Why 30-40 Minutes of Cardio May Be a Perfect Goldilocks Zone

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Throughout my ultra-endurance sports career, I pushed my body to extremes that were dangerous and unhealthy; I took my love of running and my passion for Ironman triathlons too far and almost died prematurely in the process.

The duration (often 7-9 hours per day) and "pedal-to-the-metal" exertion level of my daily cardio workouts were maniacally intense. After breaking a Guinness World Record by running six back-to-back marathons nonstop on a treadmill, I found myself in the ICU on the verge of dying young and becoming a mortality statistic.

Although many people assume that "more is better" when it comes to high-intensity interval training (HIIT) and moderate-to-vigorous physical activity (MVPA), this is not the case. On both ends of the daily physical activity continuum, too much and too little exercise can increase someone's risk of morbidity and mortality.

For me, being a "cardio fanatic" backfired. I learned the hard way that there's a sweet spot of exercise intensity and duration that's healthy and promotes longevity. Just like sitting too much can escalate mortality risk, doing too much exercise can also increase one's odds of premature death.

This intro leads us to the million-dollar question: What is the perfect "Goldilocks zone" or sweet spot of daily cardiovascular exercise that's just right for reducing all-cause mortality? For the record, this is a hotly debated topic with a wide range of different (and sometimes contradictory) evidence-based findings. (See here, here, here, here, here)

As one example, a meta-analysis of 16 different studies published a few years ago (Ekelund et al., 2016) concluded that 60-75 minutes of daily MVPA was ideal for eliminating the increased risk of premature death associated with excessive sitting. The research used for this meta-analysis was based on self-reported data, which often skews the results because most people know they should "sit less, move more" and inadvertently exaggerate how much time they spend moving vs. sitting each day.

Now, a new meta-analysis of nine different studies (Ekelund et al., 2020) reassesses the "Goldilocks zone" of daily MVPA associated with lower all-cause mortality based on accelerometer measured (not self-reported) physical activity and sedentary time. This paper was published on Nov. 25 in the British Journal of Sports Medicine.

Notably, when people's daily habits were gauged using fitness tracking devices, the data showed that less than 60-75 minutes of daily exercise seemed to offset the detrimental associations of too much sitting. As the authors explain, "This finding may suggest that relatively low levels of MVPA, lower than the current recommendations, reduces the risk of death if combined with low levels of sedentary time."

In a recent New York Times article, "11 Minutes of Exercise a Day May Help Counter the Effects of Sitting," Gretchen Reynolds reports on this new (2020) meta-analysis. Overall, her excellent reportage represents the research findings perfectly, but, in my opinion, the title itself is slightly sensationalized. After reading this open-access BMJ paper, it seems to me that from a public health perspective, highlighting "34 minutes" as the sweet spot for increasing longevity would be a more beneficial summation of this meta-analysis' key findings than "11 minutes."

As Reynolds writes in her Dec. 2 NYT article:

"People in the middle third for activity, who exercised moderately for about 11 minutes a day, were significantly less likely to have died prematurely than people who moved less, even if all of them belonged to the group that also sat the most. Crunching the numbers further, the researchers concluded that the sweet spot for physical activity and longevity seemed to arrive at about 35 minutes a day of brisk walking or other moderate activities, an amount that led to the greatest statistical improvement in life span, no matter how many hours someone sat."

Although the above-mentioned NYT headline spotlights that relatively small amounts of physical activity can pay big dividends (which is an important message), the title also inadvertently promotes the absolute minimum amount of daily MVPA that appears to offset the negative health consequences and all-cause mortality associated with too much sitting.

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In the meta-analysis that Reynolds is referencing above, the authors (Ekelund et al., 2020) write:

"Our joint analysis results suggest that about 30-40 minutes (median of medians=34 minutes; interquartile range: 26-48 minutes) of MVPA per day (online supplemental table) attenuated the risk of death in our most conservative model. Those with as little as 11 minutes per day of MVPA in combination with 'low' sedentary time (i.e., <8.5 hours per day) did not differ in risk compared with the referent group (i.e., highest third of MVPA in combination with lowest third for sedentary time). This finding may suggest that relatively low levels of MVPA, lower than the current recommendations."

In their conclusion, Ulf Ekelund of the Norwegian School of Sports Sciences and co-authors sum up the key finding of their most recent (2020) meta-analysis: "About 30-40 minutes of MVPA per day attenuate the association between sedentary time and risk of death, which is lower than previous estimates from self-reported data."