It can be difficult to really know whether your fasts are helping you achieve your health goals when you’re solely relying on subjective ratings of how you are feeling (unless of course your goal is just to feel good!). However, tracking metrics like heart rate during your fasts can allow you to further analyze how fasting (and feeding) is affecting your metabolism.

How does fasting affect heart rate?

Heart rate is one of those sneaky metrics that can actually provide a lot of insight into how we are treating our bodies, and our heart! It’s tough to pin down because it’s incredibly variable -- Everything we do from walking up the stairs, to high-intensity exercise, eating and fasting, and even sleep changes our heart rate. Heart rate at rest is the measurement that we’re really talking about here. Resting heart rate is associated with our overall physical fitness and cardiovascular risk factors, among other health markers, and tracking it can help us evaluate lifestyle choices, including our fasts.

In the context of evolution, fasting was a part of life for our hunter-gatherer ancestors. They didn’t fast by choice, they fasted because food was scarce. It’s only in the last century that food has become industrialized, with 24/7 access. Our bodies adapted to feast-and-famine over millennia. Initially, the body may respond by activating our sympathetic nervous system, otherwise known as our “fight or flight” mode, which can certainly increase heart rate. Sympathetic activation serves a couple of functions: one, it prevents hypoglycemia (low blood glucose) by signalling the breakdown of liver glycogen to glucose, and two, it increases alertness and food-seeking behavior. However, as mentioned in our article on fasting and sleep, this effect is only temporary.

In the long term, fasting is more likely to lower heart rate. This is our parasympathetic nervous system at work, otherwise known as our “rest and repair” state - slowing metabolism and reducing heart rate in an attempt to conserve energy for survival’s sake. Caloric restriction is known to produce this same effect, which is a frequent by-product of different types of fasting. Interestingly, caloric restriction seems to also produce reductions in heart rate at night, which may offer an advantage as far as sleep goes, but again, that’s a topic for our sleep discussion!

But it’s not all about calories. Timing of meals matters, too.

Recent research into circadian biology has identified that metabolism is tightly linked to time of day, and when we eat can have profound effects on the body, heart rate included. In one study of alternate-day-fasting (ADF), researchers observed a reduction in heart rate in the group that consumed all of their calories at lunch, but not at dinner, relative to three small meals spaced out throughout the day. This is an example of early-TRF, or “Early Time Restricted Feeding” involving a condensed eating window earlier in the day with a fast usually beginning around 3 - 5 PM.

So, while you may be able to achieve a reduction in heart rate by simply reducing calories, opting for an eating window earlier in the day may be a more sustainable approach that shifts your awareness from counting calories to time of day.

Further, if fasting reduces heart rate, it may not surprise you that feeding can increase heart rate. Every time we eat we put our bodies to work. Blood flow diverts to the gut to participate in digestion and energy delivery from food to body tissue.  This requires some serious heart pumping which in turn yields an elevated heart rate. The effect appears to be dose-dependent, meaning the larger the meal, the greater the increase in heart rate.

Tracking your resting heart rate, especially at night, can help you gauge how your meals and meal timings are affecting you, especially your sleep.