What Happens During a Fast
So what exactly happens to your body during fasting? This timeline will describe
1) what is happening inside of your body throughout your fast.
2) and how you may be feeling.
The timeline below is merely meant to serve as a general guideline, and these processes and the rate at which they occur are highly individual based on a number of factors including genetics, activity levels, gender, diet, and whether or not calories are being consumed.
0-4 HOURS
What Happens: At this stage, your body is still breaking down your most recent meal into glucose, amino acids, and fatty acids to either use for immediate energy or to store for later use. If your last meal was high in carbohydrates, then you’ll have a lot of glucose in your bloodstream, which causes a spike in insulin. In metabolically healthy people, insulin and blood glucose levels will return to normal after between three and four hours. During this time, you’ll also experience a decrease in ghrelin and an increase in leptin, the balance of which signals to your brain that you’re full.
How You Feel: Because you just ate, you’ll feel full, satiated, and energized as your body metabolizes your most recent meal for energy. The fun has only just begun.
4-14 HOURS
What Happens: After about 4 hours, your body will have finished processing your most recent meal, your blood sugar will drop, and you’ll start tapping into stored glycogen for energy. Since glucose is still your body’s main fuel source at this point, the body will metabolize as much glycogen as it needs to keep your blood glucose in the “normal” range. Many benefits of fasting—including mild ketosis and fat burning start to occur after you’ve abstained from food for 12-14 hours. Though the body prefers glucose and glycogen at its primary source of energy, it will up regulate fat metabolism and the production of ketone bodies to make up the energy difference when it starts to run low. The exact time that your body will make the shift to burning more fat for energy depends on how much glycogen you have stored and how quickly you run through it. You’ll make the switch into fat-burning faster if you exercise in this timeframe, as exercise depletes your glycogen stores more rapidly.
How You Feel: Hunger should be minimal, especially since most of the fast should be taking place overnight during sleep. If you’re used to eating breakfast, you may feel hungry when you wake up, but it may be more psychological hunger from engrained circadian cues.
14-24 HOURS
What Happens: Mild autophagy tends to activate after 16 hours of fasting. After 18-24 hours, your body will have burned through the majority of its glycogen reserves, and it will enter a state of gluconeogenesis. Gluconeogenesis allows the body to create its own glucose by breaking down stored body fat in the absence of food. It can produce about 80 grams of glucose per day through this process, most of which is used by the brain. Because that glucose is being made from stored body fat, this time period is the sweet spot for short-term fasting if better body composition is your main goal.
How You Feel: You’re likely to feel an increase in energy levels at the 18 hour mark as your body shifts into burning more fat. If you’re new to fasting, you may feel hungry, lethargic, and irritable as you approach your usual meal times. This is because the body is still in the process of switching from its glycogen reserves to fat metabolism. This period of time is usually most mentally difficult for first-time fasters, but things will improve!
24-48 HOURS
What Happens: After 24 hours of fasting, the body’s glycogen reserves are completely depleted. And although the body has started producing ketones, production isn’t at a level that’s high enough to meet all of its energy needs. This causes a faster rate of gluconeogenesis to keep your energy levels up and your organs fueled. During this time, the body also increases production of human growth hormone to help maintain lean tissue.
How You Feel: It’s common to feel hungry and physically weak during this phase of the fast, and many have reported experiencing headaches and trouble sleeping. You can combat these side-effects by supplementing with electrolytes and staying adequately hydrated
48-72 HOURS
What Happens: After 48 hours of fasting, your levels of growth hormone could rise to as much as five times higher than normal. At the 60 hour mark, fat metabolism increases and the body starts using fatty acids to meet about 75% of its energy need. 72 hours into the fast, you can experience a noticeable increase in metabolic rate.
How You Feel: Congratulations, you’ve gotten over the hump! You should be feeling better. On a psychological level, you’re likely to be less attached to your circadian-entrained mealtimes, which means you won’t feel as much hunger as dictated by the clock. You can expect to experience more stable energy levels due to an increase in metabolic rate and improved utilization of ketones for energy.
72 HOURS AND BEYOND
What Happens: Day three is where things really begin to shift. At this point, the body enters yet another uptick in ketone production and fat-burning to meet the energy needs of your heart, liver, and brain. During the 48–72 hour timeframe of fasting, blood ketone levels can measure somewhere between 0.5 and 2 mM, which qualifies as nutritional ketosis. There’s also an increase in growth hormone production, which helps the body maintain muscle mass and other lean tissues, as well as a major improvement in the regulation of ghrelin. And on top of that, there’s a short-term reduction in insulin like growth factor (IGF-1), which has been associated with reduced inflammation and oxidative stress, and may even play a role in anti-aging and longevity. 72 hours of fasting has also been shown to increase cellular resilience to toxins and stress and lower levels of circulating insulin and glucose, which can reduce the risk of developing metabolic disease and has benefits for immunity, inflammation, and neurogenesis.
How You Feel: Most people typically feel more energetic due to the body’s improved utilization of fatty acids, and experience even less hunger due to better hormone regulation. Many experienced fasters have reported feeling like they could “fast forever” after this point…but we definitely don’t recommend that. Unless you’re on a spiritual quest, or are working with a doctor for a specific medical reason, fasting for three to five days is the perfect range for hormone regulation, fatty acid metabolism, and autophagy.