Controlling body mass
The systems controlling our body mass are more complicated than many diet and weight loss programmes suggest. These materials allow you to explore how much (or how little) body mass changes as we change the amount of food we eat and the exercise we take. The teachers notes explain the involvement of our endocrine system – specifically the hypothalamus in the brain. The student information sheets present a significant amount of data for analysis and interpretation. This is an ideal opportunity to present ideas about How Science Works related to:
- issues of anonymity or confidentiality for subjects involved in this kind of research
- difficulties of collecting reliable and relevant information in complex situations where there are many variables
- validity of data which rely on personal responses from the subjects involved
- ethical use of animals for research.
These activities could also lead to discussions about:
- how obesity develops
- attitudes to obesity
- what should our attitude be to people who are trying to reduce their body mass?
- the routes to body mass reduction that we think are appropriate
- what we know about healthy eating and how easy we find it to eat well.
The information and data provided in this practical have been supplied by members of the British Society for Neuroendocrinology, working at the Rowett Institute of Nutrition and Health in the University of Aberdeen.
Introduce the topic in the context of looking at healthy diets, or the energy content of foods, or the maintenance of steady states in the body.
Present data for the students to interpret and analyse. Discuss the data in the light of information about the reactions of the hypothalamus to changes in our energy input or expenditure.
Consider collecting food diary, exercise diary and body mass data from adults or students within the school – either before this activity to add to the data presented here, or as a follow up activity to investigate ideas further.
Apparatus and Chemicals
For each group of students:
Copies of the student information sheets
Food diaries (optional)
Stepometer (optional) – to measure daily physical activity
Body mass scales (optional)
For the class – set up by technician/ teacher:
Slide presentation including pictures of children with leptin deficiency, before and after treatment (Note 1)
Health & Safety and Technical notes
There are no health and safety issues associated with this procedure. It involves data analysis, or monitoring ‘normal’ activity.
1 Pictures of children with leptin deficiency: Scientists and doctors investigating leptin deficiency make photographic records of their patients as part of the medical record and to monitor the effects of treatment, not for general publication. Therefore we have not reproduced those images on this site, and do not recommend reproducing the images on worksheets for students. However, the photographs are informative and interesting when presented in context. So, we suggest that you visit the images online during your teaching session to discuss the effects of leptin on body mass. Several versions of these images are available online – many significantly distorted. Be aware that most pairs of pictures show a 3-year old before treatment and a 6- or 7-year old child after treatment. At 3 years old, leptin deficiency causes obesity – but not exceptional height. Yet most pairs of pictures show children at the same height, so the photograph of the younger child is not shown to the same vertical scale. The images on the website of the Geneva Foundation for Medical Education and Research show relatively little distortion, but are still a bit off scale
This topic needs to be presented with awareness of, and sensitivity to, the reactions of some students to discussion of body mass and mass reduction. Be on the look out for any signs of distress. Decide in advance how you might approach any students who reveal an eating disorder or a significant concern about body mass or dieting for mass reduction. Be particularly careful if you want students to collect body mass and food diary information and ensure that any students who take part are enthusiastic, interested and genuine volunteers. The activity provides opportunities for discussing the ethical issues associated with using animals in research programmes.
Introduce images of children with leptin deficiency in an appropriate way, encouraging students to look at them thoughtfully and with consideration for the difficulties such conditions may bring to an individual.
SAFETY: There are no specific health and safety risks associated with this work.
a (optional) Collect data about body mass, energy expenditure and food intake from volunteers (adults or students) for a period of a week or more. There is an example set in the teaching notes – but it could be more interesting to follow a subject closer to home (anonymised if necessary) over a period of weeks. It would also be easier for you (or a relative or colleague) to observe the effects of missing a meal, or of eating/ exercising more or less than usual.
b If your body mass is normally stable then you are normally consuming the same number of calories as you use. Present the diagram of energy balance and discuss the consequences of imbalance.
c Present data about how many calories are used just lying or sitting. Ask students to speculate how much energy might be used up in different forms of exercise. Present data about the real energy expenditure of exercise. Relate the caloric content of various foodstuffs to the calories used during exercise. Is it surprising to the students how much extra exercise we have to do to ‘burn off’ extra food? Discuss the consequence of not burning off any excess – which is that our mass would increase.
d Present the two photographs – of obese mice and of the leptin-deficient human, before and after treatment. Discuss the relationship between the mice, and the child in the each picture. Answer the question “What is going on here?” Discuss the impact of leptin treatment on the human subject.
e Analyse the data on body mass for the adult human (20 KB).
f Study the data about body mass, food intake and energy expenditure from the other subjects. Is there a clear correlation between changing energy intake, changing energy expenditure and changing body mass? Over what time scale does the body react to readjust any imbalance? Compare your students’ typical food intake with that of Michael Phelps while training for Olympic swimming events.
Obesity is an issue of great concern for our society today, as worries about the health of our population and the costs of health care increase. Many people try to lose body mass by following calorie-controlled (low energy intake) diets and/ or by increasing the amount of exercise they take and hence their energy expenditure.
A typical loss of mass curve shows an initial rapid loss, then reduced rate of loss and finally an increase in mass again. It seems to be very difficult to maintain a steady rate of loss and, for most people, to maintain the lowest body mass achieved. Why is this the case?
Our brains receive signals from hormones and metabolites in the blood and from our nervous system. These signals provide information about how much food has been eaten and whether it has all been used or if there has been some to store. The signals are read by a specific region of the brain – the hypothalamus – where they are translated into further signals to the body. Those signals control subsequent food intake (or energy intake – EI) and energy expenditure (EE). These physiological systems work with considerable precision to maintain a fairly stable body mass. For mass stability, EI must equal EE. It is not possible to gain mass unless EI is greater than EE.
EE is made up of three components – Basal Metabolic Rate (BMR), thermogenesis (heat generated/ energy used during the digestion of food) and physical activity. The relative contributions of these components to overall EE are important in considering strategies to manipulate energy balance (for example, when trying to change body mass). In most people BMR accounts for 70% of EE, thermogenesis 10% and physical activity 20%.
Thermogenesis is the result of the stimulation of energy-requiring processes after eating. These processes include the intestinal absorption of nutrients, the initial steps of their metabolism,, and the storage of the absorbed but not immediately oxidized nutrients. Energy expenditure linked to thermogenesis varies from nutrient to nutrient – 0-3% for fat and up to 30% for protein – averaging at 10% for a mixed diet. (See reference in links below.)
When studying energy balance, it can be very difficult to make reliable assessments of energy intake (food and drink intake) unless subjects are closely monitored. Most self-reporting is compromised by the subject.
The relative importance of controlling EI to reduce body mass is made clear when you look at the amount of additional physical activity that would be required to burn off excess calories.
This Energy inputs and outputs table available from Parliament's Publications & Records, shows the nutritional content of various foods alongside the amount of activity required to ‘burn off’ the calorific value of the food.
The first activity on the student sheet involves calculating how much extra activity would need to be done to ‘burn off’ an extra item of food in the daily (or weekly) diet.
When we adopt a conventional calorie-controlled diet and try to reduce our calorie intake, brain signals change and result in a reduction in energy expenditure and an increase in powerful feelings of hunger associated with a drive to eat.
The fact that brain signals change when the calorie value of the diet is changed is confirmed by studies in animals. If the ‘low calorie diet’ signal molecules are injected directly into an animal’s brain, then the animal will take in more food (increase in EI) and the energy expenditure will reduce. Similarly, if an animal is put on a lower-calorie diet, the concentration of signal molecules increases.
When laboratory rodents are placed on a calorie-restricted diet, the strength of blood-borne hormonal feedback signals such as leptin declines. This signal, and others, are then integrated in the hypothalamus and change the activity of a number of brain signals related to energy balance. These studies are mainly done on post-mortem tissues where gene expression and neurotransmitter levels can be measured, but the effect of changes in the strength of these signals has also been shown directly following the administration of relevant molecules directly into the brain.
There is often argument as to whether the observations made in animal studies can be applied to the human situation. However, there is strong evidence from human and other animal investigations of obesity that the same systems operate in several organisms. The photographs below show first an ob/ob mouse – a mouse with a genetic disorder in its leptin hormone gene that prevents signalling from fat stores to the brain – and the second a leptin-deficient human, with a similar leptin gene mutation. In both cases, the mutation in the leptin gene prevents the brain obtaining vital information about the overall state of energy balance in the body, and the size of body fat stores.
The original data for this graph are in the Excel spreadsheet linked below – adult male.xls (20 KB).
The short term variations are accounted for by short-term changes in EI or EE. Physical activity accounts for only 20% of total EE, but is variable on a day-to-day basis. The body is controlled to maintain a stable mass and compensates for changes in EE or EI by reducing or increasing appetite and changing BMR. The body tries to defend energy balance but it is easy to slip into a small positive energy imbalance over time. However, when we slip into a negative balance, for example, when we try to lose weight, lots of physiological signals are generated that tell us to eat more. It is more difficult to eat less and lose weight in comparison to overeating, which is easy to do.
There are opportunities to analyse this dataset in a variety of ways. The student sheet suggests plotting graphs of key time periods, and calculating change in mass as a percentage of maximum and minimum over a short term and over a longer period.
It is interesting to try to collect information about activity, food intake and body mass, if only to show how difficult it might be to collect accurate and detailed information. You can get a measure of activity by wearing a stepometer (or pedometer), and assess food intake by recording the content and approximate scale of meals, snacks and drinks. It is possible to weigh (or estimate) portions and make a more accurate record, but this requires some commitment. In the student sheets attached below there are examples of
a food diary (436 KB), data inputting instructions (317 KB) and spreadsheet to make energy calculations from input food information (0.9 MB). These documents have been provided by the Rowett Institute of Nutrition and Health at the University of Aberdeen and you may use them freely.
A consistent diary, even if inaccurate, is likely to illustrate a day-to-day variation, possibly with marked differences for weekends compared to weekdays. Add to this some assessment of feelings about appetite and energy levels. When you see the amount of variation from day-to-day and consider whether most people pay attention to their daily diet’s detailed composition (in terms of energy content or mix of protein, carbohydrate and fat) it is reasonable to ask the question: is our food intake consciously controlled, or controlled by involuntary physiological systems?
This table gives some information about the Olympic athlete Michael Phelps and his typical diet during training (as reported in the press at the time of the Beijing Olympics). It is interesting to compare this with a typical student’s diet of around 2000 to 3000 kcal/day.
|Typical food intake on one day|
Once people become overweight, it seems it is difficult to regain an ideal body mass. If we take on a reduced calorie diet, the hormonal and brain control systems of the body try to maintain a balance on the reduced diet, by reducing metabolic rate. In principle this would make it necessary to reduce calorie intake further to continue to lose body mass at the same rate. Similarly, when we increase our rate of energy expenditure, with no corresponding increase in food intake, the body’s hormonal control systems may adjust in an attempt to maintain the balance again. A common pattern for people who are overweight is to reduce their mass on a certain diet, maintain a lower body mass for a time and then to regain more than was lost.
In terms of health benefits, there is a significant benefit for an overweight person in losing a comparatively small percentage of their body mass e.g. 5%. This is often achievable and can be maintained. However, people often want to lose unrealistically large amounts of mass e.g. 20-30 kg and this is harder to maintain, even with the ongoing support of nutrition experts.
Top tips for weight loss and weight maintenance
1 Count calories: look at food labels and get used to knowing what you are eating and drinking.
2 Enjoy food and eating: buy a cook book and start making your own healthier versions of your favourite foods. Losing weight doesn’t have to mean lettuce leaves, but it does involve some planning for shopping and eating.
3 Keep a food diary as a record of what you eat, how much and when. Keep it for a week and record body weight for a week to get an idea if these habits are in the direction you want.
4 Portion control: keep an eye on how much is on your plate. Use smaller plates.
5 Diet composition: look at what’s on your plate – protein rich foods minimise feelings of hunger.
6 Weigh yourself regularly and keep a record of weight. If you don’t have scales, use a tape measure and measure waist circumference.
7 Body weight: set limits for weight gain – it is easier to lose half a stone rather than wait until you need to lose a stone.
8 Expectations: be realistic about weight loss and set realistic targets for weight change.
9 For weight maintenance, don’t go back to old eating habits, because this will lead to weight regain.
10 For weight maintenance, find a form of exercise to increase physical activity levels. This doesn’t have to be the gym, must be something you enjoy and will maintain over time. Walking, pilates or swimming – have fun!
Health & Safety checked, December 2010
Third report from the Select Committee on Health – May 2004. This detailed report discusses a range of issues related to food, nutrition, diet, obesity and health.
A report on the BBC website of the diet of an Olympic athlete in training – swimmer Michael Phelps.
An article in Nutrition and Metabolism (2004), reviewing research into energy expenditure by thermogenesis. The author is Klaas R Westerterp at the Department of Human Biology, Maastricht University. There are clear descriptions of methodology and of the limitations of some studies, making it a useful example of scientists at work.
Eduweb is a collection of resources for education from the Rowett Institute of Nutrition and Health. This includes a series of podcasts featuring scientists who work in nutritional health talking about their daily work and about good nutrition.
(Websites accessed November 2011)