Diets Dissected: Intermittent Fasting
Welcome to Diets Dissected, a new blog series where we team up with Western’s registered dietitians to take a closer look at popular diets. Our goal? To give you clear, fact-based information so you can make informed decisions about what works best for your body and your lifestyle.
Before we dive in, a quick note: At Western, we don’t take a one-size-fits-all approach to nutrition. In fact, we’re not big fans of restrictive diets. Our philosophy is more holistic, body-positive, and rooted in long-term health, not quick fixes. That said, we also know that some diets can offer benefits when used thoughtfully and with good guidance. That’s why we’re here to unpack the science, talk through the pros and cons, and help you sort out what’s worth considering.
In this post, we’re exploring intermittent fasting—a way of eating that’s gained a lot of attention in recent years. Maybe you’ve heard about people skipping meals to “reset” their metabolism or following time-restricted eating windows to support weight loss or energy levels. But what does the research actually say? Let’s take a closer look.
What is intermittent fasting?
Intermittent fasting isn’t so much a traditional “diet” as it is an eating pattern. Rather than focusing on what you eat, it centers around when you eat. The idea is simple: you cycle between periods of eating and fasting over a set schedule.
There are a few different ways people approach intermittent fasting, but all of them involve dividing the day or week into eating windows and fasting periods. For example, a common method is the 16:8 approach, where you fast for 16 hours and eat only during an 8-hour window (say, between 12:00pm and 8:00pm). Another method is the 5:2 plan, where you eat normally five days a week and limit calories significantly (around 500–600 calories) on the other two days. There’s no single “right” way to do it. It depends on what feels sustainable and aligns with your routine.
But why would anyone willingly go hours—or even a full day—without eating?
The idea actually has roots in human history. Long before supermarkets, meal prep apps, and snack drawers became part of daily life, humans were hunters and gatherers who often went long stretches without food. Eating three meals a day (plus snacks) is a relatively modern invention. Our bodies have adapted over time to handle gaps between meals, tapping into stored energy when food isn’t readily available.
Fasting has also played a significant role across cultures and religions for centuries. People have fasted for spiritual reasons in practices linked to Islam, Christianity, Judaism, Buddhism, and others. Today, some people turn to intermittent fasting for more secular goals, such as improving energy levels, simplifying their eating habits, or supporting metabolic health.
In short, intermittent fasting is a structured way to limit when you eat, with the idea that giving your body regular breaks from digestion can have physical and possibly even cognitive benefits.
Types of intermittent fasting
Intermittent fasting isn’t a one-size-fits-all approach. There are several different methods, and each one structures fasting and eating windows a little differently. If you’re curious about trying it, it’s helpful to understand the various styles so you can find one that suits your body and lifestyle.
Here are some of the common types:
16:8 Method (Time-Restricted Eating)
This is one of the most popular (and beginner-friendly) forms of intermittent fasting. You fast for 16 hours and eat during an 8-hour window each day. Some people skip breakfast and eat between 12:00pm and 8:00pm, while others shift the window earlier or later to fit their schedule. The idea is to give your body a break from digestion for most of the day while still eating balanced meals during your eating window.
5:2 Method (Twice-a-Week Fasting)
With this method, you eat normally five days of the week and significantly reduce your calorie intake on the other two (nonconsecutive) days. On fasting days, many people aim for around 500–600 calories total. The rest of the week, there are no time restrictions. This approach can be a good fit for those who prefer more flexibility day-to-day.
Eat-Stop-Eat
This method involves fasting for a full 24 hours once or twice per week. For example, you might eat dinner at 7:00pm and then not eat again until 7:00pm the next day. While it’s a more intense form of intermittent fasting, some people find it works well when done occasionally. It’s generally recommended for those who already have some experience with fasting and feel comfortable going longer stretches without food.
Alternate-Day Fasting
As the name suggests, alternate-day fasting involves alternating between days of normal eating and days of fasting. On fasting days, some versions allow around 500 calories, while others eliminate food entirely. While it can be effective for some people, this approach may be more challenging to sustain and isn’t right for everyone, especially if you’re very active or sensitive to dips in energy levels.
Crescendo Method
This gentler approach involves fasting for 12 to 16 hours, two to three days a week. The fasting days are spaced out—say, Monday, Wednesday, and Friday—giving your body time to adjust. It’s often recommended for beginners or those who want to ease into intermittent fasting without diving in all at once.
While these are some of the most commonly practiced forms of intermittent fasting, it’s important to note that longer fasting periods—like 36, 48, or even 72 hours—can carry risks. Going too long without food can cause your body to conserve energy by storing fat, and may lead to dizziness, fatigue, or even nutrient deficiencies if not done under medical supervision.
As always, if you’re thinking about trying intermittent fasting, it’s a good idea to check in with a healthcare provider or registered dietitian, especially if you have underlying health conditions, are pregnant, or have a history of disordered eating.
Potential benefits of intermittent fasting
Intermittent fasting has gained a lot of attention for its potential health benefits, and while the research is still evolving, there are some promising findings. Below are a few of the most commonly discussed benefits, many of which go beyond just weight loss.
Weight Loss and Body Fat Reduction
One of the main reasons people try intermittent fasting is to support weight loss, and for some, it works well. By shortening the window of time in which you eat, you may naturally take in fewer calories without consciously cutting back. It can also help regulate hunger hormones and reduce mindless snacking.
Some research suggests that alternate-day fasting, in particular, may be especially effective for weight loss. In one study, participants who alternated between 24 hours of fasting and 24 hours of unrestricted eating consumed significantly fewer calories overall and lost an average of over seven pounds in just four weeks. While results vary, many people find that fasting helps them simplify their eating habits and create a calorie deficit without feeling overly restricted
Improved Insulin Sensitivity
Intermittent fasting may also help your body respond more efficiently to insulin, the hormone that helps regulate blood sugar levels. When your body becomes less sensitive to insulin, blood sugar can rise and increase the risk of type 2 diabetes. By giving your body regular breaks from food, some studies suggest that intermittent fasting can reduce insulin resistance and help keep blood sugar levels more stable over time.
Reduced Inflammation
Chronic inflammation has been linked to a range of health issues, including heart disease, cancer, and autoimmune conditions. Some early studies suggest that intermittent fasting may reduce markers of inflammation in the body, though more research is needed to fully understand how and why this happens. Still, the preliminary findings are encouraging.
Brain Health
There’s also growing interest in how intermittent fasting might support cognitive health. Animal studies (particularly in rats) have found that fasting can increase levels of a brain-derived hormone known as BDNF, which supports the growth of new nerve cells and may protect against neurodegenerative diseases. While human studies are more limited, the connection between fasting and brain health is an exciting area of ongoing research.
Physical Performance
For athletes or active individuals, intermittent fasting may offer some performance-related benefits. Some small studies in humans have shown that fasting—especially the 16:8 method—can lead to fat loss while preserving lean muscle mass. In animal studies, fasting has been linked to improvements in endurance and energy efficiency. However, results can vary depending on the individual, and it’s important to time workouts and meals in a way that supports recovery and energy needs.
Potential downsides of intermittent fasting
While intermittent fasting offers some potential benefits, it’s not without its challenges, and it’s certainly not the right fit for everyone. Like any significant change to your eating habits, it’s important to consider both the risks and how your body might respond before diving in.
Hunger and Adjustment Period
Let’s start with the obvious: you might get hungry. Really hungry.
One of the most common early side effects of intermittent fasting is increased hunger, especially if you're used to grazing throughout the day. It can take anywhere from two to four weeks (or longer) for your body and brain to adjust to a new eating schedule. During that time, it’s normal to experience hunger pangs, irritability (hangry anyone?), and low energy. For some people, this side effect fades as their body adapts; for others, it remains a dealbreaker.
Fatigue, Dizziness, and Mood Swings
In addition to hunger, some people experience fatigue, lightheadedness, headaches, or mood swings, especially during the fasting window. These symptoms can make it harder to concentrate, work out, or just get through your day. If you're someone with a demanding schedule or who is highly active, this may be a significant drawback.
Digestive Changes
Digestive changes are another potential downside. Some people experience constipation while fasting, likely due to reduced food and fiber intake during shorter eating windows. Staying hydrated and eating fiber-rich foods during your eating window can help, but it’s something to watch for.
Menstrual and Hormonal Disruption
Intermittent fasting doesn’t affect all bodies the same way, and research suggests that it may have a different (and sometimes more negative) impact on women. Female bodies are particularly sensitive to energy restriction, and fasting for extended periods or too frequently can disrupt hormone signaling.
The hypothalamus, a part of the brain that regulates reproductive hormones, can be affected by low calorie intake. When this happens, the body may reduce the production of key hormones like luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which can lead to irregular periods, fertility issues, and even weakened bone health.
That said, not all research shows negative outcomes. A small 2021 study of women with PCOS (polycystic ovary syndrome) found that intermittent fasting helped regulate menstrual cycles and reduce body fat. This highlights how individual factors, like health status and fasting frequency, can shape outcomes.
Blood Sugar Complications
While intermittent fasting may improve insulin sensitivity in some people, there’s evidence that this benefit isn’t universal. One study found that blood sugar control actually worsened in women after three weeks of intermittent fasting, while men in the same study saw improvements. This difference underscores the importance of a personalized approach, especially for women or individuals managing blood sugar–related conditions.
Possible Heart Health Concerns
Research is still emerging when it comes to how intermittent fasting affects heart health, and results have been mixed. While some studies have linked intermittent fasting to improvements in blood pressure, cholesterol, and inflammation, others suggest that certain fasting patterns—like 16:8—might be associated with increased cardiovascular risk. At this stage, there’s no clear consensus, and more long-term research is needed.
Lack of Long-Term Data
Speaking of research, one of the biggest limitations of intermittent fasting is that we simply don’t have much long-term data. While short-term studies show some promising effects, we still don’t know how sustainable or safe intermittent fasting is over the course of many years. This is especially important for people with underlying health conditions, athletes, or anyone considering intermittent fasting as a permanent lifestyle change.
How to know if intermittent fasting is right for you
While intermittent fasting can work well for some people, it can also pose risks for others. It’s important to consider not just the potential benefits, but also how this type of routine might affect your body, energy levels, hormones, and mental health.
Who Should Not Try Intermittent Fasting:
Some people should avoid intermittent fasting altogether, including:
Children and teens under 18 – Growing bodies need consistent nourishment throughout the day to support development, activity, and cognitive function.
People who are pregnant, breastfeeding, or trying to conceive – Fasting can interfere with important hormonal and nutritional needs during these stages. Nutrient intake should remain steady and sufficient.
Individuals with type 1 diabetes who take insulin – Skipping meals or fasting can lead to dangerous drops in blood sugar and make diabetes management more difficult.
Anyone with a history of eating disorders – Intermittent fasting may trigger disordered behaviors or reinforce a restrictive mindset, especially in those with past struggles around food and control.
Who Might Be a Candidate for Intermittent Fasting:
That said, intermittent fasting may be worth exploring for some people, especially when done under medical guidance. You might consider it if:
You’re generally healthy and looking for a structured way to reduce snacking or late-night eating.
You find that limiting your eating window helps you feel more mindful about food choices or simplifies your daily routine.
You’re interested in potential metabolic benefits, such as improved insulin sensitivity or fat loss, and are working with a provider to monitor those outcomes.
You’re not experiencing significant stress, fatigue, or hormonal imbalances, and your energy needs are relatively stable throughout the day.
If you do try intermittent fasting, it’s important to monitor how your body responds. Are you feeling energized and balanced, or are you constantly distracted by hunger or struggling with mood swings? Do you find yourself overly focused on food during fasting windows? These are signals worth listening to.
Considerations from a dietitian
Thinking about trying intermittent fasting? Before you jump in, it’s important to remember that how you eat during your eating window is just as important as when you eat.
Here are a few key things Stephanie Nelson, one of Western’s registered dietitians, encourages you to consider:
Quality Still Matters
Just because you're limiting the number of hours you're eating doesn't mean it's a free-for-all during that window. Think whole grains, lean proteins, healthy fats, and plenty of fruits and vegetables. “Balanced eating habits are still essential,” Stephanie explains. “You just have less time to fit them in—so planning is key.”
Snacking Can Be Tricky
If you’re someone who naturally snacks throughout the day, adjusting to a condensed eating window can be a real challenge. “It can be difficult to meet your calorie and nutrient needs with small snack-sized meals alone,” she says, “You may need to be more intentional about eating complete, satisfying meals that support your energy and activity levels.”
Find the Right Eating Window for You
There’s no one-size-fits-all when it comes to choosing an eating window. Stephanie recommends finding a schedule that matches your energy needs and lifestyle. “For some people, that means starting the eating window earlier in the day, especially if you’re active in the morning or afternoon,” she says. “A well-balanced breakfast-type meal at the start of your window can set the tone for sustained energy throughout the day.”
Meal Planning Is Your Friend
Since intermittent fasting limits the number of opportunities you have to fuel your body, planning ahead becomes even more important. If you have specific nutrition goals, like increasing protein or getting more fruits and vegetables, you’ll want to be intentional about what you’re eating and when.
Bottom line from Stephanie? Intermittent fasting might be worth exploring, but only if it supports, not restricts, your overall nutrition and well-being.
Thinking about giving intermittent fasting a try?
Intermittent fasting isn’t a magic solution, but for the right person, it can be a helpful tool to support overall health, weight goals, or mindful eating habits. Like any approach to nutrition, its success depends on your individual body, lifestyle, and relationship with food.
If you’re interested in learning more about intermittent fasting or just want help figuring out a sustainable, personalized nutrition plan, Western’s registered dietitians are here to help. Our team can work with you to create a plan that honors your goals, your health history, and your daily life.
Let’s take the guesswork out of nutrition together. Schedule a one-on-one consultation with a Western dietitian today.
