Exercise and metabolism

Put simply, your metabolism refers to all the processes in your body that help you to function normally. This requires energy, which your body gets from the food you eat. The rate at which your body uses up kilojoules (or energy) to carry out these vital functions is called the metabolic rate. Around five to 10 percent of your energy is used to eat, digest and metabolise food, another 20 percent to burn kilojoules during physical activity (if you are a normally-active person), and the remaining 50–80 percent is the amount of energy used while you are at rest. This is also known as your basal metabolic rate or BMR. The faster your BMR, the more efficiently your body burns up kilojoules.

Your BMR can be influenced by many factors including your body size, age, gender, genetic predisposition, hormones and what you eat. The amount of exercise you do can have an effect as well.

How does my gender affect my BMR?

It’s all to do with muscle. Compared to women, men’s bodies generally have more muscle and less fat which makes a difference to your BMR. While fat burns very few kilojoules, muscle is an active, ‘hungry’ tissue that uses up kilojoules even when you’re just sitting around.

How does exercise help increase your BMR?

Exercise increases the amount of muscle you have—and the more muscle you have, the faster your BMR will be. Although exercise will generally help increase muscle, studies show strength training (also known as resistance training or weight training) builds muscle more effectively and increases BMR.

Other factors that can influence your BMR include:

• genes. The way you use energy can be influenced by inherited characteristics
• your age. Metabolic rate tends to slow down with age. This is now thought to be related to muscle loss rather than to the ageing process itself
• your size. People who are larger tend to burn up more kilojoules
• crash dieting or fasting. Dramatically restricting kilojoules can trick the body into thinking there’s a famine. This encourages the body to slow down the BMR in order to conserve fat stored in the body.

Tips for boosting your BMR

• Avoid quick weight loss diets or detox diets that cut too many kilojoules – they can slow your BMR down and make it harder to lose weight.
• Even steady weight loss can affect your BMR. At some points, you may hit a ‘plateau’ – a point when weight loss slows down or stops. Increasing physical activity or changing your exercise program may help. Eating a little more may also work by tricking the body into thinking the ‘famine’ is over.
• Exercise regularly and include two sessions of strength training each week. Building more muscle will boost your BMR.
• Eat breakfast – research from the Third National Health and Nutrition Examination Survey (NHANES III) suggests a relationship between eating breakfast and lowering your BMI, which is believed to happen in part by kick-starting your metabolic rate.
• Some medical conditions such as hypothyroidism (underactive thyroid) can cause weight gain due to a slow metabolism – if you’re concerned, check with your doctor.

Further information

Better Health Channel: Metabolism explained http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Metabolism_explained

Sources

Bouchard C Pérusse L Dériaz O et al. Genetic influences on energy expenditure in humans. Critical Reviews in Food Science and Nutrition. 1993; 33(4): 345–350.

Cho S Dietrich M Brown C et al. The Effect of Breakfast Type on Total Daily Energy Intake and Body Mass Index: Results from the Third National Health and Nutrition Examination Survey (NHANES III). Journal of the American College of Nutrition, 2003; 22(4): 296-302.

Dolezal BA Potteiger JA. Concurrent resistance & endurance training influence BMR in non-dieting individuals. Journal of Applied Physiology. 1998; 85: 695-700.

Hensrud D. Slow metabolism: Is it to blame for weight gain? [online] Mayo Foundation for Medical Education and Research. c1998-2010 [last updated 27 Aug 2009, accessed 17 Aug 2010] Available from: http://www.mayoclinic.com/health/slow-metabolism/AN00618

Last Published: 31 October 2011

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