First, a definition. Metabolic rate is simply the amount of energy expended over a given period of time. This energy is released as heat. You can measure your metabolic rate at a single point in time using different methods, but the rate will vary throughout the day based on a variety of factors including activity level. Here are some factors that affect metabolism.
Factors That Determine Your Metabolic Rate
Race
According to a study published in Obesity Research, black women have a resting metabolic rate that’s about 5% slower than white women.
Gender
Men have a metabolic rate that’s 10 to 25% higher than women. This is at least partially due to greater muscle mass since muscle is more metabolically active than fat tissue. You can’t control your sex, but women can increase their lean body mass through exercise.
Age
Resting metabolic rate drops by as much as 2% each year after the age of 20. Both men and women also lose muscle mass as they age, which accounts for some of this decline. You can’t control the aging process, but you can do strength training to increase how much muscle you have.
Size
Larger people have higher metabolic rates because they have a greater total mass. You can’t determine your height or the size of your frame, but you can alter your body composition and how much mass you carry to some degree through diet and exercise.
Genetics
Genetics play a role in determining metabolic rate too. Most people know someone who can eat anything they want without gaining a pound. Unfortunately, it catches up with them as they age, and their metabolism starts to slow down. Some people aren’t able to adapt to their changing metabolism and gain significant amounts of weight as they grow older. You’ve probably heard formerly thin people say, “I was as skinny as you when I was younger.” They probably were.
Hormones
The primary hormone responsible for regulating metabolic rate is thyroid hormone, but sex hormones such as estrogen, progesterone, and testosterone play a role too. This may be due to their effects on lean body mass. Most women experience a steeper decline in metabolic rate after menopause, but the accelerated loss of muscle mass also contributes to this slow down. Growth hormone and other fat-burning hormones likely play a role in regulating metabolism, and growth hormone levels decline with age.
Other Factors That Affect Your Metabolic Rate
Factors such as ambient temperature affect metabolic rate. Colder temperatures boost the metabolic rate by causing shivering, which produces more heat. Turning down the temperature in your home can subtly boost your metabolism.
If you’re anxious or have a fever, you have a faster metabolic rate and produce more heat. Some medications can alter it too. Thyroid hormone, nicotine, and caffeine raise it, and some medications such as antipsychotic drugs lower metabolism.
A high-intensity exercise that uses the anaerobic energy system such as heavy weight-lifting and sprinting boost metabolism for hours to days afterward, and when you build lean body mass you burn more calories. Moderate-intensity endurance exercise has less of an effect on metabolic rate.
What and how you eat plays a role too. Restrict calories too much and your metabolism slows down to protect you against starvation.
What You Can Do to Boost Your Metabolism
Certain factors such as genetics, sex, race, and age you can’t control. But you can boost your metabolism by doing high-intensity exercise and resistance training to increase lean body mass. If you restrict calories, never go below 1200 calories a day to avoid slowing down your metabolism. Focus on eating smaller meals more frequently that contain small amounts of lean protein. Drinking caffeinated beverages and green tea may also subtly increase metabolism.
References:
Ann Intern Med. 1995 Nov 1;123(9):673-5.
Obesity Research (2002) 10, 726-732.
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3 Factors That Determine Daily Energy Expenditure and Metabolic Rate
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Is Muscle Loss the Only Reason Your Metabolism Slows with Age?
Why Metabolic Rate Calculators Aren’t Always Accurate
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I think it’s interesting how many women carry extra weight (from what I’ve observed, usually as a result of poor diet and lack of exercise, despite their claims to otherwise) and want to blame it on their thyroid, and try to convince their docs they have some hidden thyroid issue. It’s a small percentage of women that truly have hypothyroidism (I don’t know the exact number but I think it’s around 5%), and yet fully half of the overweight women I know think it’s their thyroid…
Amy, I have done a fair amount of research on this topic. Granted, most of it has been on hyperthyroidism, rather than hypo, but they do overlap in that many journals cover both topics. While it’s not good for people to make excuses for poor habits, and I’d bet many of the women you’re referring to are doing just that, there is concern that there are a good number of undiagnosed hypothyroid people trying to function when they’d be better off with some supplemental hormone. Part of the problem is that the normal range for testing in the US is somewhat skewed by the number of obese and overweight people in the general population. Many people feel their best on the low end of normal, which is to say on the hyper side of normal, rather than the hypo side of normal. Endocrinologists and gps are reluctant to treat people within the normal range, which is completely understandable, but they also undertreat those already diagnosed as well. Many endos test only once, and only for TSH, rather than also testing free t3 and/or free t4 or testing at intervals, which gives a poor picture of the body’s hormonal state. Meanwhile, the ATA estimates that as much as 12-18% of the US population are hypothyroid if you include those that test subclinical, or within normal range but symptomatic. That’s a lot of people who could be helped to function better.