Intermittent fasting has recently gained in popularity as a weight loss and health improvement strategy. Proponents claim that it can help you lose weight, improve your insulin sensitivity, reduce oxidative stress and cellular aging, improve cardiovascular health, and more. In this review of intermittent fasting studies, we take an in-depth look at the evidence behind these claims so that you can decide whether or not intermittent fasting is right for you.
#1 IF increases fat burning
Multiple studies show that intermittent fasting increases the body’s ability to burn fat for energy during periods of fasting and can help you lose weight. One study found that people who followed an intermittent fasting lifestyle lost more weight over a year than those who followed a traditional diet. Another study found that after six months, participants following an IF diet had significantly lower insulin levels than those following a normal diet and had significantly improved glucose tolerance. Yet another study looked at how intermittent fasting impacts weight loss, blood pressure, heart rate, cholesterol levels, and other biomarkers in obese children and found that IF is a feasible and effective option for reducing cardiovascular disease risk factors without causing excessive hunger or malnutrition.
#2 Cholesterol improves on IF
Intermittent fasting can improve cholesterol levels by reducing LDL or bad cholesterol, and increasing HDL or good cholesterol. Lowering LDL levels is a primary goal of many weight loss diets in order to reduce the risk of heart disease. This is because elevated LDL levels are linked to clogged arteries and an increased risk for heart attack and stroke. Studies found that intermittent fasting significantly improved total, LDL, and HDL cholesterol after 10 weeks. The number of subjects varied from 4 to 334 across the 40 studies reviewed, with results ranging from a 7-pound weight loss to 11 pounds over 10 weeks in the average study participant.
#3 Blood pressure improves on IF
Intermittent fasting has been shown to reduce blood pressure in both healthy individuals and those with hypertension. One study found that after three weeks, systolic BP decreased by an average of 7.5 points and diastolic BP by 5.5 points in the IF group. The control group experienced no significant changes in blood pressure. Another study showed that participants who practiced IF had lower rates of stroke, heart disease, cancer and diabetes than participants who followed a standard diet. Still another review of the literature concluded that IF led to improvements in cardiovascular risk factors including weight loss; reductions in total cholesterol levels; reductions in LDL cholesterol levels; reductions in triglycerides; and increases in HDL cholesterol levels. In one 6-month randomized controlled trial, 64 overweight adults were assigned either a 20% calorie restriction (CR) or alternate day fasting (ADF). The CR group consumed a maximum of 400 kcal per day for 3 months while the ADF group consumed 25% fewer calories every other day for 3 months.
#4 Glucose improves on IF
In a study published in the journal Cell Metabolism, researchers put mice on one of two diets for three months: either a high-carb or a ketogenic diet. Mice on the ketogenic diet had increased levels of beta-hydroxybutyrate (BHB), which is a ketone body that is produced during times of low food intake and fasting. The high levels of BHB in this group led to decreased glucose production by the liver. The high-carb group experienced an increase in glucose production, as expected. The ketogenic group also had lower insulin levels and higher fat oxidation rates than those on the high-carb diet, showing that the effects go beyond just weight loss and into reduced risk factors for diabetes and heart disease.
#5 Thyroid function improves on IF
An American study found that intermittent fasting improves thyroid function, which may be related to weight loss. Of the subjects who fasted intermittently for 24 weeks, those with hypothyroidism had a greater decrease in thyroid-stimulating hormone (TSH) levels and an increase in free thyroxine (T4) levels. No changes were seen in those without hypothyroidism.
Thyroid hormones are important in regulating metabolism. When thyroid hormone levels decline, metabolism slows down, resulting in weight gain. By increasing free T4 levels, it is possible that intermittent fasting helps improve weight loss by improving metabolic function. It’s also possible that weight loss caused by intermittent fasting leads to a decline in TSH and an increase in free T4 levels.
#7 Eating less reduces diabetes risk: A review of multiple studies suggests that eating less can help prevent type 2 diabetes.
#6 IGF-1 increases on IF (surprise!)
Fasting decreases levels of the hormone insulin, which in turn causes increases in growth hormone (GH). Levels of GH rise sharply during fasting and remain high throughout the day. Increased GH is associated with an increase in IGF-1 production. And while there are many benefits to fasting, weight loss is a huge one.
Perhaps equally as important, fasting increases levels of norepinephrine. Norepinephrine is a hormone that helps to regulate metabolism, and also has an appetite suppressant effect.
#7 Hunger decreases on IF
In the first study, 3 days of fasting decreased hunger and increased fat burning in men. In another, a 6-day fast decreased hunger more than a 12-hour fast. In the third, men who ate only one meal per day for 2 weeks had a lower appetite than when they ate 3 meals per day. It makes sense that fasting would decrease appetite: in the absence of food, our body will burn its own fat stores for energy instead of constantly demanding food to make up for what we don’t have!
#8 Muscle mass increases on IF
Muscle mass increases during intermittent fasting because the body is able to use its own fat stores for energy and doesn’t need to break down muscle as much. The study showed that after 24 weeks of a strict fast, participants lost between 5-8% of their lean body mass, but regained almost all of it within six weeks of returning to normal eating patterns. The authors concluded: Intermittent fasting may be an effective strategy for reducing excess fat mass.