When to eat, what to eat, and how much to eat are decisions that play a role in our lives every day — and what we decide can play a big role in our overall health.
I am a board-certified internal medicine physician who earned an additional certification in culinary medicine from the Culinary Medicine Institute. I am particularly interested in the vital role food plays in our lives, and I keep up to date on the latest research and trends in nutrition and diets. One trend that is gaining attention is intermittent fasting. Intermittent fasting is safe for many people and has many short-term health benefits, but many who try it have a hard time sticking to the plan for more than a few months.
Here is information about intermittent fasting that may be helpful if you are trying to decide whether to give it a try.
What is Intermittent Fasting? Is Intermittent Fasting Safe?
Intermittent fasting is a method of restricting calories during some time periods and eating normally during other time periods. The focus of intermittent fasting is more on when you eat rather what you eat — though devouring unhealthful foods during eating periods still carries health risks, such as developing Type 2 diabetes or heart disease.
Intermittent fasting is a way to push the body’s metabolism from a glucose-based metabolism to a ketone-based metabolism. Usually, our bodies use glucose as its main energy source. But after about 12–14 hours of fasting, our bodies start breaking down stored fat cells instead, a process called ketosis. When our bodies are in ketosis and burn fat cells, we lose weight.
If you are considering trying intermittent fasting, be sure to check with your doctor. Though the practice is safe for most people, some people shouldn’t attempt it. Children should not attempt intermittent fasting; they need a consistent supply of calories throughout the day. Intermittent fasting can also be dangerous for pregnant women, lactating women, women trying to conceive, people with a history of eating disorders, people who are underweight, and people who are diabetic and dependent on insulin. (Some people with diabetes can try intermittent fasting, but they should be sure to work closely with their doctor or nutritionist.)
Health Benefits of Intermittent Fasting
Weight loss itself is generally associated with a lot of health benefits. Maintaining a healthy weight helps us prevent heart disease and stroke. It lowers our blood pressure, cholesterol levels, and improves our mood. Losing weight also improves our mobility, making it easier to move around without knee or back pain.
Research is showing that intermittent fasting has the same benefits of weight loss with some additional health benefits, such as making the body more sensitive to the effects of insulin. This is an important factor in preventing Type 2 diabetes: those who develop Type 2 diabetes lose their sensitivity to insulin.
Intermittent fasting also has been linked with lower levels of an inflammatory biomarker called C-reactive protein. High levels of C-reactive protein mean there is inflammation in the body. Inflammation in the body is associated with coronary artery disease, dementia, diabetes, cancer, and arthritis, for example.
Common Types of Intermittent Fasting
There are a few different ways to do intermittent fasting. One approach is known as alternate day fasting, or ADF. People using this method consume about 25 percent of their daily calorie requirement on fast days — roughly 500 calories — and 125 percent of their daily calorie requirement on nonfast days. (Get a sense of your daily calorie needs.) Fast days are every other day, and “feast” days are on the alternate days.
Another approach is the 5:2 method. Using this method, people fast two days a week by eating about 25 percent of their daily calorie requirement, and they eat regularly the other five days a week. The two fast days shouldn’t be consecutive days.
While many people like these approaches, others prefer the time-restricted eating method. One example is the 16/8 regimen. People choose an eight-hour window in which they eat, and then eat nothing for the remaining 16 hours a day. Some people may choose to eat all meals between 8 a.m. and 4 p.m.; others may prefer to eat between 10 a.m. and 6 p.m.
A modified version of the 16/8 method is the 14/10 method, where people choose a 10-hour window to eat, perhaps between 8 a.m. and 6 p.m., and fast for the remaining 14 hours.
The idea is to be finished eating by early evening when we are less active. It’s important to fast for at least 12–14 hours so body metabolism shifts from burning glucose to burning fat cells.
People trying the time-restricted eating method may do this a few times a week or every day.
There is a lot of research looking at intermittent fasting. One study in particular showed that over a three-week period, people who tried alternate day fasting had the most weight loss, followed by those who tried time-restricted fasting. Those in a control group — people who restricted calories but didn’t try intermittent fasting — lost the least amount of weight. In the same study, those in the intermittent fasting groups also saw significant decrease in blood sugar and triglycerides compared to the control group.
However, when intermittent fasting was compared to traditional calorie restriction — consuming 75 percent of total energy need daily — the weight loss was similar between the two methods after 12 months.
No matter what approach you try, remember to drink noncaloric beverages during fasting periods. Water, coffee, and tea are fine to drink, but don’t add sugar or cream.
Side Effects of Intermittent Fasting
People who try intermittent fasting may experience side effects. Many are mild, but some can be serious. Side effects may include:
- Feeling tired
- Feeling cold
- Thin skin
- Hair loss
- Increased risk for gallstones. If you feel abdominal pain, seek medical care.
- Starvation ketoacidosis. This can be a serious condition and warrants immediate medical attention. Symptoms include confusion, stomach pain, nausea, and a low pulse.
Many people can tolerate some of these symptoms as they are getting used to their intermittent fasting plan — side effects often ease up in a few weeks — but if you are too nauseous to eat, have severe abdominal pain, or have any symptoms of starvation ketoacidosis, intermittent fasting probably isn’t the right plan for you and you should seek medical care.
While intermittent fasting has many health benefits, many people have a hard time sticking with an intermittent fasting plan in the long term, and may eventually gain the weight back. The best plan to maintain a healthy weight is one that is sustainable to you. You may need to experiment with different eating plans, but when you find one that works, you will reap the benefits.
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