Everyone knows that exercise is good you. It’s that one thing that has no down side: It makes you stronger, quicker, and smarter. It fixes your metabolism, protects your nervous system, and boosts your mood.
It even has an anti-aging effect, keeping bone strong and dense, muscle intact, and fat at bay. Exercise also protects the skin, improving the inner layer of collagen that makes skin supple and buoyant rather than old and dry.
How does it do all this goodness?
Emerging research shows that exercise rejuvenates the mitochondria—a part of your cells known as “energy factories” that have the task of converting the fat, carbs, and protein from the food we eat into usable energy. With aging, mitochondria die off or become dysfunctional, starving cells of energy.
Loss of mitochondria has a markedly negative effect on physical function. For example, as muscle cells lose mitochondria, they become weaker and do not regenerate as quickly after trauma, whether from injury or just a tough, muscle-damaging workout.
Loss of mitochondria also has an aging effect: Scientists illustrated this in a study that used mice who were genetically designed to have fewer functioning mitochondria. Compared to a group of normal mice, they lived half as long and experienced visible signs of aging with a graying of fur, loss of hearing and eyesight, loss of lean muscle and bone, and accumulation of body fat. Researchers theorized that the decrease of functioning mitochondria produced an “energy crisis” that caused a progressive decline in tissue and organ function, ultimately resulting in aging.
Fascinatingly, exercise was found to halt the aging effect. When the low-mitochondria mice performed 45 minutes of running daily, three times a week, they not only looked as young as healthy mice, but biopsies showed the exercise stimulated the growth of new mitochondria in cells throughout the body. Cell death, which is linked to dysfunctional mitochondria, was also reduced. Although cell death is an important physiological process that can help avoid cancer and other diseases, if too many cells die, the organ and overall health are affected.
Research on humans further explores which types of exercise are best for protecting the mitochondria and slowing the aging process. This is an important question because public health recommendations typically recommend aerobic exercise as the be-all, end-all for older adults. However, the two strongest predictors of longevity are muscle mass and strength, neither of which is improved with aerobic training.
Not only is strength associated with greater ability to perform essential functions of everyday life, but the stronger you are, the lower your chance of falling. Muscle mass also has a protective effect, helping people survive cancer and other ailments such as kidney disease.
Muscle and strength are a product of overload: You overload the body with weights that you aren’t accustomed to and recruit higher threshold muscle fibers that allow you to increase your strength. Lifting weights also stimulates protein synthesis so the muscles maintain or increase in size. This is the opposite of what typically happens with aerobic training—an exercise mode that trains efficiency, while degrading muscle mass over long periods.
Therefore, it’s essential that we identify an exercise prescription that allows trainees to maintain muscle and strength, while also giving love to our mitochondria. A study from the Mayo Clinic provides some guidance: Researchers divided young (under age 30) and older (over 64) adults into 4 groups:
1. A control group
2. A weight training group
3. An interval training group (4 minutes on, 3 minutes active rest)
4. A combined training group (30 minutes of steady-state exercise and 2 days a week of strength training)
As you’d expect, the weight training group gained the most muscle, with the young trainees getting the most buff. Interval training resulted in the strongest increase in endurance. When scientists tested biomarkers related to mitochondria, they found that interval training reversed many of the age-related decrements that lead to lower energy, strength, and function in the elderly. In the older adults, interval training increased mitochondrial function by 69 percent, whereas the younger adults had a 49 percent increase. Their improvement was lower because baseline function was higher (that is, they had less room to improve). Aerobic training and weight training also had a protective effect, positively impacting mitochondria, though it was much smaller compared to the interval group.
Why do mitochondria respond differently to strength vs interval training?
The theory goes that interval training stimulates a pathway known as AMPK, which is necessary for the growth of new mitochondria. Strength training stimulates mTOR, which is the pathway that increases muscle mass. The mTOR pathway is thought to inhibit AMPK, which is why you don’t get the same robust growth of new mitochondria with strength training.
Additionally, interval training leads to an increase in both oxidative and fat burning enzymes that optimize the function of both the oxidative aerobic energy system and the glycolytic anaerobic system. In contrast, strength training is more reliant on a third energy system, the phosphagen system that engages when you need to produce a lot of power.
The take away is that to get the greatest protective, anti-aging benefits from your training you want to do the following:
Perform interval training to sustain fitness and optimize mitochondria. Training 2 to 4 days a week is ideal, with workouts ranging from using short, very intense intervals (30 seconds all-out) to longer, more moderate efforts (up to 3 minutes at a high-intensity pace).
Do strength training to maintain strength and muscle. Training 2 to 4 days a week does the trick, with workouts ranging from using heavy loads and fewer reps (2 to 8 reps) to lighter weights and more reps (8 to 15 reps).
If your goal is general fitness, you could do to non-consecutive days of interval training and two non-consecutive days of strength training. It’s okay if strength and interval are one after the other as long as you are recovering well and not having reduced strength or lingering soreness.
For goals geared at body composition, increasing your training days may pay off. Depending on preference and time constraints, you could either increase strength workouts to 4 a week with 2 interval sessions or do the opposite: 4 interval sessions and 2 strength workouts.
What about endurance training? If this is your bread and butter, you can incorporate endurance workouts into your program. It’s worth including at least one interval session for the mitochondrial benefits and a minimum of two strength workouts for the muscle mass benefits but after that, enjoy all the endurance training you want. Always check in with yourself and make sure you are recovering effectively and not having nagging soreness, increased appetite, or injuries.