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# 2.2 Experimental studies of energy balance

The energy that a human takes in, primarily in food or nutritive fluids, has to be balanced by the energy lost in various ways. Some energy is used to maintain basic metabolic processes, some in physical activity while the remainder is lost as heat, or in the faeces or urine. If energy input and output do not balance, then the residue must either result in a loss or a gain in body weight.

One way of estimating these energy fluxes is to use the technique of whole room indirect calorimetry. Volunteers live on their own in a small suite of rooms which are sealed from the outside environment. The room can also contain exercise equipment. Food and water are provided through an airlock, and the exact amounts of carbon dioxide produced and oxygen used are measured. It is then possible to calculate an overall energy budget for each individual.

In one experiment of this type, participants were divided into three groups, each group receiving a diet of different fat content (Poppitt and Prentice, 1996). The other constituents of the diet, which were mainly protein and carbohydrate, were varied in such a way as to keep the overall energy density of the diets constant.

## Activity 18

What is the energy density of a diet?

The number of calories (or joules) in a standard quantity (often 100 g) of that food.

The participants in this experiment were not aware that their diets were being manipulated in this way. In practical terms this kind of manipulation is achieved by, for example, substituting fat for carbohydrate within a yogurt while, at the same time, ensuring that the sensory characteristics (e.g. taste, smell and feel) do not change.

The results of this experiment are shown in Figure 6(a).

## Activity 19

How did the voluntary energy intake of the participants vary as a function of the fat content of the diets when energy density is constant?

The overall result was straightforward: the energy intake of the volunteers hardly varies as a function of the amount of fat in the diet.

Figure 6 Voluntary energy intake and dietary energy density in energy-manipulated diets. In (a) the other dietary constituents were manipulated so that the overall energy density remained constant as the proportion of fat was increased. In (b) no change was made in other dietary constituents as fat was added and as a consequence the overall energy density of the diet increased.

The apparent difference between the 20% and 40% groups was not significant. In addition there was no significant change in body weight over the experimental period of a few days.

In a second experiment the participants were again divided into three groups. Now each group received a diet in which the fat content had been covertly manipulated so as to vary the total energy density. One group received a diet which was relatively low fat and low energy, in a second group it was relatively normal whereas the third group received an energy-dense, high-fat diet. Other factors, such as the palatability of the foods provided, were again held constant. Surprisingly, the participants in the different experimental groups ate about the same number of grams of food each day regardless of its energy content.

## Activity 20

Look at Figure 6(b). What was the consequence of this behaviour for the energy intake of the participants?