Over the past few years, intermittent fasting (IF) has become a popular discussion point in mainstream news and culture. But the media coverage only tells part of the story surrounding IF, leaving several unanswered questions. For people considering IF, that can make it hard to know where to start or what to expect.
If you’re contemplating giving IF a shot but need more information, we have you covered! Follow this comprehensive guide to learn the different IF cadences, benefits and special population considerations!
What is Intermittent Fasting?
Intermittent fasting (IF) is an umbrella term for various ways to fast, or rather, go for periods without food or calories. When we sleep at night, we go hours without consuming food. But it’s when we prolong the fasting period for 12 hours or more that we start to see the heightened benefits associated with IF.
It takes about 12 to 24 hours of fasting for the human body to experience a significant decrease in blood sugar and stored glucose, which switches the body’s metabolic state. However, this amount can vary based on the person’s daily activity level. From there, the body uses ketone bodies and free fatty acids as its primary energy source. This state is known as ketosis.
Intermittent Fasting Timing & Cadences
True to its name, a Full-Day Fast involves 24 hour periods without eating. Full-Day Fasts can be practiced anywhere from once a month to twice a week. Your goals and lifestyle should determine the frequency.
The shift from glucose to fat as your primary energy source is most pronounced after 18 hours of fasting. As such, an occasional Full-Day Fast can provide a significant metabolic shift. Research suggests that the occasional Full-Day Fast may also improve cardiovascular health. However, studies also suggest it can be challenging to follow this fasting schedule consistently. But as long as you discontinue the fast after 24 hours, it’s generally safe for healthy people without pre-existing health conditions. 
Alternate-Day Fasts involve one day of fasting, followed by one day of normal consumption. The fasting day can include 20 to 25% of your daily calorie needs. So rather than eliminating food entirely during these fasting periods, you’re just severely limiting it.
Studies found that Alternate-Day Fasts combined with an overall calorie reduction resulted in moderate weight loss. They also found that subjects held on to more lean body mass during weight loss when following an Alternate-Day Fast compared to traditional calorie-restricted diets. However, Alternate-Day Fasts didn’t provide any additional weight loss benefits when compared to conventional calorie-restricted diets.
The 5:2 Fast
5:2 Fasts are a slightly different approach to IF. They involve two nonconsecutive days of energy-restricted eating per week with normal eating patterns during the other five days of the week. Generally, 5:2 Fasts limit calorie consumption to 20 to 25% of daily energy needs on regularly scheduled fasting days. For some, 5:2 Fasts are easier to follow than Alternate-Day Fasts since they offer additional flexibility and require fewer fasting days per week.
Studies show that 5:2 Fasts are equally effective when compared to traditional calorie-restricted diets when the goal is obesity-related weight loss. However, fasting without overall calorie reduction did not produce weight loss. 
Time-Restricted Feeding is a style of IF that has grown in popularity. Many people who follow this type of fast adhere to a 16:8 cadence, which involves 16 hours of fasting and 8 hours of feeding over 24-hours. For example, if you finish your last meal of the day at 7:00 p.m., then hold your fast through 11:00 a.m. the next day, that’s a Time-Restricted Feeding pattern.
Fasting overnight can improve many metabolic markers that are linked to chronic diseases, such as insulin and glucose. Studies found improvements in insulin sensitivity when subjects followed a Time-Restricted Feeding pattern. Other studies have found mild improvements in weight loss and blood pressure markers, but no other significant improvements when compared to traditional calorie-restricted diets. 
One study found that regardless of what time of day you pick for the 8-hour feeding window, glucose tolerance improved. But no study found there to be significant changes in lean body mass, which suggests the weight loss was primarily comprised of body fat. These results also indicate that this style of IF may help people lose weight while maintaining muscle mass better than a traditional calorie-restricted diet. 
Overall, studies found that Time-Restricted Feeding was associated with slight reductions in body weight, total cholesterol, triglycerides, glucose, insulin, inflammatory markers and improved insulin sensitivity. 
Potential Benefits of Intermittent Fasting
The majority of intermittent fasting (IF) studies resulted in moderate weight loss among participants. But when compared to traditional calorie-restricted diets, no significant weight loss benefits were identified. Also, when there wasn’t an overall calorie reduction IF did not provide any marked weight loss benefits.
Important note: Even while fasting, your metabolism is adaptive. Any drastic caloric reduction for long periods can cause a metabolic decrease.
Improved Cardiovascular Disease Risk
The proportion of small LDL particles that can lead to cardiovascular disease significantly decreased in a few studies done on IF. There was also a decrease in triglycerides, which can impact cardiovascular health. However, it is unclear if the improvements were due to the fasting or the resulting weight loss.
Studies also found that IF resulted in increased insulin sensitivity. High insulin sensitivity allows the cells of the body to use blood glucose more effectively, which helps reduce blood sugar.
Some studies showed that IF could improve people’s circadian rhythm, the body’s 24-hour cycle that tells us when to eat, sleep and wake. One study suggests that aligning IF feeding windows with daylight hours could help regulate the body’s sleep-wake rhythms.
Glucose, lipids and energy metabolism are all regulated by the circadian system, which increases and decreases them at certain times of the day. As such, these findings suggest that eating more calories at breakfast and fewer calories at dinner may help regulate the body’s sleep-wake rhythms.
Lean Body Mass Retention
The breakdown of lean body mass doesn’t occur until a prolonged fasting period (about the third day of fasting). As such, the occasional Full-Day or Alternate-Day Fast appears to help people maintain more lean body mass than long-term fasts or traditional calorie-restricted diets.
Special Populations Who Should Avoid Intermittent Fasting
Despite its potential benefits, intermittent fasting (IF) isn’t for everyone. There are also specific populations that should avoid IF altogether due to possible health complications.
Under Stress or Rehabbing an Injury
People suffering from severe stress or chronic fatigue, an additional stressor like IF may be unwise, or even dangerous. Those recovering from a bodily injury should also consider waiting to try IF until they have fully recovered. In both cases, it would likely be more advantageous to focus on consuming adequate amounts of protein and eating whole foods to nourish your body.
Pregnant or Breastfeeding Women
If you are pregnant, breastfeeding or trying to maximize fertility, IF is not recommended. People that fall under these populations have additional energy needs and should focus on consuming enough calories, not restricting them.
Underweight or History of Eating Disorders
If you are currently underweight or have a history of eating disorders, IF could be harmful to your recovery. Any diet emphasizing long periods of caloric restriction may encourage binge eating or other behaviors associated with eating disorders. As an alternative, consider a diet that emphasizes whole foods and nutrient-rich ingredients.
Every person’s body is different, and it’s essential to consider that before trying any new diet. Anyone considering following IF should seek professional guidance before beginning their journey.