Intermittent fasting (IF) refers to a range of eating patterns that cycle between periods of eating and fasting without prescribing specific foods. It gained widespread mainstream popularity with the 2012 publication of The Fast Diet by Dr. Michael Mosley, a UK physician and journalist, and co-author Mimi Spencer, who popularised the 5:2 diet, five normal eating days and two “fast” days with limited calorie intake. Variants such as 16/8 fasting, fasting for 16 hours and eating within an eight-hour window also became common soon after.
Intermittent fasting is distinct from traditional calorie-restricted diets because it emphasises when to eat rather than what to eat. Major forms include:
5:2 diet: Two days per week of very low calories (about 500–600) and five days of regular eating
16/8 time-restricted eating: Daily fasting for 16 hours with an 8-hour eating window
Alternate-day fasting: Normal eating every other day, with fasting or very low-calorie days in between
Short-term clinical studies suggest that intermittent fasting can be effective for weight loss. A 16-week clinical trial using a 5:2 fasting pattern in people with newly diagnosed type 2 diabetes reported significant weight reduction and improved glycaemic control compared with standard drug therapy, with sustained improvements maintained eight weeks after the diet ended.
Other research, including studies comparing daily calorie restriction to IF, has shown similar weight loss outcomes between the two approaches, indicating that total calorie reduction is a crucial factor.
The American Heart Association (AHA) has noted mixed evidence, and preliminary research presented at scientific meetings indicated that strict daily time-restricted eating (eating within less than eight hours per day) was associated with higher rates of cardiovascular mortality in observational data, though these findings require peer-reviewed confirmation.
The AHA has also emphasised that reducing total calorie intake may be more influential for long-term weight and cardiometabolic health than limiting eating windows alone.
According to Dr. Trisha Pasricha, MD, a physician at Beth Israel Deaconess Medical Center, assistant professor of medicine at Harvard Medical School, and columnist for The Washington Post, human studies show that intermittent fasting may lead to short-term weight loss but is difficult to sustain, with weight commonly regained after discontinuation. Current research has not demonstrated clear benefits for preventing heart disease, cancer, or extending lifespan. Instead, Dr. Pasricha highlights evidence supporting consistent, high-quality dietary habits, including eating a fiber- and protein-rich breakfast soon after waking and avoiding late-night eating. Research suggests that overall diet quality, emphasizing fruits, vegetables, whole grains, legumes, nuts, and fewer ultra-processed foods plays a more significant role in long-term metabolic health than strict meal timing.
Public reporting on heart health and IF has been mixed. The Washington post overview noted that some studies linking time-restricted eating to long-term cardiovascular risk are preliminary and may be influenced by investigation limitations.
Elsewhere, nutritional guidance highlights that skipping meals or sustained fasting may potentially impact circadian rhythms and metabolic responses, though definitive human evidence linking IF to improved longevity or reduced risk of chronic disease like heart disease or cancer is lacking.
Professional organisations such as the American Diabetes Association (ADA) maintain that evidence about the safety and effects of intermittent fasting for people with type 1 diabetes remains limited, and controlled research is ongoing. For those with type 2 diabetes, some preliminary findings suggest potential benefits for weight and blood sugar control, but long-term safety and risks are not fully determined.
Clinicians often advise individuals considering intermittent fasting especially those on medications, with cardiovascular disease, or with a history of eating disorders to consult healthcare providers before adopting IF regimens.
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