Dieting affects Your Metabolism

Hormones cause weight gain triggered by dieting

So how does dieting affect your metabolism? To answer this question, we firstly need to discuss what your metabolism actually is. Your metabolism refers to the chemical processes occurring inside the body allowing life and normal functioning. These processes require energy from food. The amount of energy (calories) your body burns at any given time is regulated by your metabolism. You can not control your metabolism, but you can make it work for you!

Your metabolism is controlled by hormones and the nervous system. Its functioning can be influenced by a number of things and among these are genetic disorders and hormonal problems.

The two main processes of your metabolism are:

  • Anabolism - energy is stored in fat cells or used to repair your body.
  • Catabolism - the breakdown of food components (such as carbohydrates, proteins and fats) into their simpler forms, to be used to create energy.

Your Metabolic Rate is the rate your body uses or expends energy; this is particularly relevant when you are trying to lose weight.  There are three aspects that influence your metabolic rate that are helpful to understand. 

  1. Basal Metabolic Rate (BMR) – Your BMR determines the amount of calories burned at rest by your body and the amount of energy your body needs to maintain itself. It usually contributes 50-80 per cent of your total energy used. Muscle mass is largely responsible for the BMR, so anything that reduces your lean muscle mass will reduce your BMR.
  2. Energy used during physical activity - Energy used during exercise is the only form of energy expenditure that you have any control over. In a normally active person, this contributes 20 per cent of daily energy use.
  3. Thermic effect of food - this is the energy you use to eat, digest and metabolise food. It contributes about 5-10 per cent of your energy use. Your BMR rises after you eat because you use energy to metabolise the food. The rise occurs soon after you start eating and peaks two to three hours later.

Your BMR, the major contributor to your metabolic rate, is influenced by a number of factors:

  • Muscle Mass – the more muscle you have, the faster your BMR. People with more lean muscle mass burn more energy even when asleep!
  • Age - metabolism slows with age, due to muscle loss and hormonal changes.
  • Genetics - your metabolic rate may be partly decided by your genes.
  • Amount of body fat
  • Hormonal and nervous controls
  • Dietary deficiencies - for example, a diet low in iodine reduces thyroid function, which slows metabolic functioning.
  • Crash dieting, starving or fasting - eating too few kilojoules encourages the body to slow the metabolism to conserve energy; BMR can drop by up to 15 per cent. There is also loss of lean muscle tissue, which further contributes to the drop in BMR.
  • Amount of physical activity - hard-working muscles need plenty of energy to burn. Regular exercise increases muscle mass and 'teaches' the body to burn kilojoules at a faster rate, even when at rest.
  • Drugs - some drugs can influence your BMR.

The human body is infinitely intelligent and has the potential to adapt to its environment perfectly.  For example, when you do not eat or dramatically reduce your food or calorie intake, the body senses that you may not have enough energy to sustain normal activities.  So, the thyroid gland slows your metabolism down to accommodate this; it lowers the energy requirements needed by the body.   This is what is commonly referred to as a 'slow metabolism'. 

This is the perfect response - it is trying to keep you alive!   It’s like a car running on fewer cylinders and therefore your body shifts gears and then starts to use less fuel.  It’s actually a fantastic survival mechanism, but is also a disaster if you are trying to lose weight!

For some, decreasing the amount of food they eat or consuming fewer calories can certainly help weight loss.  But if done excessively it also can have a slowing effect on your metabolism and how your body utilises food and energy.  In other words, your body won’t respond to the diet in the way that you want it to; it actually may flip the other way and make you more prone to weight gain.  Successful dieting is far more complex than just reducing food input and increasing your energy output through exercise.  If you disrupt the normal balance too much - you will disrupt your body's metabolism.

Starving your body, radical diets and skipping meals can actually create a harmful cycle in your body that can make weight loss even more difficult than before.

If your body goes into a "Starvation Mode" and slows your metabolism it means that your system now needs even less food as energy to survive than it did before.  The main effect of drastic dieting is the effect it has on the thyroid gland.  This is the gland that keeps your metabolism kicking along at a healthy rate and if your thyroid function slows down, so does your metabolism.

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