Does this cliche fitness statement hold true? Like pretty much everything in this world, it depends and is highly contextual. So, picking a set percentage doesn't make much sense.
Losing weight simply requires an individual to be in a calorie deficit and can be achieved through diet manipulation, exercise, or a combination of both. Nutrition's importance also depends on the goal at hand- pure weight loss or improved body composition (increasing muscle mass). Let's examine both scenarios.
Pure weight loss
If this is your goal, it is possible to get results without any exercise at all. Example- Donald Trump requires 2,000 calories per day to maintain his body weight based on a sedentary lifestyle with no exercise (hypothetically). By simply dropping calories to 1,500 per day (500 calorie daily deficit needed to lose roughly one pound per week), Mr. Trump will lose weight. No elliptical, no running, no lifting weights, nothing. So in this situation nutrition would be considered to be 100% of the equation.
However, adding in 3 to 5 hours of exercise per week will increase your calorie expenditure by a few hundred per day making it easier to diet because more food can be consumed. So, if Trump walked for an hour each day after his campaign speeches were over, he could probably get away with eating 1,800 calories instead of 1,500 calories and still lose a pound per week. Now, nutrition and exercise are blended (probably 70-90% nutrition, 10-30% exercise).
On the flip side, if Trump decided to ignore tracking his calories and just increase exercise frequency in an attempt to drop weight, it probably won't work. It doesn't matter how much you exercise if you are still in a calorie surplus. You see this a lot with people looking to lose weight- first instinct is to ramp up the cardio with no awareness of how many calories they are actually consuming.
Improved body composition
If your goal is to lean out but increase muscle mass, exercise (weightlifting specifically) plays a much larger role than in pure weight loss. Muscle tissue simply cannot grow without an appropriate stimulus. Lifting weights provides the stress muscles need to break down, repair, and ultimately grow. So, in this case nutrition is still important but you can argue that exercise (weightlifting) is just as important (50% nutrition and 50% exercise).
I'd advise most people, guys in particular, to prioritize body composition over just losing weight. If you are a guy and drop 50 pounds with no foundational base of muscle, it's not going to look good (think skinny fat). Fortunately, it's very easy to put a base of muscle on by just incorporating some full body weight training into your schedule 2-3 days per week.
What's the verdict?
Again, it's hard to say that nutrition is 80% of the equation in all cases. You can see from the above examples that the percentage swings based on the individual. However, for the general population just looking to lose some weight, nutrition is going to provide the best bang for your buck. And that's great news! You know why? Because eating food is something we already do everyday and not additive like starting to workout. So, instead of saying I HAVE TO do more cardio to lose weight, start saying I HAVE TO be more responsible and aware of the amount of calories I consume each day. The body fat will start peeling off with much less effort and stress!
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