To Fast or Not Too Fast


What about intermittent fasting? It has been a hot topic in recent years and not an uncommon question in our clinics. Public information on the topic is abundant and purported results are nothing short of miraculous. However, in the medical literature, good data can be relatively scarce. It is argued that from an evolutionary perspective humans have probably survived long periods of time with little or no food. In the animal world, predators may go days or weeks before their next meal while maintaining the ability to chase down their prey. Historically, for religious and cultural reasons, fasting has been used for millennia. Medically, a hundred years ago fasting was the treatment for diabetes. Some may point to the adverse events of “therapeutic fasts” in the 1960s which consisted of extended fasts for weeks if not months and could result in side effects such as acidosis, renal failure, dysrhythmias, and death. These therapeutic fasts are probably not a good idea generally speaking — at least not without very close medical involvement. These therapeutics fasts are not what we are talking about right now.

So what is intermittent fasting? Part of the problem gathering good data is that there are so many ways to intermittently fast. In general, intermittent fasting is voluntarily eating no or few calories for periods of time ranging from 12H to several days in a recurring fashion. Alternate day fasting is probably the most well studied of late and involves no or few calories on fasting days alternating with a day of unrestricted or somewhat restricted food intake. Time-restricted feeding restricts meals to a certain time of day with the rest of the day fasting. For example, having meals between 12p and 8p (an 8-hour window) would produce a 16-hour fasting interval. Periodic fasting may involve severe restriction for 1-2d/week with ad libitum consumption the other 5-6 days. Thus, an endless combination of feeding and fasting protocols can be devised and popularized such as The Warrior Diet, Eat-Stop-Eat, The 5:2 Diet and The Alternate-Day Diet.

Intermittent fasting capitalizes on our physiology. Fasting allows significant lowering of glucose and insulin levels to achieve a hormonal quiescence – rare in our high-carbohydrate society—that hopefully reduces insulin resistance. Also, with no intake, our glycogen stores are depleted in about one day, and gluconeogenesis and then lipolysis/ketosis take over to maintain glucose levels. Nutritional ketosis contributes much to the beneficial effect of fasting. Of course, intermittent fasting is not the only way to achieve ketosis. Any number of low carbohydrate nutritional plans or 20 miles of continuous running should achieve “fat burn.” However, current evidence seems to support the hypothesis that any eating pattern that reduces nighttime eating or prolongs nighttime fasting may result in improved health.

The strongest evidence of the health benefits of intermittent fasting comes from animal studies which show improvement in glucose and insulin sensitivity, improvement in levels of leptin and adiponectin, retention of lean mass, reduced risk of obesity and obesity-related conditions such as NAFLD, diabetes, cancer and even an increase in lifespan. Results are less clearly established in human investigations. Most human studies are observational, cross-sectional or interventional. The few interventional studies tend to have small sample sizes of less than 50 participants. The majority of these studies suggest weight loss, loss of fat mass and improvements in glucose and insulin metabolism. About half had no comparison group. When compared to continuous calorie restriction, intermittent fasting studies have not confirmed superiority regarding weight loss. Some of those studies showed better results for insulin resistance with intermittent fasting, but others showed no significant differences. There is an overall suggestion that LDL, HDL, and triglycerides improve but in some studies either no change was found, or there was actually an increase in cholesterol, LDL or triglycerides. The limited data showing changes in inflammatory markers is decidedly mixed.

In regards to weight loss, almost any type of intermittent fasting appears to reduce weight including fat mass and probably improves health. Further, intermittent fasting regimens appear to be safe. Intermittent fasting may be a promising tool among many that could be used depending on the patient and their medical history. What is clear is that more rigorous, larger scale, and longer-term clinical investigations are needed. In the meantime, clinical judgment matters and having a thorough open conversation with our patients to determine their needs and goals matters most of all.

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