Activity level is a number used to express how physically active someone’s lifestyle is. It is derived by dividing total energy expenditure by basal metabolic rate and is typically between 1.4 and 2.1.
Most people will never know there physical activity level with any accuracy because calculating it requires sophisticated laboratory techniques. Nonetheless it remains the backbone of almost every calorie or weight loss calculator on the web.
In this post we’ll look at why your activity level is so important, how is it used in calorie calculators and how it varies.
How Does Activity Level Affect Calorie Needs
Activity level is the most important driver of calorie needs. It’s a stronger driver of energy expenditure that age, height and weight. Here’s how that relationship looks based on real world data of 645 people.
How is Activity Level Used in Calorie Calculators
The vast majority of calorie calculators on the web use the same formula. They first estimate basal metabolic rate and then multiply to activity level to estimate calorie needs. Although the results may seem accurate, they are anything but. Let me explain why.
Men: REE = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5
Women: REE = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161
We know studies that 70% of estimates for the obese using this equation were accurate within ±10%, while the total range was ±20% or so.
The activity multipliers you typically see used are as follows:
Sedentary = 1.2 (little or no exercise, desk job)
Lightly active = 1.375 (light exercise/sports 1-3 days/week)
Moderately active = 1.55 (moderate exercise/sports 6-7 days)
Very active = 1.725 (hard exercise every day, or 2 xs/day)
Extra active = 1.9 (hard exercise 2 or more times per day)
Although I realise you need to make an estimate to put a calculator together possible, but getting these multipliers right is very hard. Pretending they are largely determined by how much you exercise is plain false. Activity level is a complex function of genetics, work life, exercise patterns, muscle mass, postural control, propensity to fidget . . .
How Activity Level Varies Among People
Here’s what physical activity level looks like when graphed against age.
The average physical activity level (PAL) in this dataset is 1.7, so is the median. Over 80% of people fall somewhere between 1.4 and 2.1. The small orange ‘line of best fit’ gives you an idea of the average, but this barely explains 6% of variation.
The things likely to take a person above 2 are a physically active job, endurance training or being one of those lucky fidgety folk that are wired to burn excess food. Going below 1.5 is likely to involve a desk job and much more common in older people.
Your activity level is determined by many things including genetics, occupation, exericse, posture control and fidgeting. It rises with overfeeding and falls during dieting.
The large uncertainty involved in estimating it should make you sceptical of all calorie calculators.