What is Metabolism and a safe Calorie levels for successful weight-loss

Dieting has consistently been shown to cause a decrease in metabolic rate as the body strives to conserve fuel.1

At the outset I need to state that you can diet unhealthily and healthily. First let’s talk about metabolism.

What is metabolism or metabolic rate?

Metabolism is the rate you burn calories. You measure it in calories. Metabolic rate is the daily calories you need to maintain your weight. Calories are numbers which is great because we are able to set and measure numbers.

There are other factors that affect metabolism

  1. If you move more, you burn more.
  2. The more muscle you have, the more calories you burn.
  3. The thyroid gland produce hormones that drive the conversion of food into energy. Under or over production of these hormones affect calorie burn rate.
  4. High Stress levels affect both thyroid hormones and food stores, adding to central obesity
  5. Infection, disease and ill-health affect everything including metabolism
  6. Age – everything slows as we age. Just accept it but we don’t want a 20 year old body at 60. We just want a healthy body.

So, should we ever Diet?

In short, yes. I prefer to call it cutting back or ‘cleaning up my act’! We all do it. However please avoid extreme measures, restrictive food plans and low-calorie diets.

Metabolism appears to return to normal once the diet comes to an end1 However eating below a healthy number of calories can leave some nasty after taste!

  • cravings for the stuff you are trying to avoid
  • binge eating
  • unhealthy gut – indigestion, bloating – IBS type symptoms
  • weaker muscles ad feeling cold
  • an increased intolerance to certain foods
  • unable to lose weight in later years

What is too Low?

Below 1200 calories – woman              Below 1800 calories – men

The American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 per day for men.

Some people like numbers so here you go….

Our basal or resting metabolic rate (BMR) is the amount of calories we need to keep our bodies running before we move (sleeping).  There are formulas to calculate it.

Calculate your BMR rate here:  step 1, 2, 3

Step 1) BMR

(men and women)         370 + (21.6 X lean mass in kg)   (Katch & McArdle formula)

If you do not know your lean mass, use the following formula

Men: BMR = 66 + (13.7 X weight kg) + (5 X height cm) – (6.8 X age)

Women: BMR = 655 + (9.6 X weight kg) + (1.8 X height cm) – (4.7 X age)

(Harris Benedict formula)

Step 2) Moving metabolic rate

Sedentary = BMR X 1.2 (desk job, no exercise)
Lightly active = BMR X 1.3 (walk/leisure cycle/sports 1-3 days/wk)
Mod. active = BMR X 1.5 (moderate gym & outdoor activity/sports 3-5 days/wk)
Very active = BMR X 1.7 (hard exercise/sports 6-7 days/wk)
Extr. Active = BMR X 1.9 (hard daily (marathon etc) exercise/sports & physical job)

Step 3: Weight-loss rate

Above Rate – 15% + weight training + cardio activity = strong body with sustainable weight for the long-term.

My female clients are typically on 1450-1500 kcals and male clients on 2100 – 2500 kcals for weight loss.

In case you are curious, here is a little bit more about the thyroid gland a the stress hormones.

The Thyroid Gland

Thyroid hormones drive the conversion of food into energy, very much like the role of a spark plug to petrol in a car. If there is not enough thyroid hormone, your engine may not burn fast enough. You can get your thyroid hormones checked at your GP. There is also a simple test you can do at home – Barnes Basal Temperature. This underarm temperature test, taken over 5 consecutive days is considered to be quite sensitive as it measures the body’s response to thyroid hormones, regardless of blood levels.

The thyroid gland produces hormones in response to another hormone, TSH that comes from the brain. Two types are produced, T4 and T3. T4 (known also as inactive thyroid) is produced in greater quantities than T3. T4 is converted into T3 in the liver or kidney. If liver and kidney health is poor, it may affect the thyroid along with other bodily functions. Micro-Nutrient deficiencies may adversely affect this process as adequate iron, zinc, selenium, iodine, potassium, Vit A, E and B2 are needed to convert T4 to T3. This is another reason why bad diets involving alot of processed foods are not a good idea.

 How STRESS impacts the Thyroid and Food Stores

Our Adrenal glands produce our stress hormones. Another possible cause for slow weight loss is Adrenal Gland insufficiency or high-stress. If you are stressed, you produce high levels of cortisol and this hormone converts T4( inactive) and T3 (active) to rT3 (very inactive), therefore stress reduces the activity levels of the thyroid, reducing your metabolic rate. In addition, Cortisol releases stored fats and sugars when you are stressed and if you have a sedentary role, these can be re-stored centrally around the belly area.

In Summary

Low Calorie diets tend to lead to a feast and famine or binge style of eating known as yo-yo dieting. Yo-Yo dieting has been linked to an increased risk of heart disease, gall bladder disease and type 2 diabetes. Many clients recall their dieting history and when they regain weight, they get bigger in different parts of their body, notably the middle. Potentially, new fat is deposited inside the belly area, closer to the liver rather than on the hips and thighs where it is safer. A large waist size is a risk factor for type 2 diabetes and heart disease.

Possible explanations are that fast weight-loss (>than 1.5lb – 2lb after week 3) involves a loss of lean tissue (from muscles and organs) rather than just fat from your fat stores. Many clients have noted that they are less physically strong after a severe weight-loss programme. They also report feeling colder. This is why I recommend weight-training to avoid lean-muscle tissue loss, eating your body weight in protein and a slow but steady weight-loss of 1lb to 2lbs per week.

The most damaging effect of yo-yo dieting is psychological – it makes you feel like a failure. It makes weight-loss feel unachievable whereas it is Just Weight. Eat whole food. I offer 2 programmes, a Low Carb and a Low Fat. Both involve eating real food.

If you would like help losing weight, contact me.


 1.Connolly J. et al (1999) Selections from current literature: effects of dieting and exercise on resting metabolic rate and implications for weight management. Fam Pract.16(2):196-201

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