**Introduction** Food consists of three basic macronutrients: fats, carbohydrates and proteins. Some of these components are essential aspects of our diets. We eat food to obtain them and also to provide the energy we need to carry out our basic metabolic functions including thermoregulation, and also to perform any behaviours: such as physical activity and reproduction. It is widely considered that animals have some sort of regulatory system that controls how much food we eat to match together our energy requirements with our energy intake. Different macronutrient compositions of food however seem to override this control system which leads to the deposition of body fat (obesity). At present it is unclear what macronutrients cause us to deposit excess fat. For a long time between the 1950s and 1970s it was felt that foods high in fat promoted obesity, but then ijn the 1970s attention shifted to carbohydrates and in particular refined carbohydrates. Fat beame the culprit agian in the 1990s until recently when attention turned back to carbohydrates as the problem. Some much needed clarity was brought to this area by the introduction of nutritional geometric approach by Simpson and Raubenheimer. Emerging from this framework was the protein leverage hypothesis. The protein leverage hypothesis posits that we eat food primarily to obtain protein rather than energy. Hence if the protein content of foods declines we are compelled to overconsume calories in order to obtain the protein. If this excess energy consumption cannot be burned off, by for example upregulating energy expenditure, then the result is fat storage. This project involves a number of sub-projects that concern the roles of macronutrients in the regulation of body weight in the mouse. These consist of a main experiment involving a panel of 24 different diets that vary in the three major macronutrients. There are then 2 follow up experiments, one in which the sucrose content of the diet is varied and a second in which the range of protein levels in the diet is extended to very low levels. **Methods** **1) main experiment** *Diets* Most studies of dietary manipulation in mice use a 3 diet series constructed by the company research diets (often known as the yellow, pink and blue diets). These contain 10%, 45% and 60% fat respectively. The main problem is that the levels of other macronutrients also vary enormously between these different diets. For example the sucrose content of the lowest fat diet is very high (37%) compared to the higher fat diets. As people started to recognise that sucrose may be a factor driving weight regulation the company brought out a second low fat diet with much lower levels of sucrose. Nevertheless this is not the only problem. Other components of the diet change in paralell with the fat content (eg maltodextrose), and the composition of the fat itself is different between the different diets. It is unclear then when using these diets whether one is detecting an effect of fat content, or fat composition, or a response to another dietary component. We have therefore designed a panel of 24 diets with the help of Research diets which overcome these issues. The panel consists of 4 series of 6 diets. The first two series involve escalating protein contents from 5 to 30%, with the fat content fixed at 20% or 60%. The second two series involve escalating fat contents at fixed protein levels. Across all diets the sucrose levels are fixed at 5%. In addition the composition of the fat is the same independent of the fat content, and has been designed to mimic the n3:n6 ratio in modern human diets of 1:12, plus the broad make up of mono- and polyunsaturated fat contents. The detailed dietary compositions of these 24 diets are in uploaded files J01 to J04 with their corresponding diet codes at research diets. These diets can be ordered direct form Research diets by quoting the appropriate code. *Mice* We have exposed mice of 5 different strains to these diets. The strains are C57BL/6, BALB/c, DBA2, FVB and C3H. The main experiment has been conducted using C57BL/6 mice with the core findings in C57 mice replicated in the other strains. *The primary questions being asked are* a) what is the impact of changing protein content in the diet when fat content is constant. In particular testing the hypothesis from the protein leverage hypothesis that declining protein content will stimulate appetite to meet a protien target and hence lead to overconsumption of calories and thus elevated adiposity - or mechanisms enabled to burn off the excess consumption. b) what are the roles of fat and carbohydrate contents at fixed levels of protein. **2) Impact of variable sucrose content** *Diets* In the main experiment the level of sucrose was fixed at 5%. we have designed a further panel of 12 diets where the protein content is fixed and the sucrose content of the carbohydrate portion escalates form 5 to 35%. In the first six diets the fat is fixed at 20% and in the second six the fat is fixed at 60%. **3) impact of very low protein diets** *Diets* In the main experiment the lowest protein content we used was 5%. we have designed a further 4 diets with 2.5% and 1% protein (20 and 60% fat) to extend the range of protein contents in the diets and explore the impact of such low protein levels on energy regulation.