Pathways are presumed to exist that transmit information concerning the metabolic effects of a food to the brain to allow development of learned associations with one or more sensory properties of the food. The simplest such pathway would be via the blood. For example, a low blood calcium level arising from eating a low-calcium food might be sensed by the brain, which would respond by increasing the intake of an alternative, high-calcium food.
There have been a number of proposals as to the involvement of neural transmitters in the control of diet selection in rats. For example, carbohydrate intake appears to be enhanced by hypothalamic injections of noradrenaline and neural peptide Y (NPY) and inhibited by 5-HT. There is a steady rise in hypothalamic NPY concentration from birth to puberty, at a time when protein intake increases; however, the optimal protein:energy ratio in the diet declines during this period (see Chapter 8), so the relationship between hypothalamic NPY and selection for protein:energy is not in the direction expected if NPY is indeed solely or mainly responsible for selection between carbohydrate and protein.
There have also been theories of the involvement of individual amino acids in the control of food choice. Given a choice between a tryptophan- deficient food and a control food, chickens preferred the latter but, when the piriform cortex was separated from the rest of the brain, they had difficulty in discriminating between the two foods (Firman and Kuenzel, 1988). It is possible that the piriform cortex monitors amino acid levels, or that its separation disrupted the sense of smell (it is part of the olfactory system), or both of these. However, lesions of the nucleus taeniae, which is also a part of the olfactory system, did not affect ability of birds to choose the right food.
As so little is known about neural pathways and transmitters in farm animals, the interested reader is referred to a book reviewing the neurobiology of food choice in laboratory animals (Berthoud and Seeley, 1999).
Timescale of control of diet selection
It might be supposed that animals that start to eat a meal from one food will tend to find the other food more attractive as the meal progresses. Sixty per cent of the meals taken by broilers given free access to high- and low-protein foods are from both foods, i.e. there is a strong tendency to change foods during the meal (Shariatmadari and Forbes, 1992a). As metabolic receptors would not have had time to be significantly influenced by the food eaten earlier in the meal, this suggests that the choice of food is predominantly controlled by learned associations between the foods and their hedonic properties rather than by immediate feedbacks.
In order to look for an explanation as to how short-term feeding behaviour is involved in food choice, Yeates et al. (2002) have analysed large amounts of meal data collected automatically from cows given free access to foods high
(HP) and low (LP) in protein, in three experiments. It was concluded that the cows did not attempt to select a consistent diet in terms of protein:energy ratio either within a single meal or in a short sequence of meals, as there was no difference in the proportion of visits to HP and LP during meals compared with random sequences of feeding bouts. The authors concluded: ‘Our present analysis does not suggest what the most relevant time scale is, except that is must be longer than a meal’. Further detail is given in Chapter 2.
As an illustration of the greater stability of choice over longer periods, Fig. 7.9 shows the proportion of HP taken by eight cows offered a choice of HP and LP over a period of 28 day (Kyriazakis et al., 1999). Choice in consecutive 8-h periods is very variable and ranges from wholly HP to mostly LP. Cumulating these into 24-h periods greatly reduces, but certainly does not eliminate, the variability, while 7-day means show great consistency, with a steady decline in selection for HP as milk yield declines in mid-lactation. It can be concluded from this that diet selection was being controlled in these cows over a timescale of 1–7 days.
Other research with cows shows selection changing in response to changes in protein content of the foods on offer, starting within 1 day after the change and being complete by about 4 days later (Tolkampet al., 1998b, Fig. 13.2). It has to be said that in this latter case the changes in food composition were by means of addition or removal of urea from both foods, which leads to rapid changes in ruminal ammonia, whereas in the earlier example (Fig. 7.9) the only change was the very gradual one of advancing lactation.
In sheep the timescales are similar, i.e. regulation of protein content by selection between normal foods with different protein contents to produce a
0 100 200 300 400 500 600 700 800 900 1000
0 5 10 15 20 25 30
Day
Diet choice (g HP/kg TFI)
Fig. 7.9. High-protein (HP) food intake as a proportion of total food intake (TFI) by eight lactating cows given a choice of HP and LP over periods of 8 h (䡲), 24 h (䡩) and 7 days (●) (from Kyriazakis et al., 1999).
stable diet over 3–7 days (Kyriazakis et al., 1994), but a much more rapid adjustment as a consequence of feeding on a rapidly fermentable food (James and Kyriazakis, 2002). In the opinion of Kyriazakis et al. (1999): ‘… the question to be asked in relation to diet selection is not “what time period matters to the animal?” but “how much change or deviation in the internal state is the animal prepared to accept?”’.
In the medium to long term, internal changes in the animal do lead to appropriate changes in selection, presumably in order to control some aspect of the animal’s state rather than to stabilize some short-term changes in blood hormones or metabolites. Examples are given in Chapter 8.
Nutrient infusions and preloads
In order to understand the link between metabolism and learning, numerous experiments have studied the effects of loading a substance or a certain type of food into the digestive tract or bloodstream of animals and observing the effects on diet selection.
Poultry
Mechanisms of diet selection have been investigated in more detail by giving overnight-fasted broilers a meal of either HP or LP, to which they had previously been accustomed, and then offering both a choice. Whether the choice was given immediately after the initial meal or delayed for 1 h, significantly greater amounts of the opposite food were eaten (Forbes and Shariatmadari, 1996) (Fig. 7.10). When the initial meal was given by tube into the crop there was no significant preference subsequently, so it seems as if it is necessary for the bird to taste the food in order to predict its protein content according to its previous experience of the two foods.
The independent control of energy and protein intakes was further demonstrated by force-feeding broilers with 40 g of food containing 60, 135, 215 or 295 g CP/kg daily for 10 days while given free access to foods with 60 and 295 g CP/kg (Shariatmadari and Forbes, 1992b). During the force-feeding period, the higher the protein content the greater the voluntary food intake, but the proportion of HP chosen was lower, resulting in an almost constant level of protein intake across the treatments. After the cessation of force-feeding, the birds which had been force-fed with the two highest levels of protein continued to have higher voluntary food intakes but with a lower proportion of HP, and deposited less protein in the carcass, than birds force-fed with lower-protein foods.
This suggests that, although compensating only partly for the protein given by gavage during the force-feeding part of the experiment and thus ending this period with more carcass protein, the birds force-fed with the higher levels of protein then responded to their higher body protein content by voluntarily consuming a diet with a lower protein concentration than those given lower protein by tube.
Sheep
It might be expected in ruminant animals that products of digestion in the rumen would affect diet selection. Infusion of sodium acetate or sodium chloride intra- ruminally in sheep gave large reductions in the intake of concentrates but had no effect on hay intake (Engku Azahan and Forbes, 1992). The effect was almost as great for chloride as it was for acetate, so the major reason for the decreased intake was probably osmotic (see Chapter 3).