Nutritional Assessment

20 April, 2011

Optimal health, wellbeing and body weight are all about balance. Our body requires a balanced diet which provides us with sufficient macronutrients (protein, carbohydrate and fat) and micronutrients (vitamins and minerals) in response to its needs.  These needs are a function of the lifestyle we lead and the level of phyiscal activity we undertake; the more physically active we are, the more energy we require from the food we eat.

Of course our body needs energy to maintain functionality when at rest; to support structural integrity, detoxification and elimination, immune and inflammatory processes, hormonal and neuronal communication, energy production.

The amount of energy required for this purpose is defined as Basal Metabolic Rate (BMR) which is the energy we expend when at rest after 8 hours of sleep, provided our last meal consumed prior to sleep has been fully digested and absorbed.  BMR is typically given for a 24-hour period (kcals/day) and accounts for 60-75% of our total daily energy expenditure.

A further 5-10% of our daily energy expenditure is required for digesting and absorbing the food we eat, while 15-30% is needed for exercise and physical activity.

For weight management, it comes down to a simple equation.  If energy intake through our diet is equal to energy expended while living, breathing and going about daily activities, then our body weight will remain stable.  If energy intake is greater than energy expenditure our body will be on a weight-gain trend, if energy intake is less than energy expenditure our body will be on a weight-loss trend.  Changes in body composition will also have an impact on our energy requirements; an increase in lean body mass (i.e. skeletal muscle) will demand a higher basal metabolic rate.

To determine whether we are in energy balance or not, and maybe later to adjust that balance in order to achieve our health and weight management targets, we need to measure both the energy we intake through our diet and our energy expenditure, typically over a period of 3 to 7 days, and ideally considering both weekdays and weekends.

Energy Intake

To assess average daily energy intake, we need to keep a record of everything we eat and drink.  Given there is quite a bit of work involved, a 3-day food diary is a good place to start, ideally including 2 weekdays and 1 weekend day.  There are 4 key steps to this process:

  1. weigh/measure the amount of each food item consumed in a single day
  2. use nutritional data to translate food consumed into amounts of protein, carbohydrate and fat
  3. estimate the day’s energy intake (kcals) remembering 1g carbohydrate = 3.75kcals, 1g protein = 4kcals, 1g fat = 9kcals, 1ml alcohol = 7kcals
  4. calculate the average daily energy intake (kcals) over the 3 days

There is a wealth of information available in the public domain which documents the nutritional composition of individual food items (raw and cooked) and specific dishes and recipes.  One obvious source is food packaging, but comprehensive databases can be found online.  Probably the most widely used food table is the McCance and Widdowson (2002) which forms the data behind many dietary software packages.  A spreadsheet to help you build a food diary can be downloaded here.

Energy Expenditure

The starting point for this calculation is your BMR.  You can find yours on the Walking Course page of this site.

There are several equations available to estimate BMR, but one set that is widely used is that proposed by W. N. Schofield (1985), based on age, gender and body weight.  Further details of these equations can be found here.

Remembering that your BMR is the energy cost of lying at rest for 24 hours, you can determine your lowest energy expenditure rate in kcals/min by dividing BMR by 1,440, the number of minutes in a day.

For example, if a woman of 40yrs weighing 57kg has a BMR of 1,308.8kcals/day, she consumes 0.9kcal/min when lying at rest.  Then if everything she does during the day can be expressed in energy expenditure terms as a multiple of this number, keeping a physical activity journal will enable her to estimate her total energy expenditure.  A minute by minute account would be too detailed, but recording activities in blocks of 15mins is realistic.  There are 5 key steps to this process:

  1. record all activities in blocks of 15mins
  2. determine the total time (mins) spent undertaking each activity for a single day
  3. assign a physical activity ratio (PAR) to each activity; PAR figures are provided here
  4. compute (time spent undertaking activity x PAR x BMR/min) for each activity
  5. sum across all activities

As an illustration, suppose the woman referred to above spends an hour walking uphill carrying a load, she would estimate her energy expenditure during this time as 60 x 6.9 x 0.9 = 372.6kcals.

There is quite a bit of work involved, but the results can be extremely useful in supporting our weight management.  Of course this is not an exact science and our bodies don’t all react the same when performing similar actvities.  But having a better understanding of our energy requirements, possibly recognising that we need to increase the level of physical activity and/or reduce the amount of food we intake is a first step in enabling us to reach our target weight.

Case Study

To understand the process better, it helps to see it in action.  Jack, a 52yr Caucasian male living in Hong Kong is a little overweight and wants to reduce his body mass by 5%.  In order to achieve his goal, Jack has decided to undergo

  • a comprehensive energy intake and dietary asessment
  • an analysis of daily energy expenditure and activity levels
  • an assessment of body composition

A detailed analysis of the results and recommended lifestyle chances for Jack can be found by following this link: Nutritional Assessment