Cardio and aerobic fitness

Cardiorespiratory fitness is the ability of the heart, lungs, blood and muscles to supply and use oxygen during sustained physical activity. Its standard measure is VO2 max, the most oxygen the body can use during all-out exercise, often expressed in METs (multiples of the oxygen used at rest). Fitness is among the strongest known predictors of mortality (risk of death): in a meta-analysis (a study that pools the results of earlier studies) of 33 studies and 102,980 adults, each 1-MET higher level of fitness was associated with 13% lower all-cause mortality (death from any cause), and adults with low fitness had 70% higher mortality than those with high fitness [1]. Among 750,302 US veterans, mortality kept falling up to about 14 METs, and being unfit carried more risk than any cardiac risk factor examined [2]. VO2 max declines with age, by more than 20% per decade from the 70s onward [3].

Aerobic activity is usually grouped by intensity. The World Health Organization recommends 150 to 300 minutes a week of moderate activity or 75 to 150 minutes of vigorous activity, or an equivalent mix [4]. Zone 2 training refers to steady exercise at the upper end of moderate intensity. In six pooled studies of 661,137 adults, meeting the minimum recommended amount of leisure activity was associated with 31% lower mortality, and the association levelled off at 39% lower at three to five times that amount [5]. High-intensity interval training alternates vigorous bouts with recovery and raises VO2 max more than moderate continuous training on average [6], although the one long-term randomised trial (participants assigned to groups by chance) with death as an outcome found no statistically significant difference (none clearly beyond chance) [7].

Walking and daily steps are the most common form of aerobic activity. In pooled data from 15 studies of 47,471 adults wearing step counters, the most active quarter (median 10,901 steps a day) had 53% lower mortality than the least active (median 3,553), with risk levelling off at 6,000 to 8,000 steps a day in adults aged 60 and over [8]. Nearly all of this evidence is observational, from tracking people rather than assigning them to exercise: fitness is partly inherited, and illness lowers both fitness and activity before it is diagnosed, so the associations probably overstate the causal effect. The broader relationship between physical activity and lifespan is described under exercise and longevity.