Select Committee on Health Minutes of Evidence

Memorandum by Adrianne Hardman (OB 73)


  I have 30 years of experience with research in exercise metabolism, energy expenditure and adaptations to training. I was a member of the Scientific Advisory Board for the 1992 Allied Dunbar National Fitness Survey. I have published extensively and was the only UK participant in the 2000 Consensus Symposium convened by Health Canada and the US Centers for Disease Control and Prevention Dose-response issues concerning physical activity and health. My book Physical activity and health: the evidence explained is to be published in September.


  The Committee will have heard evidence for the rapid increase in the prevalence of obesity in the UK. They may also be familiar with the evidence that proxy measures of physical inactivity are more closely related than indices of dietary change to changes in the prevalence of obesity (Prentice and Jebb 1995). This implicates inactivity as the dominant factor in the aetiology of obesity. Prospective studies lend support to this proposition, showing that people who report low levels of physical activity are more prone to weight gain than those who report high levels. For example, in one large study, men and women with low levels of leisure time activity were more than two and a half times as likely as highly active people to experience a clinically important weight gain (>5 kg) over 10 year follow-up (Haapanen et al 1997).

  Factors that have probably contributed to the decrease in levels of physical activity were summarized in the report of National Audit Office (2001) and will be familiar to the Committee. The effects of these changes are not trivial; total daily energy expenditure has declined by around 20% ( 800 Calories) during the second half of the 20th Century—the equivalent of walking 10 miles.


    —  explain the relationships between physical activity[1]/exercise[2], energy expenditure and body weight; and

    —  provide illustrative examples of the potential of physical activity to improve weight regulation and prevent weight gain.

Basic principles

  An individual's weight remains stable only if they remain in energy balance over a long period. That is:

    energy intake from the diet = total energy expenditure.

  Just a small imbalance between energy intake and expenditure makes a large difference to body fat stores in the long term. For instance, a positive energy imbalance of just 1% will increase body weight by around 20 kg (more than 40 lb) during adulthood.

  Three components make up total energy expenditure, ie the resting metabolic rate, the energy associated with digesting, absorbing and storing food and the energy expended during physical activity. Thus:

    Total energy expenditure = resting metabolic rate + thermic effect of food + energy expended in activity.

  The resting metabolic rate accounts for 60-70% of total energy expenditure and the energy costs of feeding are about 10%. The energy expended in physical activity is the most variable component, contributing 10-15% in sedentary people but 30-40% in normally active people. (It can be much larger in some athletes or in individuals engaged in endurance challenges.) Importantly, it is the only component of total energy expenditure that one can increase voluntarily, through behaviour. Even walking at an ordinary pace increases energy expenditure three-fold.

Factors that influence the energy expenditure of physical activity

    (i)  For weight-bearing activities, energy expenditure is proportional to body weight.

          For a 70 kg (11 stone) individual, walking at 3 mph (a "normal" speed) expends about 4 Calories/min gross or 2.8 Calories net (more than at rest). For a 90 kg individual, this rises to 5 Calories/min gross, or 3.5 Calories/min net.

    (ii)  Energy expenditure increases with the intensity of activity.

          A 70 kg individual expends 3 Calories/min walking at 2 mph, 4 Calories/min at 3 mph and 8.6 Calories/min at 5 mph.

    (iii)  Energy expenditure increases in proportion to the duration of activity.

    (iv)  Exercise induces a small increment in energy expenditure in addition to that incurred during the exercise because the metabolic rate remains elevated for some time afterwards. For activities such as walking, this is about 15% of the net energy expenditure during exercise.

  THUS: For the typical range of walking speeds, the net increment in energy expenditure, ie above the resting level, associated with walking one mile is 60 Calories for a 70 kg individual, 50 Calories for a 57 kg individual and 75 Calories for a 90 kg person.

What determines an individual's perception of how "hard" exercise is—and thus their capability/willingness to engage in it?

  This depends on the individual's capacity for exercise, usually measured as his/her maximal oxygen uptake. The level of "physiological stress" elicited by a specific activity depends on what proportion of the individual's total capacity this demands. Activity demanding less than 45% of capacity is regarded as light, 45-60% as moderate and levels above that as vigorous or hard. The maximal oxygen uptake varies a lot between individuals, is higher in men than in women. It declines on average by 10% per decade after the age of 30.

  Thus, walking at 3 mph demands an oxygen uptake that constitutes less than 30% of capacity for an average young man and is therefore light exercise. For a 75-year-old, with a lower capacity, walking at this speed may represent 60% or more of total capacity and thus be moderate to vigorous exercise.

Is physical activity an effective way to lose weight?

  A lot of activity is needed to expend the energy in body fat stores. In the short-term increasing activity looks an unpromising means of weight loss. For example:

    —  Adipose tissue (body fat) contains about 7,740 Calories per kg (3,518 per lb). Our 70 kg person therefore has to walk 129 miles in order to expend the energy stored in one kg of fatty tissue (81 miles to expend the energy in one pound of fat. Since a typical confectionery bar contains 300 Calories, he/she has to walk five miles to expend this amount of energy.

  BUT—Obesity takes years to develop. The average 35 year-old man gains 0.2-0.8 kg every year (0.4-1.8 lb per year). Gains in women tend to be even greater. Even small amounts of activity, taken regularly, have considerable influence on energy balance long-term.

    —  By increasing energy expenditure by walking just one mile extra every day, our 70 kg individual expends an additional 21,900 Calories in one year—the equivalent of 2.8 kg (6.2 lb) of fatty tissue[3].

  What tends to happen in practice?

    —  Overall, experiments on weight loss through exercise have found only modest reductions in body mass—of the order of 0.09 kg per week (0.2 lb per week). This is equivalent, of sustained, to a loss of more than 4.5 kg or 10 lb in a year. Such studies typically last months rather than years, however, and the rate of weight loss may be expected to decrease over time.

Physical activity in people who are obese

    —  Weight-bearing activity is fatiguing for obese persons. For many, even walking constitutes vigorous exercise, because it demands a high proportion of their poor functional capacity. They also experience problems with joints. Thus, although obese individuals expend more Calories per mile than normal weight people, they find it difficult to expend a lot of energy through physical activity.

    —  Activities like swimming or cycling, where body weight is supported, may be better tolerated.

What type of activities are effective?

  Aerobic or endurance activities usually lead to the greatest energy expenditure. This does not have to be vigorous or sporting, however (Table). For weight regulation, it is the total energy expenditure that matters. To improve weight regulation, activity needs to be regular and frequent. It has to become a habit. Walking in particular can be recommended (Morris and Hardman 1997). It is readily incorporated into the lifestyle and incurs little risk of injury—one main reason why people give up exercise regimens based on sports.


Rate of energy expenditure (Calories/min)


per week

Total net energy expenditure (Calories)

Casual soccer
8.7 60 min, twice a week897

7.7 60 min, twice a week777

Walking ("normal" pace, 3 mph)
4.530 min, twice a day, to and from work, 5 days/week 982

Cycling ("commuting" pace, 5.5 mph)
4.530 min, twice a day, to and from work, 5 days/week 982


    —  Over months or years, increasing physical activity is associated with modest weight loss.

    —  More importantly, high levels of physical activity improve weight regulation and contribute to avoidance of weight gain.

    —  Current recommendations for physical activity are widely interpreted as 30 minutes of moderate activity per day. This is probably not sufficient to avoid further increases in population levels of overweight/obesity.


  Haapanen, N, Miilunpalo, S, Pasanen, M, Oja, P and Vuori, I (1997). "Assocation between leisure time physical activity and 10-year body mass change among working-aged men and women." International Journal of Obesity 21: 288-296.

  Morris, J N and Hardman, A E (1997). "Walking to health." Sports Medicine 23: 306-332.

  National Audit Office (2001) Tackling obesity in England. The Stationery Office.

  Prentice, A M and Jebb, S A (1995). "Obesity in Britain: gluttony or sloth?" British Medical Journal 311: 437-439.

1   Physical activity-any bodily movement that is produced by the contraction of skeletal muscle that substantially increases energy expenditure. Back

2   Exercise (or exercise training)-a subcategory of leisure-time physical activity in which planned, structured and repetitive bodily movements are performed to improve or maintain one or more components of physical fitness. Back

3   Actual loss of weight/fat will be less than this if an associated increase in energy intake reduces the energy deficit. Back

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