When I was a student athlete in college, I was warned against going for too long without food for fear that my body would enter “starvation mode”. Rumors had it that even going a day without much food would prompt my body to start pulling from its muscle reserves or, worse yet, start storing all of my energy as fat when I started eating again. Regardless, not eating for anything more than a few hours would surely reduce my athletic performance. Sugary Powerade was always available and encouraged before practices, and we rushed to “carb up” after workouts. 

Science research is beginning to reveal just how wrong this thinking was. (By the way, we now know that it’s all that sugar that promotes fat deposition.) Today, athletes work out fasted and trade sugars for ketones – chemicals produced by the liver from fatty acids during periods of low food intake. Healthy people are reducing their heart disease risk and levels of inflammation by eating every other day. What?!

“Pre-clinical studies have shown that [dietary interventions including intermittent fasting] reduce oxidative stress and metabolic rate, activate autophagy [a process of cellular recycling], and expand healthspan and lifespan in various model organisms [including mice and monkeys].” – Stekovic et al., 2019

Alternate day fasting is a form of intermittent fasting that involves eating less than 500 calories every other day, or more strictly fasting for 36 hours followed by 12 hours of eating normally every two days. A small number of clinical trials have now shown this form of intermittent fasting to be safe and to promote weight loss and metabolic health, but among mostly overweight people. Now, a new clinical trial suggests that many healthy adults could probably benefit from alternate day fasting. 

A new research study published in Cell Metabolism suggests that for healthy, non-obese adults, not only is eating every other day safe to practice for at least several months, but it also has weight loss, general health and potential anti-aging benefits. For example, alternate day fasting appears to increase levels of healthy fatty acids (like those found in olive oil!) and antioxidants like vitamin E in the body. It also appears to kickstart recycling of old proteins and to lower blood levels of “pro-aging” protein bits such as methionine, an amino acid found in meat and dairy products that when restricted from the diet is associated with longevity in animals

The new study, conducted by researchers at the National Institute on Aging and the Medical University of Graz in Austria, reports changes in markers of general health and aging among people who practiced alternate day fasting for either 4 weeks (as part of a randomized controlled clinical trial) or for more than 6 months.

green tea or coffee on their fast days. Photo by Alexandru G. STAVRICĂ on Unsplash.
In the Cell Metabolism study of alternate day fasting, individuals in the fasting group couldn’t have any calories but could consume water, flavored carbonated water, unsweetened black or green tea or coffee on their fast days. Photo by Alexandru G. STAVRICĂ on Unsplash.

Compared to 30 people who ate normally for 4 weeks, 30 people who fasted every other day consumed fewer calories even though they were told to eat as much as they wanted on non-fasting days. They also lost a healthy amount of weight, lowered their fat mass – especially in the trunk region, increased the levels of ketones in their blood even on their “feast” days, lowered their blood pressure and lowered their risk score for developing cardiovascular disease. They also had lowered levels of T3 in their blood, a thyroid hormone that is often found in lower levels in centenarians and family members of long-lived individuals

Amazingly, only one individual dropped out of the fasting intervention (which does cause a lot of hunger on fasting days, especially at first), while none reported any issues or had any adverse health events happen while practicing alternate day fasting. 

Compared to people eating normally, people who had practiced alternate day fasting for more than 6 months also had lower levels of cholesterol, triglycerides, T3 thyroid hormone and sICAM-1 – a marker of inflammation. This soluble intercellular adhesion molecule-1 (sICAM-1) is a biomarker “for inflammatory processes involving activation or damage to cells such as platelets and the endothelium.” If this is too much science, no worries – it basically means that when sICAM-1 is around, inflammation is as well. People with asthma, like me, tend to have higher levels of sICAM-1 circulating in their bodies. It therefore makes sense that fasting would improve asthma symptoms.

Some other interesting findings: People practicing alternate day fasting for more than 6 months had lower levels of stress response proteins after a fast day, such as proteins involved in inflammation and programmed cell death.

People who had practiced alternate day fasting for more than 6 months also didn’t experience any of the downsides associated with long-term restriction of calories, such as loss of bone mass (a risk factor for osteoporosis and frailty later in life) or a lowered number of immune cells. Immunosuppression is a downside of chronically consuming fewer calories than you need, which makes only intermittently restricting calories a promising alternative for healthy aging.

Is Alternate Day Fasting Right for You?

There is still a lot we don’t know about how alternate day fasting could impact different people in different ways. It probably isn’t a good idea for people at risk of becoming underweight or frail for various reasons. Any form of intermittent fasting isn’t appropriate for people with type 1 diabetes, people with eating disorders or women who are pregnant or breastfeeding. But for otherwise healthy individuals seeking to improve their weight, metabolic health or inflammatory status, alternate day fasting could be a safe and beneficial alternative to restricting calories. 

“Preclinical studies suggest that the beneficial effects of intermittent fasting cannot be solely attributed to a reduction of total caloric intake.” – Stekovic et al., 2019

However, we don’t know if (and for whom) alternate day fasting is more or less beneficial than other fasting schedules, such as early time-restricted eating (eating early in the day with a 16 hour or longer fasting period overnight). If you do want to practice alternate day fasting, it is best to start slow with overnight fasting and to ultimately stick to a schedule that leaves you feeling your best. If you can, also monitor your blood glucose and ketone levels to ensure that you are staying within healthy ranges during fasting and eating periods.

“The optimal length of recurring fasting periods in humans may depend on the desired effect and parameters measured and be subject to individual differences.” – Stekovic et al., 2019

Some potential issues with alternate day fasting may be short-term reductions in insulin sensitivity and blood sugar spikes when people break longer (more than 24 hour) fasts. While this is likely a short-term, transient effect of longer fasting durations, it might mean that people practicing alternate day fasting need to be more careful to eat lower glycemic index foods when they break their fasts. On the other hand, daily early time-restricted eating typically improves insulin sensitivity the morning after a fast.

Fish, veggies, olive oil and herbs - an example of a healthy, low glycemic index and anti-inflammatory meal. Photo by Ive Erhard on Unsplash.
Fish, veggies, olive oil and herbs – an example of a healthy, low glycemic index and anti-inflammatory meal. Photo by Ive Erhard on Unsplash.

“In humans, prolonged fasting (>48 hours) induces insulin resistance; this is likely a protective mechanism to spare glucose for the central nervous system.” – Hutchison et al., 2018

“Importantly […] a wholesome and balanced diet is likely crucial to foster the beneficial effects caused by ADF [alternate day fasting]. Thus, appreciable clinical support and a generally healthy lifestyle should be considered before starting ADF.” – Stekovic et al., 2019

We need more studies of long-term alternate day fasting to determine its safety and health impacts as a lifestyle. But many users of our LIFE Fasting Tracker and LIFE Extend apps currently successfully practice this style of intermittent fasting for weight loss and other health reasons. It’s also a great way to quickly raise levels of brain-protective ketone bodies in the blood.

“[P]eriodically elevated levels of ketone bodies, such as b-hydroxybutyrate, might contribute to long-term healthspan improvements and cardioprotective effects.” – Stekovic et al., 2019


Have questions about alternate day fasting? Let us know! Join the First Time Fasters Circle in the LIFE Fasting Tracker of LIFE Extend app.