The production of animal-derived products can be seen as a linear process in which inputs such as feed are converted to outputs such as meat, milk, or eggs. Feed efficiency is then just the ratio of outputs over inputs. However, there are many more inputs and outputs in the production process, and not all are desirable. The idea behind a circular (bio)economy is to consider undesirable outputs from one subsystem as useful inputs for another subsystem, thereby eliminating waste and closing the loops. The circular flow of material and energy has been described by concepts such as ‘urban metabolism’ or ‘agricultural metabolism’. The metaphor with metabolism is interesting, because circularity exists widely in biology. The Krebs cycle and the urea cycle are examples of this, but the storage and mobilization of nutrients can also be seen as circular processes. However, these circular processes are linked to rather ‘linear’ objectives such as the production of ATP from acetyl CoA, the elimination of ammonia as urea, or to account for time differences between nutrient supply and demand. Circularity in metabolism is thus not necessarily a goal by itself, but a means to achieve a specific linear goal. Metabolism is also very well controlled, involving circular feedback loops, which occur at different levels of organization. For example, the functioning of an adipocyte is controlled both locally and centrally. Adipocytes can have a life of their own (e.g. they can be cultured in vitro), but they also respond to the energy supply and demand of the organism. The combination of linearity and circularity allows us to control our metabolism within very narrow limits, making the system remarkably robust. However, a 0.5% difference between energy intake and expenditure (i.e. the equivalent of one peanut per day), results in gaining roughly 20 kg of lipid between 20 and 60 years of age. Our metabolic system is thus very robust, but not fail-proof. Agricultural metabolism and livestock production systems are probably also very robust, but nether fail-proof nor fool-proof. Because we have some control over it, let us try not to be the fools that caused the disruption.