This Is What Happens To Your Body Physically (And Emotionally!) When You Fast
Whether you’re brand new to fasting or have been doing some type of intermittent fasting for years, it’s no secret that the popular diet trend is everywhere nowadays. Intermittent fasting (IF), in which you eat and abstain from food during certain times of the day. But what actually happens to your body as it moves through different stages of fasting?
Fasting may help activate your metabolism, increase longevity, and can even mimic the health benefits of aerobic exercise, says Kelsey Costa, MS, RDN, a dietitian and nutrition consultant for the E-Health Project. That said, fasting isn’t for everyone. There are many potential side effects, and limiting your diet may not be best for everyone's body or lifestyle. So, you should always consult with your doctor or a registered dietician before starting a fasting protocol.
Below, experts explain the four stages of fasting, what happens during each part of the fasting timeline, and who should (and shouldn’t!) try fasting.
Meet the experts: Raj Dasgupta, MD, is a physician and the chief medical advisor for Sleep Advisor. Kelsey Costa, MS, RDN, is a dietitian and nutrition consultant for the E-Health Project.
Potential Benefits Of Fasting
Fasting is widely known for its ability to help jumpstart weight loss. “Some studies suggest that intermittent fasting is about as effective as a typical low-calorie diet for weight loss,” says Raj Dasgupta, MD, a physician and the chief medical advisor for Sleep Advisor. “That seems reasonable because reducing the number of calories you eat should help you lose weight.”
Apart from weight loss, following a fasting schedule may encourage a calorie deficit or simply help you follow a more consistent diet routine. Short-term fasting may also help boost metabolism, per a recent study, and research has shown that people can lose up to 13 percent of their body weight through intermittent fasting, according to a recent review of 27 randomized controlled trials. “Fasting can [also] help reduce inflammation and promote weight loss, lowering fasting insulin levels and decreasing cardiovascular risks,” Costa adds.
Stages Of Fasting
“During a fast, the body transitions between the fed and fasting states, experiencing distinct metabolic and hormonal alterations,” Costa says. This process can trigger your body’s metabolism and promote autophagy—a process in which unnecessary or deteriorating cells are recycled—and accelerate lipolysis, which helps break down fat stores, she explains.
The fast-fed cycle supports the body's efficient use of energy and underpins many of the physiological benefits of fasting, Costa adds. Now, let's get into the nitty-gritty of each specific phase.
1. Fed State
After you enjoy a yummy meal and your body is digesting, you’ve entered the first stage of fasting: the fed state. “Immediately after food consumption, the body enters the fed state, where insulin levels rise to manage increased blood sugar spurred by nutrient absorption,” Costa adds. During this phase, excess sugar becomes glycogen in the liver and muscles. Then, hunger-regulating hormones like ghrelin decrease, and leptin (which has an appetite-suppressing effect) intensifies, she says.
Depending on the type and amount of food you consume during this stage, a variety of reactions can occur. “Individuals may experience feelings of fullness, satisfaction, and temporary relief from hunger,” Costa says. “Emotionally, this state might offer a sense of comfort or pleasure derived from eating, though again, the fuel sources consumed and the individual's relationship with food can shape this experience.”
Consuming meals high in refined carbohydrates and added sugars can lead to a dramatic spike in blood sugar levels, followed by a crash that can affect emotional stability, Costa says. This may cause feelings of irritability and fatigue, mood swings, and exacerbate symptoms of anxiety and depression. “Choosing whole foods and maintaining more balanced blood sugar levels are fundamental for optimal brain function and emotional well-being,” she says.
2. Early Fasting State
Roughly three to four hours post-meal, your blood sugar, and insulin begin to wane and the body taps into glycogen reserves for energy, Costa says. This is the onset of the early fasting state. “This intermediary phase—escalating up to 18 hours without food—activates fat breakdown while the body gears up for alternative energy sourcing.” Think: You’re not quite fed, but not full-on fasting yet.
Because your blood glucose and insulin levels are declining during this stage, you may notice some emotional shifts, Costa says. “[Since] the body has not yet transitioned to burning fat for fuel, hunger signals intensify, leading to feelings of irritability and fatigue. It is common to observe oscillations in emotional well-being, with increased tension-anxiety and a more pronounced negative mood.”
However, as you get more accustomed to fasting, you may find that your symptoms lessen and your emotions even out a bit, she says.
3. Fasting State
During the fasting state—which can last between 18 hours and two days—the liver depletes glycogen, causing the body to metabolize fat and protein into fuel, says Costa. At around the 24-hour mark, your body may produce ketones, and you may see signs of ketosis around the two-day mark, she says. FYI: This is similar to what happens in the keto diet when your body burns through fat instead of carbs.
The hormone ghrelin (which stimulates hunger) also shifts during the fasting state. “Ghrelin levels typically peak between the first and second day of fasting before decreasing,” Costa says. You may feel intense hunger during this time before the feeling starts tapering off. “This reduction in hunger also usually coincides with the body's shift into ketosis and the subsequent use of stored fat for energy,” Costa adds.
Short-term fasting may lead to an increase in positive mood attributes and a decrease in negative ones. You may even feel proud and accomplished about your diet and routine. However, if you have no prior fasting experience, this stage might feel particularly intense with more physical and emotional challenges.
4. Long-Term Fasting State
The last stage is long-term fasting, sometimes referred to as a “starvation” state—although experts stress that this should never be the goal.
After 48 hours without food, the body enters the starvation state, where it sustains itself through gluconeogenesis—a.k.a. making sugar as fuel for the brain, Costa says. Your insulin levels decrease, ketone levels increase, and muscle breakdown may occur over time. However, this process is "not recommended for most people, and necessitates medical oversight to ensure safety and health," she adds.
In prolonged starvation mode, the body is forced to consume its muscle tissue for energy, which can lead to functional deterioration of the body's systems over time, says Costa. “Extended periods of fasting can instigate a cascade of organ failure as the body depletes its energy reserves to dangerous levels.” Extended fasting requires medical supervision, and you should always consult a healthcare provider before starting any fasting regimen, she says.
Who should avoid fasting?
If you are considering intermittent fasting, make sure to discuss it with your doctor or healthcare provider first. “Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes,” says Dr. Dasgupta. “People who take medications for blood pressure or heart disease also may be more prone to electrolyte abnormalities from fasting.”
Before trying fasting, you should also have a strong social support network to endure very low-calorie days over the long haul, says Dr. Dasgupta. “Pregnant women, people with eating disorders, or who have chronic health issues, hormonal imbalances, or nutritional deficiencies should be extra cautious and maybe [should] avoid fasting. Always check with your doctor before trying to fast to make sure it is a safe option for you."
Costa says fasting for a specific period, typically 16 to 24 hours, is generally safe for most adults and can offer significant benefits, particularly for those dealing with obesity or specific health issues. But again, pregnant or breastfeeding women, as well as individuals with a history of eating disorders, should avoid it.
“People living with conditions like type 2 diabetes should consult with their healthcare practitioners to tailor the approach to their needs,” Costa says. If you have diabetes, your doc can help ensure that your blood sugar levels are closely monitored to prevent hypoglycemic episodes.
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