March 22, 2023

Many dieters swear by it.
Intermittent fasting, a dietary regimen that cycles between periods of eating and fasting, has been a popular weight management tool for years.
The regimen does not restrict the person from any food, instead it limits the hours that eating can occur. For years, followers have been told that this method of eating could help them lose weight.   
A new study published in the New England Journal of Medicine, however, suggests that intermittent fasting may not actually help people lose weight compared to caloric restriction. Researchers in China found that when a group of obese reduced their calorie intake, with or without adding time-restricted eating, those in the fasting group did not show a greater weight loss.
Dr. Michelle Pearlman, assistant professor in the Division of Digestive and Liver Diseases at the University of Miami Miller School of Medicine, and Arlette Perry, professor and chair of Kinesiology and Sports Sciences in the School of Education and Human Development, weigh in on the findings of this new study.   
Pearlman: There are numerous trendy fad diets that are mass marketed on TV, on social media, and in magazines, many of which are not based on clinical evidence. Despite following a time-restricted diet, eating whatever you want within a shorter “eating window” does not actually mean you will lose more weight.   
Perry: I don’t agree that intermittent fasting will not help with weight loss. First, intermittent fasting is defined by alternating periods of fasting with periods of eating. There are two methods of doing this: one way is having fast days of no more than 500 calories alternated with feast days with no restrictions on quantity or quality of food. Many people do an offshoot of this, using the popular 5:2 method wherein one eats freely for 5 days alternated with 2 days of fasting. The other method is time restricted feeding, in which followers fast for about 14-20 hours/day while freely consuming foods the rest of the 4-10 hours. 
Metabolically, several hours after a meal, glucose and fats return to resting levels. However, if no food is consumed, the body’s source of stored carbohydrates (glycogen) start to get depleted which causes the body to rely more upon fats for energy.
So, that’s why intermittent fasting is reported to improve weight loss and increase the oxidation or breakdown of fats. There are many research studies showing weight loss does, in fact, occur with intermittent fasting. However, it’s not so simple. And there are several advantages and disadvantages to following this weight loss method.  
Pearlman: I find it useful in those who tend to eat mindlessly or when they are not even hungry, or those who tend to eat late before bed. If you close down the kitchen by 4 p.m., for instance, you are less likely to eat high calorie snacks while watching TV or having a bedtime snack, which may cause you to have heartburn in the middle of the night. Some people need a degree of restriction to help limit consumption of excess calories. For most people, the evidence supports that weight loss often occurs when you follow a calorie-restricted diet plan, regardless of timing or frequency of meals and snacks. 
Perry: Yes, there are advantages. The body is equipped to adapt to periods of fasting and feasting. As glycogen stores and carbohydrate oxidation decrease during intermittent fasting, fat oxidation increases and the break down of fats can be used to provide energy for the kidneys, liver, heart, and lungs. Fat metabolism leads to an increase in ketone bodies (ketogenesis), produced by the liver, when insufficient glucose is available. Therefore, the body is adjusting to reduced carbohydrates by metabolically increasing fat oxidation and ketogenesis. That is why the diet has been of interest to individuals with Type 2 diabetes.
Pearlman: Having the mentality that as long as you eat within your “fasting window” that you can eat whatever you want is not the ideal approach. Ingredients and calories still play a major role in weight and health irrespective of timing. Also, some people who suffer from common GI symptoms like heartburn or bloating or who have had certain GI-related surgeries like bariatric surgery, will often not tolerate larger quantities of food within a short period of time. 
Perry: Yes, there are disadvantages. For one, the brain and red blood cells require a continual supply of glucose. This results in a metabolic shift to metabolize substrates the body does not necessarily want to burn to preserve blood glucose for red blood cells and the brain.
There is also a temporary reduction in metabolic rate as the body tries to preserve protein for building tissue instead of using it for energy reserves. Exercise—which is recommended for metabolic fitness—healthy weight loss, and the preservation of muscle tissue may be difficult to maintain during periods of fasting.
When you break the fast, blood glucose levels and circulating lipids (fats) return to basal levels and the cycle repeats itself. However, there are very few long-term studies regarding the efficacy and safety of this type of weight loss. It is unclear if individuals are getting sufficient nutrients during fasting. Many find the diet difficult to maintain in the longterm. Therefore, once you go off this type of restricted diet, you may regain much of your weight. 
Furthermore, weight reduction on intermittent diets is no greater than standard calorie-restricted diets. In fact, the main reason for enhanced weight loss using intermittent diets is because individuals do not fully compensate for their lack of calorie intake following the fasting cycle. Additionally, changes in body composition with intermittent fasting are similar to standard weight loss diets.
Following the initial two weeks of any negative calorie balance, the majority of weight loss is roughly 75 percent fat and approximately 25 percent muscle. It is unclear whether this is the ideal weight loss method for long-term weight control.   
Pearlman: I recommend eating when you feel hungry. If you are on insulin, then timing your meals with your insulin or other medications can be very important. If you are not on insulin, however, eating at certain time intervals/frequency throughout the day is less important unless you tend to get hypoglycemia during prolonged fasting. 
Perry: If diabetics are having difficulty maintaining blood glucose control or maintaining normal levels of glycosylated hemoglobin (a long-term indicator of glucose control), then this is an acceptable approach to avoid hyperglycemia.

Pearlman: I do not recommend starting overly restrictive diets during pregnancy. I recommend consuming a whole-food, plant-based diet for most individuals, but timing of meals and snacks may vary depending on your pregnancy course and any associated symptoms. 
Perry: I would not try this type of diet during pregnancy because of the aforementioned uncertainties with intermittent fasting. During pregnancy, women are at increased risk of insulin resistance (which most women appear to handle well). However, women with a history of poor glucose control and overweight/obesity may be at greater risk for gestational diabetes. For these women, I would recommend they check with their obstetrician first before initiating this type of diet or any type of diet during pregnancy.  
Pearlman: As long as you are eating nutrient dense foods, eating less does not necessarily mean you aren’t consuming adequate nutrients. Many people who eat calorie-dense foods like fast food or baked goods are consuming excessive amounts of calories, and these foods are highly refined-processed items that often lack essential nutrients for overall health.  
Perry: Not necessarily. It is the quality not quantity of foods that are important. By eating more vegetables, fruits, and plant-based options, it is easy to pack more nutrients and fiber into lower-calorie foods.
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