Both are important for health. Cardio helps support CV health (heart and blood vessels). Strength training increases the functioning and health of the musculoskeletal system (joints, muscle, bones, connective tissue). Both have benefits in the body that stretch far beyond these body system (e.g. mental health, blood sugar regulation, hormones etc).
What about for weight loss? Well, if you are in a caloric deficit, no matter what you do, you will lose weight. This is the key principle for any weight loss regime. Exercise is just another way to help your energy output and increase your caloric deficit.
However, strength training does seem to have an advantage over pure cardio when it comes to weight loss. This is because when you go to the gym consistently and repeatedly lift moderate to heavy weights at a volume and intensity that is enough to cause metabolic stress on your muscles, you break down muscle fibres/damage your muscles. This is why you can feel sore the next day after gym.
The repair process of your muscles post workout requires a lot of protein and energy. If you are in a caloric deficit, that energy will be taken from excess body fat. This repair process can take 24-36 hours post workout; therefore, your metabolism is raised throughout this time. Essentially you are burning body fat at rest, assuming you are in a caloric deficit.
Strength training done properly (i.e. with intelligent programming), also increases muscle mass, which then increases resting metabolic rate, because the upkeep of more muscle in the body requires lots of energy. Once again, if you are in a caloric deficit, that energy will be used from body fat.
If you are not in a caloric deficit, post workout nutrition (i.e. protein and carbs) will still be used to repair the muscles and will result in increased muscle mass. Increased muscle mass still increases your resting metabolic rate. This basically gives you more “leeway” in terms of your caloric intake. It means you can eat slightly more calories without putting on weight because your resting metabolic rate is increased. Increased muscle mass also increases the release of hormones in the your body that are associated with suppressing appetite and increasing satiety.
Strength training also prevents you from looking skinny fat as you lose weight. If you eat enough protein and strength train while losing weight, your body realises it needs your muscles and therefore will not use your muscles as protein reserves. This gives you a “toned” physique as you lose weight.
If you don’t eat enough protein and don’t strength train during weight loss (i.e. caloric deficit), your body will use muscle to extract protein for physiological functions. This can result in you looking skinny fat and ‘loose.’
As stated above, if you are in a caloric deficit and decide to do cardio, you will still lose weight. However, when compared to strength training, cardio doesn’t have the same post workout metabolism-boosting benefits. This is because it doesn’t stress the muscle in the same way and doesn’t lead to muscle gain.
During steady state cardio (i.e. low-moderate intensity for extended periods), you will burn carbs (from glycogen stores) and mostly body fat. Post workout, the body will look to replenish what was lost. If you are in a caloric deficit, it won’t be able to replenish what was lost leading to weight loss. If you are not in caloric deficit, the body will replenish the fat stores lost in the workout and because there is no increase in muscle mass, your metabolic rate will not be raised post workout.
Furthermore, the more steady state cardio you do, the more efficient your body becomes in energy usage; therefore you burn less fat. In saying this, HIIT style cardio training like intermittent sprints and hill runs will increase resting metabolic rate and “fat burning” post workout, due to the different energy system used compared to steady state cardio. However, HIIT training without weights will not increase muscle mass. In order to increase post workout metabolic rate, when it comes to cardio, it is all about the intensity. Shorter sessions at a higher intensity are better than long sessions at a low-moderate intensity.
While cardio may not directly increase muscle mass, cardio has been shown to aid muscle growth in other ways. In the right amounts per week, both steady state cardio and HIIT style cardio sessions, have both been shown to increase muscle recovery and increase muscles ability to produce energy more efficiently. This translates to increased muscle mass as it allows a person to recover between strength sessions more efficiently, hence they can train at a higher intensity. It also allows them to work at a higher intensity for a longer period of time during the strength training. This leads to an overall increased performance in strength sessions leading to increased muscle growth.
In summary, both are important for health and both can play a role in weight loss. Good news is that you don’t have to do two separate sessions of cardio and strength training, just incorporate them all into one to make everything more time efficient.
The easiest way is to do strength training in a format that raises your heart rate such as multi joint movements using large muscle groups with moderate-heavy weights, high volume and minimal rests. Increasing the muscle mass and metabolic stress of large muscle groups by using targeted exercises, it increases the amount of fat burning post workout due to the fact that larger muscles burn more fat at rest. Larger muscle requires more energy to repair post workout.
Another way is to do a low-moderate intensity cardio session (i.e. 30 mins max) or a short HIIT cardio session (i.e. 15 mins max) after your strength training. Shorter session will avoid overtraining and when performed after strength training, it won’t affect the quality/intensity of the strength training session. You would not want to do cardio before strength training due to issues around fatigue effecting the quality of your workout.
Lastly, if you decided to do cardio sessions on separate days to your strength training, make sure they are not very demanding as this will affect your performance in the gym and lead to overtraining eventually. Avoid long and drawn out sessions. Cardio is there to assist your performance in the gym and support cardio health. To do this, you don’t need to smash yourself. Less sessions and less time at higher intensities are better than more sessions at lower-moderate intensities.