Traditionally it has been thought necessary to lose 2% of body mass due to dehydration, before functioning is disrupted, although recently adverse effects have been reported with a loss of 0.5-0.7%. It is, however, unclear whether the response to small decreases in mass reflects dehydration, mechanisms that help to adapt to a loss of bodily fluid, or a placebo effect. Individuals were therefore subject to a temperature of 30°C for three hours, and mood and cognition monitored. To explore the possibility of a placebo response, the consumption of plain or coloured water was compared. To consider changes in hydration status, drinks known to differ in their ability to rehydrate were contrasted. Not drinking was disruptive, although a combination of plain water and electrolyte’s most effectively prevented a decline in functioning, indicating a role for rehydration after a loss of 0.52% body mass. There was, however, also evidence of a placebo response: a combination of plain water and electrolytes was better able to prevent a decline in functioning than coloured plain water and electrolytes. As increased anxiety was a robust response, it was discussed whether the renin–angiotensin–aldosterone system might be part of the mechanism.

Consumers often cite cognitive improvements as reasons for making dietary changes or using dietary supplements, a motivation that if leveraged could greatly enhance public health. However, rarely is it considered whether standardized cognitive tests that are used in nutrition research are aligned to outcomes of interest to the consumer. This knowledge gap presents a challenge to the scientific substantiation of nutrition-based cognitive health benefits. Here we combined qualitative consumer research and expert group methodology to (1) understand how consumers comprehend the potential benefits of nutrition for brain health, and (2) determine the alignment between consumers' articulated brain benefits and validated tests and tools. We derived a ‘Consumer Taxonomy of Cognitive and Affective Health in Nutrition Research’ which describes the cognitive and affective structure from the consumers' perspective. Experts agreed that validated tests exist for some consumer benefits including focused attention, sustained attention, episodic memory, energy levels, and anxiety. Prospective memory, flow, and presence represented novel benefits that require the development and validation of new tests and tools. Closing the gap between science and consumers, and fostering co-creative approaches to nutrition research, are critical to the development of products and dietary recommendations that support realizable cognitive benefits that benefit public health.