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This question originally appeared on Quora. Answer by Bart Loews

First let’s put in the BMI reminder: BMI is an indication of nothing. The real issue is “overfat”, not “overweight”.

I could put on 20 pounds of muscle and be considered “overweight” but be completely “healthy”.

Let’s contrast the two situations then, and analyze how they came to be.

  • An overfat person doesn’t (generally) move much. Yes, yes, diet is more important than exercise, but generally speaking, most people who are overfat don’t exercise.
  • An overweight person due to muscle mass can’t get that muscle mass without exercising

Why is that important? You need the exercise to make your heart stronger in a healthy way. Additionally, progressive overload and systematic adaptations to implied demands apply. When you become overfat by eating too much and not exercising enough, you’re putting a sudden an quick demand on your heart, muscles, and bones to adapt quickly to your movements. When I gain 20 pounds of muscle, that won’t happen over night — it happens over the course of months if not years. My heart has a chance to progressively grow into it’s needs.

On top of that, CVD and a number of other conditions have elevated risk factors when you are overfat. In terms of a costs/benefits analysis, you have a lot of costs in making your overfat but very little in terms of benefits.

Some other things that might be counter intuitive: Smoking makes your heart race. Your heart gets stronger when you smoke cigarettes because nicotine is a stimulant, and your blood oxygen levels are limited because of the contents of the smoke you’re inhaling. On top of that your lungs become lined with tar, so over time they become less efficient at gas exchanges. Detecting the lower levels of oxygen, your heart is made to beat faster to more quickly circulate what it can.

Is this a healthy way of strengthening your heart? Absolutely not. The number of risk factors you incur by this strategy are astronomical. [ Smoking and Your Heart]

Why not just exercise?

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