Laurann O'Reilly: Your metabolism explained – what it is and what affects it

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Six top nutrition tips for boosting your immune health with Laurann O'Reilly

Laurann O'Reilly

When we hear the word ‘metabolism’ most people simply think of ‘weight loss’ but it’s important to understand what metabolism truly means, what affects it, as well as the truths and misconceptions surrounding it.

Here nutritionist Laurann O’Reilly and owner of Nutrition by Laurann explains everything you need to know about what your metabolism is and what affects it.

What is Metabolism?

In terms of energy, the Britannica dictionary defines metabolism as “the sum of the chemical reactions that take place within each cell of a living organism and that provide energy for vital processes”, which include basic functions such as breathing, digestion and growth and repair of tissues.

Resting Metabolic Rate: Your resting metabolic rate (RMR) also known as your ‘basil metabolic rate’ (BMR) is the calories (energy) needed to carry out the life-sustaining functions, which include basic tasks such as breathing, digestion, growth and repair of tissues when we are at rest. BMR accounts for 70% of your body’s total daily energy requirements and energy expenditure before activity.

What affects our metabolism

Genetics: Researchers from the University of Cambridge have discovered for the first time that genes can reduce basal metabolic rate. Whilst it can be rare, they found that “patients who had the mutations in the KSR2 gene had an increased drive to eat in childhood, but also a reduced metabolic rate, indicating that they have a reduced ability to use up all the energy that they consume”. However, more studies are needed in this area.

Age: As we get older our metabolic rate slows about 1-2% percent every decade after age 20. This may be as a result of a combination of decreased muscle mass, decreased physical activity levels and hormonal changes. For instance, as “muscle is the organ that consumes the largest part of energy in the normal human body” and as there is a natural decrease of muscle with age this can contribute to a natural decrease in our metabolism (Geriatrics Journal)

Gender: Due to testosterone levels men have a higher lean body mass (muscle) than women and also have larger organs than women, which means that whilst at rest men naturally burn more calories than women.

Ethnicity: Several studies indicate that metabolic rate differences exist between races and or ethnicities. For example, researchers have found that both African American men and women have significantly lower metabolic rates than white men and women across all body weights (Obesity Research Journal).

Activity Levels: The amount of energy we use in our daily lives can determine how much energy we require to function in these tasks.

Your Job: The type of work that you do whether you’re in a desk job, a busy parent, or extremely active such as intense manual labour, lifting heavy objects or where you’re on feet all day will determine how much energy you need during your working day. This is added to your resting energy requirement which I mentioned above.

Activity Outside of Work: The amount of activity that you do, or energy used outside of work also determines your energy requirement. For instance, whether you go for brisk walks, the gym, team sports or you’re training for an endurance event such as a marathon, these will all impact your energy requirement.

Fidgeting: Research indicates that people who fidget tend to burn more calories. A research study published in the American Journal of Clinical Nutrition investigated the changes in energy expenditure that accompany fidgeting-like movements in a group of healthy volunteers who varied in weight. Interestingly they found that in “all the subjects studied, fidgeting-like movements at very low work intensities were associated with substantial increases in energy expenditure”. After all, as long as you’re moving, you’re using energy.

Crash Dieting: Excess calorie restriction and crash dieting can have a negative impact on our metabolism. When you drastically lose weight, your body undergoes what is called “metabolic adaptation”. This means that your body adjusts its’ metabolism through a variety of methods in order to help your body return to your previous weight. For example, some diets entice people by promising rapid weight loss, but can be damaging at metabolic level. Crash diets can often have long lasting implications and can cause an individual to struggle with long-term weight management

Hormone Levels: Shifts in hormone levels can alter your satiety (the feeling of fullness after a meal) and metabolic rate. For example, 1) The thyroid gland releases insufficient thyroxine hormone, the basal metabolic rate slows which can result in weight gain. 2) The hormone ‘ghrelin’ is primarily secreted from the stomach and is responsible for satiety. ghrelin increases during fasting/hunger and decreases once we’re full, it’s also “diurnal in rhythm” in that it gradually rises throughout the day and before meals. Research has found ghrelin to be directly involved with the regulation of energy metabolism which may be due to the regulation of energy balance or energy stores (The Journal of Clinical Endocrinology & Metabolism) and 3) ‘Leptin’ another hormone released from the fat cells also sends satiety or ‘fullness’ signals to the brain. This particular hormone helps regulate and alter long-term food intake and energy expenditure. It has been well established that women have higher circulating levels of leptin than men due to having naturally higher levels of bodyfat.

Muscle Mass: As muscle is metabolically active, people with a higher muscle mass need more energy to function than people with a higher percentage of body fat. For this reason, it’s great to include some resistance training into your exercise regime.

The Myths

Coffee or Energy Drinks: Although caffeine in these drinks acts as an appetite suppressant temporarily, it’s effect on boosting metabolism in the long term has been found to be negligible.

Spicy Food: While some studies have shown that very spicy foods can increase metabolism, the boost won’t last and after about 30 minutes the rate of metabolism will be back where it started.

Note: Since weight loss is the result of more energy being spent than consumed, a spike in metabolic rate will help the body burn calories more quickly but will not affect overall metabolism in a noticeable way.

Green Tea: Green tea may stimulate energy metabolism, which may be caused by specific catechins (a type of antioxidant which protect your cells against damage) caffeine or their combination. However, the jury is still out on if green tea can have beneficial effects on energy balance and body fat stores over longer time periods.

Slimmer People Have A Higher Metabolism: As it turns out the opposite is true, in that the larger your body and your body weight, the higher your BMR will be. This is because a larger body requires a larger amount of energy than a smaller body does to carry out daily bodily processes. It’s important to remember that weight alone doesn’t determine metabolism but rather your muscle mass.

Eating Little and Often Speeds Is Better: It’s always been thought that eating smaller, more frequent meals can help boost your metabolism, however this is not the case. Research from the British Journal of Nutrition has found that meal frequency has no effect on our metabolic rate.

Natural ways to improve your metabolism

Include Some Cardio: Aerobic activity is a great way of increasing your heart rate and metabolism. Whether it’s power walking, dancing, running, jogging, HIIT (High intensity interval training), spin class, cycling, or team sports. For example, a study carried out by the University of South Wales found that HIIT training and maintain a high metabolic rate for up to 72 hours post-workout. Whatever it takes to get your heart rate up. Tip: If you have heart related conditions please consult with your GP or a qualified personal trainer as to what activity suits you best.

Include Some Resistance Training: Whilst the sound of lifting weights may be daunting for some there are a few ways to include resisting training as part of your exercise regime. For example, using gentle light weights, resistance bands and for those will joint related issues why not pop to the swimming pool as water can be a great form of natural resistance too. Tip: If you have any questions regarding weight or resistance training and would like to learn how to do so safely, I recommend contacting James Hickey of Specifit Cashel, who is a sports strength and conditioning coach, contact:

Reduce Your Stress: Whilst stress can mobilise the body’s energy stores in order to respond to a threatening situation, chronic stress can lead to an over-consumption of foods high in sugar and fat, resulting increased visceral (internal) fat and weight gain. This may be due to an increase a hormone called cortisol, which is often nicknamed “the stress hormone”. Whilst cortisol plays an important role in control blood sugar levels, regulate metabolism it can also cause sugar cravings. Tip: Find the stress relieving technique that suits you whether it’s gentle exercise such as walking/yoga, cardio exercise, breathing techniques or whatever suits. you.

Get Enough Sleep: Experts believe that not getting enough shut eye can cause alterations in glucose metabolism and a study in the Nutrition & Diabetes journal found that reduced sleep can lead to a reduction in resting metabolic rate. This may be because sleep deprivation can lead to an imbalance in the hormones leptin and ghrelin (mentioned above). As these two hormones act mainly to regulate appetite and fat storage, an imbalance or reduction in may result in one feeling hungry all the time and can affect your metabolism. Another reason to aim for a full night sleep.

- Staying Hydrated: It’s been said that dehydration can signal your body to slow metabolism, as your body’s biochemistry relies on water to utilise nutrients in food and burn calories. According to a study in the Journal of Clinical Endocrinology and Metabolism, people experienced a 30 percent increase in their metabolic rate when they drank 500ml of water (although the subject size of the study was quite small, so further studies will need to prove this one for sure).

Where to be extra cautious

Slimming Pills: It’s always important to be cautious of weight loss pills which are often unregulated. Also, some herbal pills claim they can block the absorption of fat. And maybe they do! But they may also block important nutrients. Remember a natural, nutritious and balanced diet is always best.

For further information contact Laurann at info@nutritionby or see  

Laurann O’Reilly is a qualified and experienced Nutritionist with a BSc. Degree in Human Nutrition from University of Nottingham and a Master’s in Public Health Nutrition from UCD Dublin.

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