This study will compare the metabolic, appetite, energy intake, and perceptual responses to a bout of exercise completed in the evening after after a low-carbohydrate lunch meal (<10% carbohydrate content / 0.2 g/kg carbohydrate; LO-CHO), with the responses to exercise performed after a lunch meal containing a high carbohydrate content (~60% carbohydrate content / 2.2 g/kg carbohydrate; HI-CHO), and after skipping lunch and fasting for 8 hours since breakfast (FAST).

Regular exercise is known to be a successful strategy for improving several facets of health and maintaining body weight. However, many people are not engaging in enough exercise, and some may not be achieving maximum benefits from the exercise that they already do. Performing exercise in the overnight fasted state has been shown to reduce energy intake over the course of a single day, without any compensatory reductions in free-living energy expenditure. Despite these promising findings, it is likely that not every member of the population is logistically able to perform exercise in the morning due to various work, family and social commitments, and exercise in the evening may be a logical alternative for these individuals.

Studies have found that exercise performed after an overnight fast may incur superior improvements in insulin sensitivity in lean individuals (Van Proeyen et al., 2010), and individuals with overweight or obesity (Edinburgh et al., 2020), compared to exercising after breakfast. These superior improvements may be mediated, in part, by an increased mobilisation and oxidation of endogenous lipid stores. Additionally, overnight fasted exercise may result in a more negative energy balance than exercising after breakfast (Bachman et al., 2016; Edinburgh et al., 2019). We recently examined whether exercise performed in the evening following an extended period of fasting (7 h) would induce similar responses to overnight fasted exercise regarding substrate oxidation patterns and subsequent energy intake (manuscript in preparation - NCT04742530). This research question was important, as we speculate that a large proportion of the population are likely unable to perform exercise in the morning after an overnight fast due to various logistical barriers. Therefore fasting prior to evening exercise could act as an alternative for these individuals.

We found that compared to consuming a carbohydrate-containing meal 2 h prior, fasting before evening exercise resulted in elevated fat oxidation rates during exercise, but was accompanied by compensatory eating at dinner. Additionally, participants reported that fasting throughout the afternoon was difficult. The long-term efficacy of fasted evening exercise may, therefore, be limited by increased hunger and compensatory energy intake.

Consuming a meal lower in carbohydrate and higher in protein and/or fat can increase rates of fat oxidation during exercise (Rowlands & Hopkins, 2002; Oliviera et al., 2021). Protein is also the most satiating macronutrient, and high-protein diets are associated with reductions in energy intake. Consuming a high-protein pre-exercise meal compared to a typical high-carbohydrate meal also led to greater exercise-induced elevations in hormones typically associated with increased satiety and reduced hunger: peptide tyrosine-tyrosine (PYY) and glucagon-like peptide-1 (GLP-1) (Oliviera et al., 2021). Therefore, consuming a meal with a low carbohydrate content and higher protein content before exercise, rather than completely fasting, could be utilised to enhance the metabolic responses to exercise, whilst simultaneously managing appetite and subsequent energy intake. Further research is needed to fully understand the metabolic and appetite-related effects of a low-carbohydrate, higher-protein meal prior to exercise in the evening, compared to a typically consumed higher-carbohydrate meal and complete fasting.

Inclusion Criteria:

1. Non-smokers (due to the well-known impact of smoking on appetite.
2. Not currently on a weight management program or have an unusual eating pattern (i.e., extended fasting periods >8 h other than overnight).
3. Have maintained a stable weight for 6 months (self-reported).
4. No history of gastric, digestive, cardiovascular or renal disease (self-reported).

Exclusion Criteria:

1. Severe food allergies, dislike or intolerance of study foods or drinks.
2. Currently undergoing a lifestyle intervention (structured diet or exercise).
3. Diagnosis of a condition or currently undergoing treatment therapy known to affect glucose or lipid metabolism (e.g., type-2 diabetes, taking statins), or contraindications to exercise.
4. Use of medication or supplements that may affect hormone concentrations and/or substrate metabolism.
5. Excessive alcohol consumption (>14 units/week).
6. Intensive training schedule (>10 hours/week).

Due to the obvious differences between fasting and consuming food, participants will be aware of when they have been assigned to the fasted condition. However, the low- and high-carbohydrate content lunch meals will be closely matched for taste, with carbohydrate content being manipulated within a drink. Participants will not be informed that the carbohydrate content of the lunch meals is being manipulated, and will therefore, be blinded to this element of the study.