What Happens to Metabolism in Fasting?

Intermittent fasting is a dietary plan where you eat at certain times and then refrain from eating completely between those periods. Following an intermittent fasting pattern can help with weight loss and positively affect other health conditions.

There are dozens of popular diet plans that claim to promote weight loss and better health. Many suggest restricting the types of food you eat in order to reduce calories. Other plans advocate merely restricting the amount of food you eat for fat loss. 

There is a growing body of research that suggests that changing the timing of eating and extending the time between meals may be a more reliable way to lose weight. Intermittent fasting is a plan that sets times when you don't eat any food at all. During those periods, your body can process all the food you ate previously and trigger metabolic changes that lead to weight loss. Some studies show that there are other health benefits as well. 

Intermittent fasting is a dietary plan where you eat at certain times and then refrain from eating completely between those periods. The periods when you don't eat are considered fasting periods.

The goal of intermittent fasting is to give your body time to fully digest the food you consume. Once your body has processed the food you eat, it can't rely on those calories for energy. Instead, you start to burn stored fat to keep your body working.

In theory, this allows your body to mimic the eating patterns that ancient humans would have followed. Hunter-gatherer peoples would have eaten during times when food was present. During the remaining hours, they would not have had access to food.

That's a contrast to modern eating patterns. In contemporary society, food is plentiful and available on demand. As a result, many people eat much more frequently than ancient humans ever did. The constant consumption of calories means that human bodies never run out of outside energy and burn only calories from food.

There are different methods of intermittent fasting. All of them suggest that you set specific times for eating and other periods where you don't consume any more calories. Some popular fasting patterns include:

Daily fasting

You select a period of six to eight hours during which you eat all meals and snacks. During the remaining 16 to 18 hours, you don't eat anything. This period can include the time when you're asleep, so it may not be as drastic as it sounds at first. It can be an easy pattern to stick to since it allows regular meals during waking hours.

The 5:2 fasting plan

For this plan, you eat however you like for five days each week. During fasting days, though, you restrict yourself to a few hundred calories per day. The fasting days do not need to be consecutive. 

Alternate day fasting

With this type of intermittent fasting, you eat whatever you choose every other day and fast the other days. On fasting days, you limit yourself to 500 calories or less.

Extended fasting

Some people engage in extended fasting periods, going 24 hours or longer at a time without food. This may become unsafe, though. Talk to your doctor before going without food for extended periods. 

While engaged in any intermittent fasting plan, no-calorie beverages such as water are encouraged. Staying well-hydrated is important to overall health, so be sure to drink enough fluids even while fasting.

Research has shown promising benefits of intermittent fasting plans. Following an intermittent fasting pattern can help with weight loss and positively affect other health conditions.

Intermittent fasting and weight loss

When you eat, your body breaks down food with digestive enzymes in the gut. Most foods are broken down into forms of sugar, which cells can use for energy. The energy from food is distributed to the rest of the body via the bloodstream. If there is an excess of the sugars from food, it gets stored in the fat cells for later use. Fat cells can expand and store more and more unneeded energy over time. 

Sugar requires an enzyme called insulin to help it get into fat cells. Insulin rises after eating to assist with the overall digestive process. After a while, insulin levels drop, and the body stops being able to store fat. Once that happens, fat cells can release the stored energy, a process known as burning fat. Intermittent fasting allows longer periods where insulin levels are low enough to allow fat loss, leading to potential weight reduction.

Intermittent fasting and cholesterol levels

Research shows that following an intermittent fasting plan can positively affect cholesterol levels. One study shows a decrease in LDL cholesterol and triglycerides after 12 weeks of alternate day fasting. Other studies show a reduction of overall cholesterol levels and improved blood lipid levels after 24 weeks of intermittent fasting.

Intermittent fasting and blood glucose levels

Studies show that intermittent fasting may be able to help people control their blood sugar levels. In several studies, researchers found that people who participated in intermittent fasting had lower blood glucose levels. The degree of blood glucose changes depended on the fasting patterns, though, and not all participants had the same results. Regardless, the research indicates that fasting can be a tool for controlling Type II diabetes in some cases.

Intermittent fasting and circadian rhythms 

Some researchers believe that intermittent fasting allows your metabolism to follow a pattern that it naturally wants to follow. The circadian rhythm is the body's natural sleep-wake cycle. Body functions change from day to night. Liver and gut functions slow, and insulin sensitivity drops during the evening and overnight. Digestion is slower at night, and blood sugar levels remain higher if you eat closer to the time when you go to sleep. Studies have found that beginning a fasting period in the evening improves the gut microbiome, sleep patterns, energy, coordination, and attention while awake.

Intermittent fasting is not the right choice for everyone. You should talk to your doctor before beginning any new eating plan. In addition, experts recommend against intermittent fasting for particular groups of people, including:

• Children and teens under age 18
• People who are pregnant or breastfeeding
• People with diabetes or blood sugar disorders
• Anyone with a history of eating disorders

In addition, some research finds that intermittent fasting has unexpected emotional effects. One study found that some people experienced negative emotions while fasting, such as:

• Anxiety 
• Anger
• Depression
• Fatigue
• Irritability
• Tension

Some people also experience physical discomfort, including headaches, fatigue, or distractibility. For some people, though, the negative effects were short-lived. They found that their moods got better after they adjusted to the new eating pattern.

Intermittent fasting can affect how your metabolism functions, but the changes aren't permanent. Your so-called fasting metabolism only occurs while you are following the intermittent fasting pattern. 

When you engage in an intermittent fasting pattern, your metabolism is able to switch between different types of fuel to provide energy to your body. When your body uses up the glucose that comes from the food you consume, it switches to a state called ketosis. Sometimes known as "keto," this refers to what happens when your body starts tapping into fat stores for energy. Some research shows that going as few as 12 hours without eating can lead to ketosis.

You can also achieve ketosis through low-carbohydrate eating plans. Proteins and fats contain much less natural sugar for your body to convert to glucose. The result is increased fat-burning ketosis. This kind of diet is hard to maintain, however, and may not offer balanced nutrition. Some experts have concerns that keto diets cause stress on the heart due to the high-fat content they require.

An intermittent fasting plan can give your body longer periods of ketosis, which can lead to increased fat burning. You go back to burning glucose from food once you resume eating again. The effects of ketosis don't last perpetually. 

Unlike keto diet plans, intermittent fasting allows you to reach ketosis while still eating a variety of foods. As a result, some people find intermittent fasting easier to maintain than a keto diet.

To fully realize the weight loss benefits of intermittent fasting, it's important to be mindful of your food choices. Eating large quantities of high-calorie or highly processed foods during eating periods conveys the same health risks that always come with such eating choices. Experts suggest choosing a balanced variety of foods, including lean meats, fruits, vegetables, and whole grains.

If you are considering an intermittent fasting plan, talk to your doctor. They will be able to tell you if changing your eating patterns will be good for your health.