Why Cardio Still Matters (Even If You Lift Hard)

If you only have one number that tells you how long you’re likely to live, make it VO2 max. It outpredicts blood pressure. It outpredicts smoking status in middle-aged adults. In a 2018 JAMA Network Open cohort study of over 122,000 patients, the difference in mortality between the highest and lowest fitness quintile was larger than the difference between smokers and non-smokers (Mandsager et al., 2018, JAMA Network Open, PMID: 30646252).

VO2 max is the maximum volume of oxygen your body can transport and use during all-out effort. It’s a system-level metric. It depends on your heart’s stroke volume, your blood’s oxygen-carrying capacity, your capillary density, and your mitochondrial machinery. Cardio improves every one of those.

Figure reference: Mandsager K et al., “Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing.” JAMA Netw Open. 2018;1(6):e183605. PMID: 30646252. Reference Figure 1: Kaplan-Meier survival curves stratified by cardiorespiratory fitness percentile.

What cardio does that strength training doesn’t

Strength training builds muscle quality and density. Cardio builds the delivery system that feeds that muscle.

Specifically, three adaptations belong to cardio and not to lifting:

The left ventricle of the heart enlarges and pumps more blood per beat. That’s why endurance-trained athletes have resting heart rates in the 40s — same blood flow, fewer beats. The heart works less for the same output.

The capillary bed in skeletal muscle proliferates. Each muscle fibre ends up surrounded by more capillaries, which means more oxygen delivery and faster lactate clearance.

Mitochondrial density and oxidative enzyme content go up. The cellular machinery that turns food into ATP becomes more abundant and more efficient. This is the part that makes you feel less tired walking up stairs at 50.

None of these adaptations show up from lifting alone. You can be very strong and still have a VO2 max in the fifth percentile. I’ve assessed it in the studio. Strength and cardio are non-redundant.

How much cardio, and what kind

The 2018 Physical Activity Guidelines for Americans recommend 150 to 300 minutes of moderate-intensity activity per week, or 75 to 150 minutes of vigorous. That’s the bare clinical minimum. It’s not what trained endurance athletes do, but it’s the dose at which mortality curves start to bend.

For most clients, that means three sessions per week of either: 45 to 60 minutes of zone-2 work (brisk walk uphill, slow jog, stationary bike — you can speak in full sentences but you’re working); or 20 to 30 minutes of mixed intervals (Norwegian 4×4 protocol, or eight rounds of 20 seconds hard / 10 seconds rest).

The Norwegian 4×4 protocol — four minutes at 85–95% max heart rate, three minutes easy, repeated four times — has the most published evidence behind it for raising VO2 max efficiently in time-poor adults (Helgerud et al., 2007, Med Sci Sports Exerc, PMID: 17414804).

The walking case

Before we get to anything fancy, walking. Specifically, walking after meals. A 2022 meta-analysis in Sports Medicine (Buffey et al., 2022) showed that even two to five minutes of light walking after a meal significantly attenuated post-meal blood glucose excursions in adults with and without type 2 diabetes. The effect was reliable. The dose was minute-level, not hour-level.

I’ve cited this in a previous paper of mine — Myette-Côté et al., 2018, American Journal of Physiology — Regulatory. The randomized trial we ran in patients with type 2 diabetes added a postmeal walk to a low-carb diet and the combination compounded the glycemic effect. Walking is the most under-prescribed metabolic intervention in modern medicine.

What this looks like in practice

If you’re doing zero cardio right now, three twelve-minute walks per week is your starting point. Not three thirty-minute runs. Twelve-minute walks. You can scale up from there once it’s a habit.

If you’re already lifting three days per week, add two cardio sessions: one zone-2 of 45–60 minutes, one shorter interval session of 20–25 minutes. That’s the configuration that produces the best body composition, the best VO2 max, and the lowest cardiovascular disease risk in the published evidence.