Intermittent Fasting 101: What You Should Know
I’ve written about fasting before, but I feel like it has been enough time that I should probably take a look at what new research is out there on the topic. I get asked about fasting a whole heck of a lot – it hasn’t decreased in popularity, and it doesn’t seem to be going anywhere.
The claims around fasting are huge – decreased risk for disease, increased longevity, decreased insulin and blood glucose, weight loss, even ‘curing’ incurable conditions like diabetes.
But are these true, and if so, are the improved metrics we see with fasting, due to the fasting, or weight loss in general?
So many questions.
There are multiple fasting configurations, with the most popular being time restricted eating (TRE). The others are alternate day fasting and 5:2, which is 2 days a week of fasting for 24 hours, (up to 25% of normal caloric intake is considered a ‘fast,’) the other days with ‘normal’ caloric intake.
When fasting for weight loss and other supposed health benefits became popular, there wasn’t a lot of human research around it. There’s more now, but it’s also important to remember that what’s out there is mostly short-term. Is fasting effective and sustainable for the long-term?
We don’t really know. As a registered dietitian, I recommend choosing the eating pattern that works for YOU. Fasting definitely isn’t for everyone, as we’ll talk about that in a bit.
Let’s go over some of the claims around fasting.
Intermittent fasting for weight loss.
Fasting often does work for weight loss, but probably not for the reasons you may think.
You don’t have to look too hard to find media headlines saying some variation of ‘Research shows that fasting is effective for weight loss!’ They make fasting seem like a magic bullet, but is it? And is it more effective for weight loss and better for health than caloric restriction?
Fasting appears to impact a lot of metrics (and here) (and here) besides weight; lipids, blood pressure, insulin sensitivity, blood sugar, inflammatory markers…but why? Are these impacts the result of fasting, or the weight loss that often occurs from it?
This is where there’s a divide in belief systems.
Weight loss that results from normal caloric restriction aka dieting, also results in improvements in all of these markers. While fasting may show a greater effect initially, in longer-term results it appears as though there’s little to no difference between fasting and calorie restriction (and here).
This 2019 study in 8 pre-diabetic men suggested that even without weight loss, IF improved insulin sensitivity, blood pressure, and oxidative stress. The problem? The study was small and short (5 weeks), and only in men. Feeding also stopped after 3pm – their fast was 18 hours long.
Besides the fact that the study had major limitations, a 6 hour eating window that stops in the afternoon is probably going to be joyless and unsustainable for most people.
The study did bring up an interesting theory, that intermittent fasting helps us eat according to circadian rhythm, which may reduce disease risk. We already know that, though, and you don’t have to eat for only 6 hours a day to achieve it.
I wrote about eating and circadian rhythm in my post about chrononutrition, here.
Intermittent fasting, for most people, ends up being a method of cutting calories. Give yourself a limited time to eat, and you’re probably going to eat less. The research is overwhelmingly congruent with the fact that fasting isn’t any more effective than other forms on caloric restriction.
A 2022 study on intermittent fasting in the NEJM showed that compared to caloric restriction, fasting yields no difference in terms of weight lost. A 2018 review of studies in the International Journal of Obesity had similar findings..
The one benefit that often comes up in the IF versus calorie restriction debate is that IF is said to be easier to stick to long-term than CR. This can be an advantage, of course, but it’s not true for everyone.
Nothing works for everyone, and the moment someone tries to sell you on a diet that supposedly does, you know it’s a red flag right there.
Still, a lot of people find it pretty simple to do a 16:8 configuration: skip breakfast, start eating at noon, and stop eating by 8pm.
Intermittent fasting and metabolic switching.
A 2020 study on intermittent fasting in the NEJM suggests that fasting results in metabolic switching, which is when our bodies use free fatty acids and ketones instead of glucose for energy. While some people believe that metabolic switching aka metabolic flexibility promotes greater health and longevity, the research behind this theory has only been done in animals.
I did a deeper dive into the research behind metabolic switching in my post about Lumen, but the TL;DR of this state is that we aren’t exactly sure if metabolic inflexibility occurs pre or post disease, and what if any overall clinically relevant benefits it really has on our health.
Intermittent fasting and disease.
Fasting has been studied as an adjunct to cancer therapy (note: I do not recommend alternative treatments in place of conventional treatment for cancer), and some research suggests that fasting may increase sensitivity to chemotherapy and a reduction in chemo side effects.
But! And there is a but…people undergoing cancer treatment may lose large amounts of weight just from that. Adding fasting to the mix can make that even worse, compromising their health and their treatment course.
CALERIE – a multi-year randomized controlled trial with an 11% calorie restriction in non-obese adults, suggested that fasting can reduce not only weight, but disease risk factors like blood glucose, lipids, fasting insulin, insulin sensitivity, and inflammatory markers.
If someone is overweight, and they subsequently lose weight – even a small amount – this can potentially change their cardio metabolic risk factors as well. Again, is it the fasting, or the weight loss?
Does it matter? It does. Because despite what some nutrition bros and ‘meno-experts’ on social media want you to believe, fasting isn’t the only way to lose weight, and it doesn’t work for everyone.
Intermittent fasting and women and menopause.
Lots of women are told to fast to lose weight in menopause and ‘turn on’ their ‘fat burning’ pathways. Let’s put one thing to rest first, when I see claims like these targeting women, it makes me see red.
Fasting doesn’t ‘turn on’ fat-burning anything. That sort of red-flag rhetoric is just bad marketing language that’s developed to sell you something.
Burning fat is never as simple as just flipping some sort of switch, imaginary or otherwise.
I’m pretty sure that every single menopause-focused weight loss program I’ve seen, includes fasting *ahem* Galveston Diet *ahem*
The problem with this is that not eating for prolonged periods of time causes stress on your body. Do you need more stress at this time of your life, along with poor sleep, a foggy brain, hot flashes, and the general anxiety of midlife?
Probably not. And just to make things clear, diets that combine IF with keto are even worse. Why in the world would you want to take fasting and layer it with one of the most restrictive diets out there?
If you believe that you have to submit yourself to a punishing regimen like that to look a certain way, please examine your intentions for how you want to live your life. Valuing thinness over everything else is not akin to living your best life.
Remember quality of life? That’s still important.
In terms of pre-menopausal women, fasting can be a bad idea, hormonally speaking. Fasting causes stress on the body, which can lead to hormonal dysfunction. This can mean:
If you’re trying to get pregnant, fasting can mess with ovulation.
If you’re struggling with irregular periods, fasting may make that situation worse.
Will this happen in everyone? Of course not. And you don’t need to be extreme about fasting, if you choose to do it. More about what that means, in a bit.
Intermittent fasting for blood sugar and insulin.
While there’s a lot of claims being made about fasting and its effect on insulin leading to weight loss, that’s not actually a proven fact (it’s more a theory, like the CIM, which has been disproven…or at least, definitely NOT proven).
These effects aren’t surprising. Let’s look at some of the reasons why these things may happen:
Fasting means that you’re not eating all day long. When you limit the duration of your eating hours, this normally has an effect on insulin and blood sugars, both of which rise (even in very healthy people) when we eat.
Fasting can also cause weight loss. Weight loss can increase insulin sensitivity and lower blood sugars.
It’s not rocket science, and guess what? You may experience the same effects, even if you lose weight without fasting.
Insulin is a fat storage hormone, but weight loss can occur regardless of insulin levels – it’s just a matter of calorie restriction. You don’t need fasting for that.
Fasting for longevity.
Biohackers love fasting. Jack Dorsey apparently eats only 7 meals a week, and going without food for any period of time is almost a given among the most hardcore keto /carnivore devotees.
Proponents claim that the increased rate of autophagy may help slow the aging process as well as help to prevent disease and improve immunity.
Autophagy is the process of cell renewal and DNA repair in our bodies. While this process occurs naturally, fasting has been shown to increase its rate by causing stress in the body.
It does seem to have this effect – in animals and cells in lab dishes. But we aren’t animals or cells in lab dishes, and a fasting regimen in humans looks a lot different in the real world compared to in the controlled environment of a lab. There’s a lot of ‘we don’t know’ where the long-term health effects of fasting in humans are concerned.
I don’t know about you, but I want to actually live, not just exist. At what point do we stop chasing the myth of eternal life and youth and make the best of the life we’re living?
If I need to not eat in order to only possibly prolong my life by some unknown duration, that’s not exactly a tempting prospect. At least, not to me.
Fasting in short.
There are downsides to any sort of caloric restriction, especially if done in an extreme manner. Intermittent fasting isn’t alone in this. Severely limiting what you eat can cause side effects including nutrient deficiencies, mood disruption, muscle loss, hormonal disruption, psychological issues around food and eating.
There’s a thin line between fasting and starving, and while I’m seeing more people going for extreme fasts – days of not eating, or a 4-hour eating window, for example – I’d never recommend those. Water fasting, forget it. There’s just no compelling evidence that severe or prolonged restriction is ‘better’ than a more moderate approach.
I don’t recommend any sort of diets, and I think fasting is more of an eating pattern than a diet. That being said, for many people, intermittent fasting is a way of life that doesn’t interfere with their physical or emotional health. For them, fasting is just another tool in the calorie-cutting toolbox.
If it works for you, that’s great.
If it doesn’t work for you, move on.
There’s no evidence that intermittent fasting is better for weight loss than any other calorie-cutting method, and you can definitely lose weight – even in menopause – without fasting. As a dietitian, I wouldn’t feel comfortable telling someone to fast. Some RDs do. If someone wants to fast, I’ll support them, IF I think it’s appropriate for them.
It’s all a tradeoff:
What are your expectations?
What outcomes do you want?
What are you willing to do to get those things?
How much will it enhance your life?
How much will it disrupt your life?
Will it leave you better, or worse, physically AND emotionally, than before you started?
If fasting ticks the boxes for you, go ahead and try it. Just know that in a lot of cases, the jury is still out in terms of effects.
Note: who shouldn’t fast.
Fasting isn’t for everyone. Here’s who shouldn’t try it:
Pregnant or nursing women
Anyone with a history of or tendency towards eating disorders or disordered eating
Anyone who is clinically underweight
If fasting makes you feel unwell in any way