What Is Intermittent Fasting?

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Medically reviewed by Isabel Smith, M.S., R.N., C.D.N.

You’ve probably heard of intermittent fasting. There are hundreds of thousands of posts dedicated to the trend on social media, and celebrities like Jennifer Aniston and Cameron Diaz swear it’s how they maintain their fit physiques.

Famous bods aside, the eating pattern has been linked to a number of health perks, including weight loss and reduced risk for Type 2 diabetes.

That said, if you were hoping that “intermittent” meant something different than regular ol’ fasting — it doesn’t. It still means that you’re not eating for long periods of time — maybe even the whole day, depending on the type of fasting method.

The practice of abstaining from food for periods of time has been around for centuries, but the long-term effects of intermittent fasting are still being studied. This is especially important to note for women and people assigned female at birth: Preliminary results from a recent study found that people who restricted their eating time to eight hours a day as part of intermittent fasting were more likely to die from cardiovascular disease, which is the number one cause of death of women in the U.S.

Read: What Women Need to Know About Cardiovascular Disease >>

Here’s what you need to know to get up to speed on intermittent fasting.

What is intermittent fasting?

Intermittent fasting is an eating plan that involves limiting your food intake to certain time periods. There are different types of intermittent fasting, but all are focused on when to eat — and when not to eat — rather than what to eat.

Types of intermittent fasting

There are several different types of intermittent fasting, which include:

Time-restricted eating: Here, you eat during certain hours of the day. For example, the 16/8 method means you fast for 16 hours and eat during the remaining eight hours left in the day. Another version, the 12/12 method, means you fast for 12 hours and can eat during the other 12-hour window.
Alternate-day fasting: This type of fasting is just like it sounds. You fast one day and eat the next with no dietary restrictions.
Periodic fasting (5:2): For two days out of the week (not back to back), you fast completely or only consume around 500 calories and, for the other five days, you eat a healthy diet.

Benefits of intermittent fasting

One of the more appealing aspects to intermittent fasting is that it’s pretty simple. You don’t need any special foods or ingredients or powders or tapeworms — just a clock and willpower.

But, does it really have any health benefits? To date, the research on intermittent fasting hasn’t been definitive. Few studies have been conducted, and the ones that have been conducted haven’t always had a lot of participants. Some studies revealed some potential benefits, but more research is needed to confirm the results.

Weight loss is one of the main benefits associated with intermittent fasting. When you’re fasting, you’re not consuming any calories, and taking in fewer calories can lead you to lose weight. That’s because your body exhausts its sugar supply/calories and starts burning fat when you go hours without food.

Less food equals less weight. This may seem like a no-brainer, but in reality, the lower-calorie approach may not work for everyone. Some studies have shown that intermittent fasting had little or no impact on long-term weight loss.

Weight loss aside, when it comes to health overall, intermittent fasting may have benefits for some people with certain health conditions.

Type 2 diabetes: Intermittent fasting has been linked to better blood sugar levels, enhanced insulin sensitivity, weight loss and a reduction in medication needed to manage the disease.
Overweight and obesity:Studies show that intermittent fasting has helped people living with overweight and obesity lose weight, although researchers noted that more studies need to be done on the connection.
High blood pressure: Intermittent fasting may help prevent hypertension. In one study, alternate-day fasting helped participants lower blood pressure and heart rate.

Intermittent fasting disadvantages

Have you ever felt positively murderous with hunger if you skip your afternoon snack or smoothie at breakfast? (Asking for a friend.) That might happen during intermittent fasting. Going without food for a long period of time can be mentally and physically trying and may cause hanger (hungry anger).

It’s worth taking an inventory of how hangry you become when you don’t eat before considering intermittent fasting.

Other side effects can include:

Changes in mood
Dizziness
Fatigue
Headache attacks
Low blood sugar
Constipation
Sleeping problems

Who shouldn’t try intermittent fasting?

Tendencies toward hanger aside, intermittent fasting can be dangerous for some people.

You should steer clear of intermittent fasting if you’re pregnant or breastfeeding because the reduction in calories can lead to nutrient deficiency. It’s also not recommended for the elderly or for people with certain health conditions. These include:

Type 1 diabetes
Osteoporosis
Disordered eating
A weakened immune system
Reactive hypoglycemia

Intermittent fasting and cardiovascular risk

Like all eating plans, there are risks and side effects to intermittent fasting, and this method may not be for everyone. As mentioned before, a 2024 study found people who limited their eating to an eight-hour window had a 91% higher risk for cardiovascular death compared to people who ate across 12-16 hours a day.

However, a 2024 review of 23 studies found that intermittent fasting could help lower total and bad cholesterol (LDL) levels and triglycerides and reduce fat tissue in the bodies of people living with overweight and obesity — all of which can help reduce the risk for cardiovascular disease.

So, that’s clear as mud, right? This is why it’s important to talk to your healthcare provider about your personal risk factors and medical history before starting any new eating plan.

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