Introduction

Physical activity as defined by the World Health Organization (WHO, 2012) is “…any bodily movement produced by skeletal muscles that requires energy expenditure.”, and physical inactivity (or lack of physical activity) has been identified as the fourth leading risk factor for global mortality (6% of deaths globally).  Put another way, of the approximately 56.5 million people who died during 2010, nearly 3.4 million died because of their lack of physical activity.

Physical inactivity is also estimated to be the main cause for approximately 21-25% of breast and colon cancers, 27% of diabetes and approximately 30% of ischaemic heart disease burden (WHO, 2012).  It is important to realise that prolonged periods of inactivity, such as sitting, can itself be a risk factor.  It is likely that even if exercisers meet the guidelines for physical activity, if they spend the rest of the day in a state of inactivity the benefits will be lost.  Therefore, it may be prudent to think of physical inactivity as a separate entity to physical activity and fitness professionals should tailor advice to address both aspects.

The WHO (2012) and Plowman and Smith (2011) both suggest that regular and adequate levels of physical activity in adults provide a number of health benefits:

• Reduction in the risk of hypertension, coronary heart disease, stroke, diabetes, breast and colon cancer, depression and the risk of falls;
• Improvements in bone and functional health; and
• It is a key determinant of energy expenditure, and thus fundamental to energy balance and weight control.

The term ‘physical activity’ should not be confused with the term ‘exercise’, exercise is a subtype of physical activity. Plowman and Smith (2011, p.6) state that “Exercise is a single acute bout of bodily exertion or muscular activity that requires an expenditure of energy above resting level and that in most, but not all, cases results in voluntary movement.”  Exercise is planned, structured, repetitive, and purposeful with the objective to maintain or improve one or more components of physical fitness (Caspersen, 1985; WHO, 2012).  For example, walking around a track at a predetermined heart rate is considered exercise, in contrast to walking to school which is considered physical activity.

Physical fitness is a set of physical attributes related to a person’s ability to perform physical activity successfully, without undue strain and with a margin of safety, and can be sub-divided into health-related physical fitness and skill-related physical fitness (both discussed later).  Physical activity and exercise both contribute to a person’s level of fitness.

Physical activity includes any activity which involves bodily movements that are performed as part of playing, working, active transportation, house chores and recreational activities.  From a physiological standpoint, physical activity and exercise both involve the process of muscle action and energy expenditure that bring about changes (acute and chronic); therefore these terms can be used interchangeably.

The WHO (2012) argues that increasing physical inactivity is a societal, not just an individual problem, and as such that a population-based, multi-sectoral, multi-disciplinary, and culturally relevant approach is required.

Physical Activity and Resting Metabolic Rate

Speakman and Selman (2003, p.621) argue that “The direct effects of physical activity interventions on energy expenditure are relatively small when placed in the context of total daily energy demands.”  They suggest that in addition to the direct energy costs of exercise itself, that the body continues to use extra energy even once exercise is complete, that is it increases your resting metabolic rate (RMR).

RMR is the largest component of the daily energy budget in most human societies and, therefore, any increases in RMR in response to exercise interventions are potentially of great importance.  Animal studies have generally shown that single exercise events and longer-term training produce increases in RMR.  This effect is observed in longer-term interventions despite parallel decreases in body mass and fat mass (Speakman & Selman, 2003).

However, Speakman and Selman (2003) also argue that studies in animals that measure the effects of voluntary exercise regimens on RMR are less commonly performed and do not show the same response as that of forced exercise, in fact these studies indicate that exercise does not induce elevations in RMR.

Data concerning long-term effects of training are potentially confounded by some studies not leaving sufficient time after the last exercise bout for the termination of the long-term EPOC.  Long-term effects of training include increases in RMR due to increases in lean muscle mass.  Extreme interventions, however, may induce reductions in RMR in spite of the increased lean tissue mass (Speakman & Selman, 2003).

Many studies of human subjects indicate a short-term elevation in RMR in response to single exercise events (generally termed the excess post-exercise O2 consumption or EPOC).  This EPOC appears to have two phases, one lasting <2 hours and a smaller much more prolonged effect lasting up to 48 hours (fitness professionals refer to this as the after burn effect).  There is a dichotomy within the research, with many demonstrating that RMR increases through long-term training, whilst many others have failed to find such effects (Speakman & Selman, 2003).

Exercising for Weight Loss

Being active is an important part of any weight loss or weight maintenance programme.  Simply put, when you are ‘more’ active, your body uses more energy and when you burn more calories than you consume, you will lose weight.

In broad terms 1 lb, or 0.45 kilogram, of fat equals 3500 calories.  In order to lose that 1 lb a person would need to burn 3,500 calories more than they consumed (of all food and drink they consume).  For example, to lose 1 pound a week, you would need to burn 500 more calories than you eat each day either by eating less, exercising more or a combination of both (500 calories x 7 days = 3,500 calories).

However, because of changes that occur in the body over time, in response to reduced energy intake, the calorie intake may need to be decreased further to continue weight loss.  Also, while diet has a stronger effect on weight loss than physical activity does, physical activity, including exercise, has a stronger effect in preventing weight gain and maintaining weight loss (Plowman & Smith, 2011; Tortora & Derrickson, 2012).

Exercisers should be wary of training providers, boot camps or otherwise, who purport to help you lose weight fast.  Dr Hensrud[1] (2011) has concerns with fast weight loss, in that it usually takes extraordinary efforts in diet and exercise (efforts that could be unhealthy and that you probably cannot maintain as permanent lifestyle changes).  He recommends a weight loss of 1 to 2 pounds a week, a recommendation backed by NICE[2] (2010).  Although that may seem like a slow pace for weight loss, Hensrud (2011) argues that it is more likely to help a person maintain their weight loss for the long term.

NICE (2010) suggests that effective weight loss programmes should have seven elements:

1. Address the reasons why someone might find it difficult to lose weight;
2. Are tailored to individual needs and choices;
3. Are sensitive to the person’s weight concerns;
4. Are based on a balanced, healthy diet;
5. Encourage regular physical activity;
6. Expect people to lose no more than 0.5–1 kg (1–2 lb) a week; and
7. Identify and address barriers to change.

Dr Hensrud also states that if you lose a lot of weight very quickly, it may not be purely fat that you are losing.  This is because when calorie intake drops dramatically, the body goes into starvation mode and burns protein from muscle along with fat.  It is difficult to burn that many ‘fat’ calories in a short period.

Dr Hensrud suggests that in some situations that faster weight loss can be safe if it is done in the right manner.  For example, your medical professional might prescribe a very low calorie diet for rapid weight loss if obesity is causing serious health problems.  However, he states that an extreme diet like this requires medical supervision.

It is advisable to seek the advice of a medical professional or nutritional specialist before altering your diet.