Metabolism - Energy Expenditure

Question 1

Energy Production

Answer 1

• One’s daily total energy expenditure depends on three variable factors:
  1. Basal Metabolic Rate (BMR)
  2. Physical Activity
  3. The Thermic Effect of Food (TEF)

Question 2

Basal Metabolic Rate

Answer 2

• The Basal Metabolic Rate (BMR)is the minimum level of energy required to sustain life; i.e. the energy used to maintain basic physiological functions such as the cardiac contractions, ventilation and cell growth.
• BMR accounts for 60-75% of total energy expenditure, mostly from the metabolic activity of lean tissue (i.e. non-adipose tissue) collectively referred to as fat-free mass (FFM).
• BMR varies between individuals, mostly due to differences in body/muscle mass.
• Physical activity can also impact BMR: exercise can elevate metabolic rate for up to 48 hours and can also increase the FFM which further elevates BMR.
• Other factors that can affect BMR include stress, caffeine, nicotine, medications, fever, injury, genetics, thyroid hormones, etc.

Question 3

Thyroid Hormones

Answer 3

• The thyroid gland is responsible for regulating energy metabolism by secreting hormones that control the metabolic rate of cells.
• Thyroid Stimulating Hormone (TSH) is produced by the anterior pituitary gland and triggers the release of Thyroxine (T4) and Triiodothyronine (T3) from the thyroid gland.
• T4 is the primary thyroid hormone and is less potent than T3.
• Iodine, tyrosine, zinc, selenium and iron are especially important for T3 and T4 production.

Question 4

Subclinical Hypothyroidism

Answer 4

• Found in 8-10% of the population (60% are unaware) – more common in women.
• It can present as low energy, reduced appetite, weight gain, feeling cold, dry skin, thinning hair, constipation, irregular periods, depression and so on.
• Underlying factors to consider may include nutrient deficiencies (especially iodine, selenium, zinc, vitamin D), autoimmunity, fluoride, mercury and exposure to endocrine disruptors such as BPA.
  Subclinical hypothyroidism is categorised as TSH levels above range but with normal levels of T4 & T3.

Question 5

Metabolism &Disease

Answer 5

• Disease or trauma can affect an individual’s metabolism:
  o Infection and fever raises one’s metabolic rate.
  o Energy requirements increase to recover after surgery or an injury.
  o The BMR of a patient with severe burns can more than double during recovery.
  o Anorexia Nervosa patients need energy intake up to 2.5 times their BMR during re-feeding.
  o Cancer patients’ energy requirements may reach 145% of BMR.

Question 6

The Thermic Effect of Food

Answer 6

• The Thermic Effect of Food (TEF) is the amount of energy required to digest and process the food you eat.
• Also referred to as ‘thermogenesis’, TEF accounts for approx. 10% of the total daily energy expenditure in an average diet.
• Different foods have different Thermic Effects, e.g.
  1. Fats= 0 - 3%
  2. Carbohydrates= 5 - 10%
  3. Proteins= 20 - 30%
  The thermic effect of highly processed food is substantially less than their whole-food counterparts.
  Leveraging the thermic effect of Protein can be helpful to weight loss programmes?

Question 7

Energy Requirements

Answer 7

• Government guidelines suggest: 2000 kcal/day for women and 2500 kcal/day for men.
• In reality caloric requirements vary between individuals.
• This guidance does not take into account many important factors such as: one’s body size, muscle mass, current state of metabolic health and whether someone has a sedentary v. active lifestyle.

Question 8

Energy Expenditure Varies

Answer 8

50-year-old, 65kg sedentary woman with an office job

30-year-old, 72kg female chef on her feet all day who trains in the gym