Acute Responses to Dietary Carbohydrate Manipulation

Dietary sugar is increasingly perceived in a negative way. This has led to taxation by government and guidelines by global public health bodies to reduce sugar intake to <5% of energy intake. All available public health guidelines regarding sugar advocate a reduction in sugar intake, despite a lack of evidence to support these recommendations. These guidelines focus on the association between sugar intake and energy intake, without regard for energy expenditure. This oversimplifies situations in which energy surplus is pathological, for example in diseases like obesity. This complexity is demonstrated by evidence that dietary sugar intake is decreasing in the United Kingdom, whilst rates of obesity have increased in the same timeframe.

It is important to consider energy expenditure in the context of health. The most variable component of energy expenditure between individuals is physical activity energy expenditure (PAEE), which varies from ~600-2100 kcal per day in men of a similar demographic. Current guidelines do not regard the effect that changing dietary sugar might have on PAEE and therefore total energy expenditure.

Carbohydrate availability dictates the capacity to perform physical work. However, the role of carbohydrate in regulating physical activity behaviours has only recently been considered. Ingestion of a carbohydrate-rich breakfast causes a significant increase in 24-hour PAEE compared with no breakfast consumption before midday. The magnitude of this difference is greatest prior to midday, near to when carbohydrate had been ingested and when glucose uptake to peripheral tissue is increased. This points towards a stimulatory role of carbohydrate or sugar on PAEE when carbohydrate is readily available to peripheral tissue. The amount of carbohydrate present in skeletal muscle is dictated by the amount of carbohydrate in the diet. As physical activity is performed by skeletal muscle, dietary carbohydrate intake may regulate physical activity behaviour. Consequently, reducing total carbohydrate intake may result in reduced PAEE.

Studies in which carbohydrate has been manipulated and physical activity has been measured have not been sufficient in answering this research question. Often self-report measures of physical activity are used, which are not sensitive enough to discern meaningful differences. Studies which have measured physical activity objectively, i.e. using pedometers or accelerometers, are confounded by a lack of information about actual carbohydrate intake or concurrent prescription of exercise interventions. Furthermore, government targets of reduced sugar intake to <5% of total energy intake are not aimed at overall carbohydrate intake per se. In the breakfast study mentioned, sugar intake was significantly greater amongst individuals who ate breakfast compared with individuals who fasted until midday. Therefore, it is also plausible that a regulatory role of carbohydrate on PAEE may be due to the type of carbohydrate rather than the absolute amount.

If the availability of carbohydrate to peripheral tissue plays a regulatory role on PAEE, then theoretically the effects of manipulating the amount or type of carbohydrate will be detectable acutely, within 24 hours.

Inclusion criteria

• Body mass index 18.5-29.9 kg∙m-2
• Age 18-65 years
• Able and willing to provide informed consent and safely comply with study procedures
• Females to maintain record of regular menstrual cycle phase or contraceptive use
• No anticipated changes in diet/physical activity during the study (e.g. holidays or diet plans)

Exclusion criteria

• Any reported condition or behaviour deemed either to pose undue personal risk to the participant or introduce bias
• Any diagnosed metabolic disease (e.g. type 1 or type 2 diabetes)
• Any reported use of substances which may pose undue personal risk to the participants or introduce bias into the experiment
• Lifestyle not conforming to standard sleep-wake cycle (e.g. shift worker)
• Any reported recent (<6 months) change in body mass (± 3%)