By now you’ve likely heard the term “intermittent fasting” or IF. What is intermittent fasting? Does it work? Does it help people lose weight or, even better, live longer? While the science is still in its infancy, I’m absolutely fascinated by the potential health benefits. Let’s explore what we do and don’t know about when we should eat

What is intermittent fasting

First, let’s get some definitions straight.

Intermittent fasting is a very large umbrella term for a type of eating that cycles between periods of eating and periods of fasting. There’s a lot of different types, many of which are all lumped together, but are actually quite different. 

Let’s discuss the most common form of fasting, daily time restricted feeding or circadian fasting. This is a type of fasting in which individuals consume all of their daily calories each day within a certain window of time. The most common form of time restricted feeding is probably 16:8, in which individuals fast for 16 hours a day and eat for any 8 hour window of their choosing. Circadian fasting is somewhat less restrictive and emphasizes eating mostly during daylight hours, with a focus on the last meal of the day ending fairly early (ie 6- 7 PM).

True intermittent fasting is considered by most to be a form of fasting in which individuals consume all of their calories on certain days and abstain completely on other days. Common variants of this include a 5:2 eating pattern, in which individuals eat 5 days in the week and fast for 2 days. Others choose to fast for longer periods of time, most commonly 5 to 7 days. This can be further sub-classified into water only fasting and a fasting mimicking diet, in which fasting is simulated by allowing very low levels of calories (usually <500 calories) to be consumed.

What is the potential health benefit of fasting

Fasting has been a part of our evolutionary development. As ancient hunterer-gatherers, we evolved at a time in which we feasted when food was readily available and fasted when it was not. It is also quite common in most of the ancient religions around the world.

There is some evidence that fasting taps into this evolutionary derived adaptive cellular responses, resulting in improved regulation of glucose, better resistance to stress, and decreased inflammation. It is postulated that during times of gut rest, intrinsic defense systems are activated - repair or disposal of damaged molecules occur (known as autophagy), a type of quality control. In times of feeding, cellular processes are then activated that promote renewed growth and synthesis.

Basically, what doesn’t kill us, makes us stronger. 

Metabolic switching

This terms gets thrown around the interwebs by the keto crowd - let’s try to understand what it really means.

Here’s the deal - glucose and fatty acids are the main source of energy for our bodies. When we are fed, our body preferentially uses glucose for energy. During fasting or energy restriction, our body will break down glycogen stored in the liver. Once this is depleted, which takes about 10-14 hours, it will then start to break down triglycerides from our fat cells to free fatty acids, which then get converted to ketones. Our body can use these ketones for energy when there is no glucose present.

Metabolic switching refers to the time when our bodies switch from using glucose to ketones, which seems to occur at slightly different times for different people. Although much of this has yet to be fully sorted out, this switch may have lots of downstream effects on certain cellular pathways, many of which have been implicated in longevity. 

Show me the evidence

The bulk of the data to date is mostly in animals. In these studies, diseases from cancer to insulin sensitivity to obesity are significantly improved. The studies in humans have been mostly short term (weeks to months). Many demonstrate improvement in various cardiometabolic risk factors (like insulin resistance, glucose, cholesterol, and blood pressure), while a few do not. Intermittent fasting has also been shown in some studies to reduce markers of systemic inflammation, changes in sleep efficiency, and increase heart-rate variability.

Many studies also demonstrate weight loss - although a study recently published in a major science journal JAMA did not see significantly greater weight loss with fasting compared to a control group.

Why the variability in outcomes? Many of these studies are quite small (which can make it more difficult to tease out meaningful differences) and blinding is impossible (ie the participants obviously know which group they are in, which is unavoidable, but can bias the results). Studies have looked at various different fasting protocols (alternate day, 16:8, etc, etc) and different time windows of eating, which may make it difficult to assess the impact of fasting - one fasting protocol may be superior to another, but it’s tough to know at this point. tt may be that different fasting protocols are effective (or not) for different people or diseases. And perhaps most importantly, most do not also manipulate what the patients ate (just when they ate), which may make it difficult to see meaningful results in the face of the Standard American Diet.

While there have been no long term studies directly assessing hard cardiovascular outcomes, I find the overall improvement in cardiovascular risk factors to be encouraging.

So where does this leave us?

Follow this space closely. In the next several years, we should learn a lot more about the timing of when we eat, not just what we eat. While what we eat remains king (and probably always will be), harnessing the power of eating windows and fasting programs will become increasingly important as we develop a better understanding of what works and why. As with everything, the ideal fasting program will likely vary from person to person, and finding what works best for each of us is key.