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The average adult in the UK consumes 200-300 calories beyond their Guideline Daily Amount. For working adults, more than one-third of calories are consumed in the workplace, making this an important environment for intervention. This thesis makes a contribution to the academic literature, by improving our understanding of how and when offering lower-energy alternatives ('swaps') is effective, and to public health by refining an intervention which could be delivered in workplace canteens. Two scoping reviews were conducted (studies 1 & 2) and pointed towards the potential effectiveness of pre-ordering lunch and offering healthier swaps as strategies that may help to improve the healthfulness of food and drink choices. When offering lower energy swaps for snacks and non-alcoholic drinks, studies 3 (n=449) and 4 (n=3,481) recruited samples of UK adults in employment to test the effect of different messages on the acceptance of swaps in an experimental online canteen. The results indicated that messages focusing on the lower-energy content of swaps offered may be an effective and acceptable approach. When highlighting the energy content of swaps offered, increasing the interpretability of this information, by providing physical activity calorie equivalent information (PACE) (i.e., the number of minutes walking required to expend the energy contained) further increased the acceptance of snack and drink swaps offered. In study 5, an online version of a real-world canteen was developed and the intervention (prompts to swaps accompanied by a PACE message) was due to be tested in a real-world trial with the healthcare organisation Bupa. However, due to Covid-19, it was tested qualitatively with employees (n=30) of this organisation across the full lunch menu to provide insights about the factors perceived to influence swap acceptance and the acceptability of the intervention. Swap acceptance was facilitated by the provision of PACE information, and swap similarity in terms of taste, texture, and expected satiety as well as the perception that alternatives provided meaningful energy savings. Overall, the intervention was viewed as an acceptable approach to help reduce energy intake in the workplace. Following refinements to the intervention, Study 6 tested the effect of offering lower-energy swaps with and without PACE messages on the energy of hypothetical lunches pre-ordered with a representative online sample of working adults (n=2,150). Offering swaps with and without a PACE message was found to significantly reduce average energy ordered at lunch compared to when no swaps were offered, the PACE message was more acceptable, and there was no evidence of significant interactions between intervention efficacy and participant characteristics. Offering lower-energy swaps in the workplace when employees pre-order is an acceptable and promising intervention to reduce the energy of foods and drinks ordered. Future work should replicate this research in real-world settings.