For a long time it was believed that your metabolism drops dramatically after the age of 20, making it harder to lose weight and stay in shape. But a recent study has shown that our metabolism – also known as energy expenditure – remains relatively stable between the ages of 20 and 60 before decreasing as we age.
The researchers looked at existing studies of energy consumption from 29 different countries. In total, they looked at data from around 6,400 people from birth to the age of 95. Each study measured energy use using a method called doubly labeled water. The participants drink a special water to which a safe, radioactive marker has been added. The marker identifies the hydrogen and oxygen present in the water, which allows researchers to track how quickly the body processes both. Urine samples are then taken from each person to track the speed both of which have traveled through the body. This gives researchers an accurate measure of a person’s metabolism – the amount of energy they used up in a day.
The analysis showed that energy consumption (metabolism) increased rapidly from birth to a year. After that, energy consumption gradually decreased up to the age of 20 and then stabilized up to the age of 60 – even during pregnancy. After 60, the energy consumption begins to decrease. These results were also correct when the researchers considered various factors – such as physical activity and body composition (how much fat or muscle a person had and how much they weighed) – that can affect a person’s metabolism.
This study builds on our understanding of human metabolism. Knowing how our metabolism may (or may not) change over the course of our lives can be important in understanding how diseases – such as obesity, diabetes, heart disease, and even some cancers – can be treated.
One problem with the study’s results, however, is that they didn’t take into account a person’s energy intake. Many people still gain weight as they age, even though their metabolism remains relatively the same throughout their lives. This suggests that the weight gain is not due to a slowed metabolism. Rather, it is because we consume more food (energy) than our body consumes.
Energy and weight gain
Metabolism is affected by many factors – including the amount of food we eat, how much physical activity we get, weight, and whether we have a lot of muscle. Metabolism can also affect how energy is extracted from food. For example, if we eat more than we need, the body is more likely to use carbohydrates and proteins from these foods for fuel and store the fat, leading to weight gain. Weight gain and obesity are both linked to metabolic rate, which is caused by the fact that we ingest more energy than we use.
With so many things to keep track of, understanding how best to manage your weight can be difficult. For this reason, experts have made national recommendations on how many calories people of different ages and activity levels should get. These recommendations are based on data on energy consumption – not energy consumption. This is because measuring expenditure is considered more accurate than measuring energy intake, which is usually done by asking people to tell themselves what they eat on a daily basis.
The problem with using energy usage data, however, is that these recommendations are based on an average value – so what works for some may not work for all. For example, some people who aren’t very active may need to eat less than the guidelines recommend. This may also be the reason why we see such a discrepancy between energy use and reported energy intake in UK adults. In fact, some studies suggest that they are eating well below the daily recommendations – and yet in the UK we see more people becoming overweight and obese every year.
Why people seem to be eating below national recommendations can be for a number of reasons. For example, research on energy intake shows that some survey respondents may misrepresent how much they eat each day. Underreporting may not be the whole story. Apps that measure food intake in detail also give lower values for the amount consumed compared to recommendations. This can make it seem like people are actually eating less than they need. Other things, like the amount of activity and height, can affect how much energy we need. Recommendations may not adequately address these things.
What is encouraging about the data from this study is that it measures energy expenditure in people of all ages more accurately than some studies have used in the past. This study shows us that energy needs do not appear to change dramatically over the course of an adult’s life, so these results can be quite useful in adjusting our current recommendations on energy needs. However, it will be crucial to tailor the new energy intake recommendations to how needs may vary from person to person based on activity level and body weight. It will also be important to develop better ways to personalize recommendations so that each person can most effectively manage their weight.
But there are still many easy ways to prevent lifelong weight gain. Physical activity and a healthy diet are both important. While there can be some margin of error, keeping a daily record of food intake can help keep track of how much you are eating and can be useful in knowing where you might need to lose weight if you gain weight.
Your metabolism changes as you get older, just not when you think
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Metabolism may not slow down after age 20, so why are we still gaining weight? (2021, August 20)
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