Rethinking Metabolism

Rethinking Metabolism

"friday night workout" by InaFrenzy is licensed under CC BY 2.0

Rethinking Metabolism

by Dale Mayo

According to the Mayo Clinic website[1], metabolism is the chemical process by which a body converts what you eat and drink into energy.  The energy your body needs for such functions as breathing, blood circulation, growing and repairing cells is called basal metabolic rate and uses somewhere around 65% of the calories you take in.  Processing food takes about 10% of the calories ingested.  Physical exercise – running, playing basketball, walking the dog – accounts for the rest of the calories burned each day and is the most variable of the factors.

A study recently published in Science[2] has upended much of we thought we knew about metabolism, which directly affects how we think about weight loss.  As pointed out in the New York Times[3], conventional wisdom about metabolism has been that people gain weight after they reach their 20s because their metabolism slows down, especially around middle age, and that women have slower metabolisms than men and therefore have more trouble losing weight (which is exacerbated by menopause). 

It is very expensive to study metabolism – Vox[4] has an interesting description of how such research is done, from the perspective of someone who spent a day in a metabolic chamber at the National Institutes of Health – so most published research has been based on very small samples.  For the study published in Science,[5] the primary author – there are more than 80 co-authors - was granted the use of data from 6 labs collected over 40 years. With the large quantity and high quality of information, the researchers could determine whether and how age, sex, and body size affect metabolism.  Here are some of their findings[6]:

  • Metabolism differs across four stages of life:  Infancy to age 1 (calorie burning is 50% above adult rate); ages 1-20 (metabolism slows about 3% per year); ages 20-60 (metabolism holds steady); and over 60 (declines by about .7% per year).
  • There are outliers – people whose metabolic rates are above or below the average for their age.
  • Weight gain cannot be attributed to a slowing metabolism.
  • There appears to be no real differences between the metabolic rates of men and women.

More about metabolism and weight

The role of metabolism in weight loss and weight control is not yet understood and new research is changing what we have taken for granted.  For example, a new study reports that we don’t burn as many calories as we think when we exercise.[7], [8]  In fact, we appear to use only 75% of the calories we thought we were using for specific activities (e.g., 75 calories for walking a mile, rather than the previously thought 100) because our bodies compensate for the additional activity.  In addition, people with relatively high levels of body fat tended to compensate for 50% of the calories they burned by exercising.  Relying on exercise alone to lose weight may not be wise.  Rather, reducing the number of calories consumed in addition to increasing physical activity will help you burn more calories than you ingest. 

What we think know now:[9]

  • Most (60-80%) of the energy you burn is from your basal (resting) metabolism.
  • Metabolism varies between people.
  • There is no food that significantly increases your metabolic rate.
  • The more lean muscle you have, the higher your metabolic rate.
  • While crash diets can lower your metabolism, healthy weight loss is possible.

Photo by mikecogh is licensed under CC BY-SA 2.0

Metabolic syndrome

According to the Johns Hopkins Medicine website,[10] metabolic syndrome is the presence of a cluster of risk factors specific for cardiovascular disease that raises the risk of developing diabetes, heart disease, stroke, or all three.  The National Heart, Lung and Blood Institute (NHLBI) lists the following cluster of metabolic factors:

  • Abdominal obesity - having a waist circumference of more than 35 inches for women and more than 40 inches for men – this is the form of obesity most strongly tied to metabolic syndrome.
  • High blood pressure of 130/80 mm Hg (millimeters of mercury) or higher. High blood pressure is strongly tied to obesity. It is often found in people with insulin resistance.
  • Impaired fasting blood glucose - a level equal to or greater than 100 mg/dL
  • High triglyceride levels - more than 150 mg/dL. Triglycerides are a type of fat in the blood.
  • Low HDL (good) cholesterol - less than 40 mg/dL for men and less than 50 mg/dL for women.

The NHLBI and American Heart Association recommend a diagnosis of metabolic syndrome when a person has three or more of the five factors listed above.  According to the Cleveland Clinic, more than 20% of Americans have metabolic syndrome including more than 40% of people in their 60s and 70s.[11]  Metabolic syndrome is typically found in people with abdominal obesity, with diabetes mellitus, with other clinical features of “insulin resistance,” older people, and in certain families and ethnic groups with higher risk.

Preventing and reversing metabolic syndrome can be accomplished by exercising and losing weight, as physical inactivity and excess weight are main contributors:

  • Lose 5-10% of your body weight (if you are overweight or obese)
  • Increasing aerobic activity (e.g., a 30-minute brisk walk every day)
  • Maintain a healthy diet with whole grains, legumes, fruits and vegetables, healthy fats (e.g., olive oil), and replace red meats and poultry with fish.

Sockeye Salmon

"Planked Alaskan salmon and asparagus" by WordRidden is licensed under CC BY 2.0

Healthy tips

While there are many diets that claim to boost your metabolism, the few foods that were thought to do so (e.g., coffee, green tea, chili, and other spices) have not been shown to have a positive effect on metabolism.[12]  On the other hand, maintaining a healthy weight can affect whether or not you develop metabolic syndrome, so following a healthy diet makes sense. 

Try to make carbohydrates 50% or less in your daily intake of calories. Limit snack foods like potato chips and reduce your intake of sugar.  Eat high fiber foods (cruciferous vegetables, nuts seeds and beans), lean protein and healthy fats.  Get plenty of sleep.[13]  Replacing red meat and poultry with fish not only helps reduce your intake of saturated fats but also increases your intake of beneficial omega-3 fatty acids.

Finally, get out and get 30 minutes of exercise every day.  Walking is perfectly acceptable as a daily exercise.  Pair it with lifting weights and you will build more lean body mass and increase the number of calories you burn in a day.  Some people swear by high-intensity interval training (HIIT) to burn more calories and potentially boost metabolism.[14], [15] 


Please talk with your health care provider about any complementary health approaches you use and before you make big changes in your usual exercise routine or your diet.




[2]   Pontzer, H., Yamada, Y., Sagayama, H., et al. (2021) Science 373,6556: 808-812.

[3]   Kolata, G. (2021). What we think we know about metabolism may be wrong. The New York Times. August 21, 2021.

[4]   Belluz, J. (2018) What I learned about weight loss from spending a day inside a metabolic chamber.  Vox, September 4, 2018.

[5]   Pontzer, H., Yamada, Y., Sagayama, H., et al. (2021) Science 373,6556: 808-812.

[6]   Kolata, G. (2021). What we think we know about metabolism may be wrong. The New York Times. August 21, 2021.

[7]   Reynolds, G. (2021) Your workout burns fewer calories than you think.  The New York Times. September 22, 2021.

[8]   Careau, V., Halsey, L.G., Pontzer, H., et al. (2021).  Energy compensation and adiposity in humans. Current Biology,


[9]   Belluz, J. (2016).  Most of us misunderstand metabolism. Here are 9 facts to clear that up. Vox. May 18, 2016 (updated September 4, 2018).



[12]   Rosenbloom, C. (2021). Why scientists now say you can’t blame your midlife weight gain on a slow metabolism.  The Washington Post. August 26, 2021.

[13]   Kirkpatrick, K. (2021) The calories in, calories out concept is 'tragically flawed,' new research suggests.  Today.  September 20, 2021.



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