In this article, drawing on ethnographic fieldwork in the Netherlands, I explore how weight management practices adapt scientific knowledge to the pragmatics of daily life. I contrast two ‘metabolic logics’: one premises calculating food and exercise to ensure energy balance; the other, operating as a critique to the first, puts its hope in activating people’s metabolic rate. Metabolic logics, I stress, do not just present ideas on bodily functioning. They are also and importantly a practical and material affair. The first approach incites a desire and sense of responsibility in people to have control over and correct their bodies, while the second, foregrounding less measurable forms of health, hinges on a person’s responsivity and trust in other active entities. Metabolic practices do not merely follow scientific insights into how fat comes about; they include estimations of what knowledge is helpful in daily life when overweight is a concern. However, innovation is difficult, as in exercise machines, recommended dietary intakes or diet shakes, figures of food as fuel and bodies as machines stubbornly sediment. In conclusion, I suggest that when ‘thinking metabolically’ we address metabolism as part of the socio-material practices that narrate eating, bodies and moving together in particular ways.