Energy Balance

Time to stop procrastinating and write about this week's content! I'm going to start with BIOC3004, because that seems to be the easiest one to write about (the summary slides are pretty clear on what we need to know). This post will be on energy balance, which unfortunately doesn't involve any kind of magical, mystical energy, but rather simply the balance between the energy we consume and the energy we expend.

Know the units to measure energy intake and expenditure

As you probably know, calories and kilojoules (kJ) are the main units used for energy intake and expenditure. The tricky part is that there are actually two types of calories: scientific calories and food/nutritional calories. Scientific calories are smaller, as scientists are often measuring things on a small scale. 1000 scientific calories (cal) make up a single food calorie (Cal). Note the capitalisation: bigger food calories are denoted Cal, whereas smaller scientific calories are denoted cal. 1000 scientific calories is also equal to 1 kcal.

One food calorie, or 1000 scientific calories, is also equal to 4.184kJ.

Know the energy density of macronutrients (in kcal)

The following lists the amount of energy (in kcal- remember, a kcal is 1000 scientific calories) per gram of a particular macronutrient:

• Carbohydrates - 4
• Proteins - 4
• Alcohol - 7
• Lipids - 9

Be able to explain the different components making up energy expenditure

Aside from physical exercise, there are many different things that we spend energy on each day. These include:

• Diet induced thermogenesis (DIT)/Thermic energy of food (TEF)- Energy expended during the digestion, absorption and storage of food. Accounts for ~10-15% of daily expenditure.
• Basal metabolic rate (BMR)/Resting energy expenditure (REE)- Energy expended in order to keep the body functioning at rest. Accounts for ~60-75% of energy expenditure.
• Non-exercise activity thermogenesis (NEAT)- Energy expenditure when we are just going about our daily business. This includes activities as diverse as simply sitting still, to doing paperwork, to cooking a meal (i.e. basically anything that isn't sleeping or vigorous exercise).

Be able to calculate BMR when given the equation (you do not need to remember the equation)

The Mifflin equation takes weight, height and age into account in order to provide an estimate of an individual's BMR. Here is the equation:

Men: kcal/day = 9.99*weight + 6.25*height - 4.92*age + 5
Women: kcal/day = 9.99*weight + 6.25*height - 4.92*age - 161

I *think* weight is given in kg and height in cm, but I've just emailed my lecturer to double check. (UPDATE: He replied and said that it is indeed in kg and cm. Also he reminded me that we don't need to memorise this equation)

Be able to explain why there are variations in energy expenditure between individuals

There are variations in energy expenditure between individuals due to differences in BMR, as I'll explain later. Additionally, different people partake in different activities throughout their day-to-day lives, and thus NEAT varies between individuals.

Be able to explain why there are variations in BMR between individuals

The BMR varies between individuals, as people have different amounts of muscle, fat and so forth, all of which contribute towards the BMR. Most variation can be explained by differing amounts of muscle. There is still around 26% of the variation that is, as of now, unexplained.

Know which tissue is responsible for most energy expenditure in the body

The muscles and liver contribute the most to overall BMR (around 22% each), with the brain a close second (around 20%).

Be able to explain the different components of physical activity

Physical activity can be classified according to how much energy is consumed per minute. This can be measured in kcal/min, but this is often impractical, so a system called "Metabolic equivalents," or METS, was developed instead. 1 MET is equivalent to the resting metabolic rate, an activity that requires 2 METS requires twice the resting metabolism, and so on and so forth. For example, a "light" activity that requires around 2.5 METS might be cleaning the house or playing table tennis. A "heavy" activity that requires 7 METS might be climbing a mountain or playing football.

Be able to explain why some foods have a higher energy density than other foods

The way a food is cooked can contribute to energy density. For example, a whole potato contains around 70 kcal of energy, but being cooked in fat or oil can increase the amount of energy for what is effectively the same amount of food. The greater the surface area of the potato exposed to the fat or oil, the higher the energy content: a roast potato, which is pretty much just coated on the outside in a small amount of oil, contains around 140 kcal, whereas a bag of potato crisps of the same weight contains around 500 kcal.