Being sedentary doesn't necessarily mean you'll gain weight.
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Mounting evidence shows that a sedentary lifestyle is associated with a host of health risks, including Type 2 diabetes, coronary heart disease, certain cancers, anxiety and depression, according to MedlinePlus. Many of these conditions are associated with being overweight and obese. People who are sedentary and do not wish to increase their activity level should reduce their caloric intake to prevent weight gain and related health problems.
Sedentary Calorie Needs
People who are sedentary burn fewer calories throughout the day than their active counterparts, so they need to consume fewer calories. Calorie intake recommendations established by the U.S. Department of Agriculture are based on gender, age and activity level. Typically, sedentary adults need 200 to 400 fewer calories each day than moderately active adults and 400 to 600 fewer calories than active adults. For example, a 27-year old sedentary female needs 1,800 calories daily, but an active 27-year-old female needs 2,400 calories.
Lack of Exercise vs. Excess Calories
Sedentary people are more likely to gain weight, according to MedlinePlus. Either through lack of exercise or consuming too many calories, sedentary people are at greater risk of a calorie surplus. Which is to blame has been well-studied without a clear consensus.
In 2014, researchers from Stanford who examined national health survey results from 1988 through 2010 found that both obesity and inactivity had increased, but calorie consumption had not. In their report published in The American Journal of Medicine, they concluded that lack of exercise was driving an increase in obesity, not extra calories.
On the other hand, other research shows that exercise plays only a small role in weight management and that diet is largely to blame for weight gain. Authors of a review in Progress in Cardiovascular Diseases in 2014 evaluated the role of exercise activity and physical activity in the prevention of weight gain, weight loss and weight management and found that, except for cases in which the volume of exercise activity was very high, activity did not play a big role in obesity.
Regardless of whether activity level or diet plays the bigger role in weight management, controlling your calorie intake to match your activity level is important. High-calorie foods are often loaded with sugar, salt and unhealthy fats that can raise your cholesterol. Sugar - especially sugar-sweetened drinks - has been definitively linked to obesity, according to a study in Public Health Nutrition in 2015, and according to study findings presented at the European Society of Cardiology Congress in 2017, a high salt intake can double your risk of heart failure.
Skipping fatty and sugary foods and beverages; eating more fresh fruits and vegetables, lean meats and low-fat dairy; and replacing saturated fats with heart-healthy fats from nuts, seeds, olive oil and avocado can help prevent weight gain from a sedentary lifestyle. A healthy diet can also help to counteract some of the deleterious effects of being sedentary.
Researchers of the review in Progress in Cardiovascular Diseases noted that, aside from the absence of impact on weight, exercise and physical activity have numerous other health benefits. Your best bet is to eat a healthy diet and try to include some measure of daily exercise to get the best of both worlds.
In order to balance calories, to something's got to give. When you consume more calories than you burn each day, you gain weight. Your body has evolved to store extra energy in your fat cells for use at a later time, should food become scarce.
Lack of Exercise
Both your activity level and your diet play a role in a calorie surplus; if you eat too many calories, you gain weight and if you are inactive, you aren't burning an excess calories -- it's a double-edge sword.