Does lowering heart rate speed metabolism?

Not exactly. Your heart is a muscle that when it works less (lower heart rate or beats per minute), it needs less fuel (blood sugar as glucose). Since the heart is not a big muscle, you are not talking about much glucose. However, your heart rate contributes only a part of the work your body does that affects your metabolic rate even though your heart has to pump blood around your body and to your lungs at rest as well as when you are running at full speed.

Metabolism is the breakdown of foods you eat which happens in your stomach and intestines as well as absorption of food nutrients in your intestines (glucose, fructose, protein amino acids, and fatty acids). Food energy is measured in calories.

At rest or sleeping (basal metabolic rate), your organs especially your brain burn more calories than your body does during physical activities during the day. Physical activity and exercise are also measured as calories burned.  It is the balance between the food calories you eat and the food calories your body burns to keep running as well as the fuel for physical activities that determines your weight. Your metabolic rate is how many calories does your body burn to keep it running and fuel your physical activities over the course of 24-hours. Actually, your metabolic rate changes over the course of a day depending on when you ae sleeping versus when you are active or exercising. Your metabolic rate determines how fast your body burns calories. A higher metabolic rate means you can eat more food and still maintain your weight assuming you don’t eat more calories than you burn because then you will gain weight.

What affects your metabolic rate is your age, gender, level of physical fitness, and amount of muscle mass. Having a larger muscle mass means you have a higher metabolism because body fat basically just sits around waiting for a famine to provide fuel so you don’t starve. Whereas muscles do work as you move and burn calories.

As we age, our metabolic rate drops when the amount of male and female hormones drop as well as loss of muscle mass due to weight loss or lack of exercise. Men are more likely to have a consistent metabolism thru life until about age 70 when testosterone levels drop and men have a higher amount of muscle mass due to testosterone than the average woman so men’s bodies burn more calories. Women’s estrogen levels drop during menopause so their metabolic rate drops around age 50.

Exercise helps sustain a higher metabolic rate for up to 15 hours after exercise. So get moving 30 minutes a day, 5 days a week.