2.8 Treatments for obesity At this point you may be wondering what the range of studies that we have examined might suggest in terms of treatment for obesity. As you have seen, weight gain essentially arises from an imbalance of energy supply and energy expenditure. Therefore it is not surprising that dieting (restriction of energy input) and exercise (increased energy output) are recommended both to reduce body weight and also for their additional health benefits. However, even a quick survey of the research literatur
2.7 A gene ‘for’ obesity? So far we have mostly emphasized the way in which different environmental factors may affect body weight and provide a partial explanation of both individual cases of obesity and the increase in average body weight that has been so clearly documented in both North America and Western Europe during the last two decades. There is also marked individual variability in body weight. For example, any weight between about 58 and 78 kg would be regarded as ‘desirable’ for a person of height 1.77
2.6 Obesity and ageing So, it seems that a part of the explanation for Ron's obesity, and the health problems that have led him to seek medical advice, may relate to the environment in which he is living, and more specifically the diet that he has chosen. It may also be a diet that is especially effective in activating the reward circuits in Ron's brain. However, Ron is also in his later middle age. A longitudinal study of people of this age in the USA suggests that average body weight increases by 1–2 kg per dec
2.4 Obesity – an evolutionary perspective If you were now to take a broader biological approach to the data discussed in the previous section you might still be puzzled. Excess body weight leads to a variety of diseases, including diabetes, osteoarthritis and so on – surely this must reduce overall biological fitness. 1.5 Obesity At the time of writing (2004) 20% of the adult population of the UK is classified as obese. The number of obese children has doubled since 1982, 10% of six year olds and 17% of fifteen year olds are now classified as obese. As shown in Table 4, obesity is recognized when the BMI exceeds 30 and occurs quite simply wh 1.3 Balanced energy intake There is a need for a certain daily energy intake to allow metabolism to occur in the body. Metabolism means all the chemical reactions occurring in the body and there are two types of process involved: catabolism breaks down larger molecules into smaller ones often with energy release and anabolism is the building up of larger molecules from smaller precursors, often requiring energy. The body requires energy to power anabolic, mechanical (for example, muscle contraction 2.4 Summary of Section 2 The water cycle involves the movement of water, in all its forms, over, on and through the rocks near the surface of the Earth in a cycle. This cycle is driven by the Sun's energy and the Earth's gravity. The total volume of water in the cycle is virtually co 4.3 Photosynthesis, respiration and decay Green plants absorb solar radiation and use its energy to fuel photosynthesis — a chem 3.3 The Earth's internal heat The occurrence of both volcanoes and hot springs shows that the Earth's interior is hot, producing molten rock at temperatures up to 1250 °C, and also superheated steam. However, these phenomena are mainly confined to several narrow zones along the world's active plate boundaries. Many measurements have now been made of the amount of heat flowing from the Earth's interior. Outside the distinctive zones mentioned above, heat flow varies from 40–120 milliwatts per square metre (mW m−2 3 Sources of energy from the natural environment The natural environment itself is bathed in energy from other sources. Standing on a cliff top on a bright spring day you can feel the warmth of the Sun and the freshness of the breeze and hear the crashing of breaking waves below. All these energetic processes can be compared in terms of Author(s): 2.2 Present-day energy use Global annual consumption of all forms of primary energy increased more than tenfold during the 20th century (Author(s): 2 Energy, work, power and efficiency In everyday speech we often refer colloquially to the powerful politician, the energetic child, the working mother and the efficient administrator. We use these terms imprecisely, and often wrongly, compared with their scientific definitions. 1 Energy use Until about 8000 years ago humans relied on hunting and gathering for food, and burning wood to keep warm. Their exact energy demands can at best only be estimated but to survive they probably needed about as much Author(s): Learning outcomes By the end of this unit you should be able to: An introduction to energy resources Understanding energy resources involves considering all types of energy source from various scientific Acknowledgements The material acknowledged below is contained in Chapter 3 of An Introduction to Galaxies and Cosmology (eds Mark H Jones and Robert J Lambourne), published by the Press Syndicate of The University of Cambridge in association with The Open University. Copyright © The Open University, 2003, 2004. This publication forms part of an Open University course S282 Astronomy. 7.5 Outstanding issues Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes. Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years. It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat 7.4 Models of active galaxies The standard model of an AGN consists of an accreting supermassive black hole (the engine) surrounded by a broad-line region contained within a torus of infrared emitting dust and a narrow-line region. Unified models attempt to explain the range of AGNs on the assumption that they differ only in luminosity and the angle at which they are viewed. One type of model attempts to unify radio-quiet AGNs. Type 1 Seyferts and type 7.3 The central engine An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt. The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System. The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius 5.1 Introduction So far we have seen how the properties of the central engine of the AGN can be accounted for by an accreting supermassive black hole. Though there are many questions still to be resolved, this model does seem to be the best available explanation of what is going on in the heart of an AGN. But of course all AGNs are not the same. We have identified four main classes and in this section we will attempt to construct models that reproduce the distinguishing features of these four classes.
SAQ 21
