of healthy fats. I think of it as the twisted sister of the 1980s low fat craze that we now know to be unhealthy. A person can eat a greater volume of food for the same calories if they avoid fats, so they are basically incentivized to avoid fat if they are restricting calories. Healthy fats, like olive oil, avocados, and nuts are nutrient-dense and thus have more calories than other foods. However, they have important health benefits. The olive oil you dribble on your salad will give you more calories than if you put on only vinegar, but olive oil is proven to decrease your chance of death from cardiovascular disease, cancer, and neurodegenerative diseases like Alzheimer’s. So you’re avoiding a very healthy food to cut calories.
But can these healthy fats also help with weight loss?
An important hormonal model of obesity points to high insulin levels, often caused by insulin resistance, as causing more fat storage in human bodies. Healthy fats do not increase insulin, so they are a helpful part of this model’s weight loss eating plan. An eating plan that includes healthy fats decreases insulin levels and leads to both healthy and more comfortable weight loss. The body releases fat naturally with lower insulin levels. On the Harvard Healthy Eating Plate, meals ideally include protein (meat or beans), fiber (vegetables, fruits, whole grains), and healthy fats. Consider the salad mentioned earlier. If you dribble on the olive oil, the healthy fat will help you feel full longer, so you wouldn’t feel hungry an hour afterward. Including healthy fats decreases the need for an insulin spiking snack between meals. So eating healthy fats aligns your hormones toward weight loss.
But what does this have to do with intermittent fasting? Doctors advising people on losing weight by decreasing their insulin levels also suggest intermittent fasting because the fasting part of the day allows the body more time with lower insulin levels. These doctors are not advising people to eat a calorie-restricted diet.
So don’t give up your eating window just yet! The good question I am hoping researchers will answer next: who loses the most weight? Is it the human who eats three meals and no snacks that include protein, fiber, and healthy fats, or the human who eats two or three meals of that same formula in an 8-hour eating window? I recommend people eat healthy fats, like olive oil, at all their meals for good health and weight loss. We need more studies on intermittent fasting that do not include calorie-restricted diets. I’m not giving up on intermittent fasting yet.
Heather Awad is a family physician.
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