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What You Really Need to Know About Intermittent Fasting

Updated: May 6

Written by: Natalie Faella, MS, RDN, LDN | November 18, 2021

empty plate with clock, fasting, intermittent fasting, what time to eat

Intermittent fasting (IF) is a more recently popular diet trend where a person follows cycles of fasting (i.e., not eating) for scheduled, extended hours throughout the day. Do I have to go on, or are you already sold that this doesn’t sound great?

If you’re not sold yet, I’ll provide a little more information. IF is sometimes described as a non-diet because it doesn’t tell you what foods to eat, just when to eat (and not eat) throughout the day. It recommends fasting for 16-hours daily, i.e., not eating from say 8PM to 12PM the next day, or to fast for 24-hours twice per week. (Ok, now are you sold?!)

There are people all over the world, without access to enough food and here we are, skipping meals BY CHOICE to follow some fad diet. Gotta love diet culture.

You might be thinking, “who would want to do that?” which is a fantastic reaction. What gets people on board, like with any diet, are the too-good-to-be-true promises it boasts. The proposed health benefits associated with IF include: weight loss, reducing the risk for and helping treat type 2 diabetes, decreasing belly fat, being good for your brain, preventing cancer and heart disease, etc.

Remember, you’re not at fault for having curiosity or trying out diet trends. Who wouldn’t be interested after reading some of these claims? Perhaps if we were told how diets, like IF, impact us long-term we wouldn’t be so keen on trying them out. So that is my goal for this blog. Let’s take a better look at each of these claims:

1. Myth: IF results in weight loss.

“But I lost weight while I was fasting and then gained it back after I stopped. Doesn’t that mean fasting works?” No, that means fasting does not provide long-term results. Weight will likely come off at first but eventually will plateau and rebound, like with any restrictive eating or fad diet. Even if you continue fasting. Beyond the fact that eating in this way may not be sustainable (what if you get invited to an early brunch? Or a late dinner? Or are just, IDK, hungry outside of the allotted eating hours?!), these long periods of fasting can set the scene for over eating when it is “allowed.” Kind of like how “cheat days” on diets can feel a little rebellious, you may overindulge (i.e., eat cheat-like meals or eat more than you normally would) when you’re within allotted eating hours.

Even if you have the strongest willpower, biochemical changes [like the increase in cortisol (stress hormone) that results from the decrease in blood sugar levels] that result from fasting trigger the body’s craving for food. The likelihood of craving simple carbs like sweets, chips, bread items, etc. is also increased because those provide quick energy and will get your blood sugar back to normal more efficiently than other foods would.

Besides its instability and resulting biochemical changes, IF will also make you less in tune with your hunger and fullness cues because this diet, like most, is asking you to ignore them and only eat within certain hours each day. So even if you aren’t “cheating” during your eating hours, you could very likely be eating past the point of fullness. These are some of the reasons why IF can eventually lead to yo-yo dieting and weight gain.

Sorry, had a lot to say about that one. On to # 2:

2. Myth: IF reduces the risk and is helpful for type 2 diabetes.

Some of the lifestyle factors associated with increased risk for type 2 diabetes include: cigarette smoking, diets high in saturated fat, frequent spikes in blood sugar (from a high sugar diet or like those that result from eating a lot after fasting), physical inactivity, and weight cycling or yo-yo dieting. (Read that last one again -— it's a side effect of IF and other diets, remember?). Diabetics have an issue with insulin, a hormone that helps take sugar (glucose) from the bloodstream and into our cells so that it can be used for energy. This process helps regulates blood glucose levels, which are naturally higher after we eat, by bringing them back to normal levels (as glucose leaves the blood stream and enters our cells). Diabetics either don’t make insulin/make enough of it, or their body can’t use the insulin they have (insulin resistance).

Regulating blood sugar levels is actually one of our body's hardest jobs; the body is constantly trying to balance these levels all day long so that our blood sugar levels don't get too low or too high. I can only see IF creating more chaos with regulation of blood sugar levels and insulin in diabetics. Think about it — you fast for 16 hours, so your blood sugar levels get real low. (And this is after being in a "fasted" state overnight while we sleep!) This alone, low blood sugar, is a dangerous thing diabetics need to avoid. (My point is essentially already made, but I’ll continue). So then you start eating and are likely to overeat or eat a lot quickly because your blood sugar is low triggering that urgency for food (plus, you’re starving), so your body releases more insulin than it would if you just had a typical meal portion. This can eventually lead to another low blood sugar level, reach for sweets/carbs to bring that level back to normal, potentially overeating again, and the cycle continues.

Plus, these blood sugar highs and lows come with a whole slew of unpleasant symptoms. The best way to prevent these highs and lows is by eating balanced meals and snacks every 3-4 hours. This is actually part of the treatment plan for diabetics — the exact opposite of the recommendations for IF.

3. Myth: IF decreases belly fat.

First, I just want to make it clear that nothing, besides liposuction, can remove fat from a specific, targeted area of the body. No food you eat, vinegar you shoot, or amount of hours not eating will reduce fat solely in the abdomen. Period! You may have noticed this yourself or heard others say that when they lose weight, they lose it from X part of their body and not Y (ex: lose weight from their face and not their hips). This is because our sex, hormones, and metabolism all play a role in where we lose and gain weight, which is why it is different from one person to the next. Plus, increased cortisol levels are associated with weight gain (fat accumulation) in the abdomen region, so IF could actually have the OPPOSITE effect.

4. Myth: IF is good for brain functioning.

Those who do IF sometimes report having more energy and alertness in the morning. This feeling is real, but misleading. As blood sugar levels remain low (from skipping breakfast after an overnight fast), cortisol and epinephrine (adrenaline) levels rise, which signal the "fight or flight" response in the body. This stress response increases alertness and energy in the short term. But chronic stress wreaks havoc on the body so that perceived benefit does not last and is largely outweighed by the cons associated with IF.

It’s probably no surprise that our brain would require a decent amount of energy seeing how it is needed to regulate every single system at play in our bodies — all day, and all night. And its preferred fuel source: carbohydrates (because remember those provide fuel the quickest? i.e., quicker than proteins or fat). This is one of the reasons why breakfast is so important. When we skip breakfast or go long hours without eating, our brain’s energy dwindles just like our physical energy can and we can notice things like: brain fog, difficulty concentrating, and feeling lethargic, irritable, and faint. With IF, we could be running on fumes for hours in the morning and into early afternoon. (Or all day if one opts for that 24 hours fast).

5. Myth: IF helps prevent cancers and heart disease.

While genetics play a major role we can’t ignore here, there are some lifestyle factors we know of that can reduce our risk for developing certain cancers, like not smoking cigarettes for example. Having a diet that is rich in fruits and vegetables is another lifestyle choice that helps to reduce risk. Diets that are not only rich in fruits and vegetables, but also whole grains, healthy fats, and lean proteins also help to reduce the risk of heart disease. I don’t think eating these foods only within allotted hours helps to reduce that risk more so than if you were to eat regularly/normally throughout the day. Also, IF reduces our number of daily eating opportunities. So by eating throughout the day (breakfast, lunch, snack, dinner), it provides more times to fit in those disease-reducing foods.

Some other IF side effects that aren’t often mentioned:

1. IF can negatively impact mood.

Putting “hangry” aside here, proper functioning and regulation of neurotransmitters, like serotonin and dopamine (the ones that help regulate our mood/make us feel happy) relies on adequate nutrient intake. Therefore if we’re fasting or not receiving proper nutrition, it can negatively impact our mood. Plus, serotonin also helps regulate appetite. So when our mood is dysregulated, so are our appetite cues. And remember how fasting can increase cortisol? That adds a layer of feeling more stress and anxiety as well.

2. IF can impair sleep.

You know how stress and racing thoughts can keep us up at night? That is because that rise in cortisol can make it harder to fall asleep and stay asleep through the night. Plus, it’s been shown that IF can reduce the amount of REM sleep we get.

3. IF can cause hair loss.

Hormonal changes can also cause hair loss. Just ask anyone you know who has been pregnant, on or off the pill, experienced a very stressful time in their life… Plus, lack of nutrients, which can result if you’re trying to cram a day's worth of nutrition into short window of time, can also cause hair loss among other things.

4. IF can cause menstrual abnormalities.

Irregular periods or loss of it altogether are another side effect of IF. Hormonal changes and calorie deficits in general are to blame here, too. And while this doesn’t sound so bad on a personal level, loss of a period reduces estrogen levels, which actually play a large role in bone health. This can contribute to weaken bones and osteoporosis at a much younger age than typically expected. (I’ve seen this in females as young as 17 who had lost their period). Another thing to note regarding females — most IF studies have been conducted on animals or males, so there is little data on how this diet specifically impacts women's health.

Please tell me you're sold now?

Bottom line, like with all fad diets and information we receive from the diet industry, the claims of IF are too good to be true. Yes, weight may come off initially as you limit intake to certain hours of the day, but the long-term effects of calorie restrictive diets are hormonal dysregulation and weight gain. You should always questions the benefits of diet trends and reach out to a registered dietitian if you are looking for answers!

Just a little food for thought — that you can have at any hour of the day :)

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Amy Adams
Amy Adams
Nov 18, 2021

Love this! :)


Nov 18, 2021

Another excellent article unpacking the negative effects of diet culture and this specific diet.

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