Effect of Nutrient Delivery Pattern on Biological Rhythms in Human Skeletal Muscle

Human metabolism is regulated on a daily basis via a circadian timing system that coordinates metabolic responses between various tissues. Skeletal muscle is a major site for the oxidation or storage of key metabolites, with recent studies at Bath and Surrey demonstrating biological rhythms in this tissue over a 24-h cycle. It is well established that dysregulated metabolism in skeletal muscle contributes to poor metabolic health and that poor diet is a causal factor in this deterioration. Diet can be characterised by three nutritional considerations: how much is eaten (i.e. quantity/dose), what is eaten (i.e. type) and when it is eaten (i.e. timing). Whilst most dietary approaches and most scientific studies have focused on the first two factors, there is convincing evidence that human health is also dictated by the pattern of nutrient delivery; this includes the frequency and regularity of daily eating occasions, along with their timing both in absolute terms (i.e. time-of-day) and relative to when other inter-related daily events occur - or usually occur (e.g. light exposure, sleep, exercise, etc.).

It remains unknown whether the interaction of such factors may affect the biological rhythms previously documented in human skeletal muscle. In particular, from a basic science perspective, the fundamental contrast between frequent versus infrequent feeding warrants investigation. To this end, 16-24 healthy adults will be randomly allocated to receive their individual daily energy requirements delivered via naso-gastric infusion either continuously over 24 hours (n=8-12; CONSTANT) or as two bolus infusions at the start (0800 h) and mid-point (2000 h) of the monitoring period (n=8-12; BOLUS). Serial skeletal muscle (vastus lateralis) biopsies will be sampled every 4 hours over a controlled 24-h sleep-wake cycle.

Inclusion criteria

• Body mass index >19 but <30 kg/m2
• Be able and willing to give informed oral and written consent, complete and meet the defined criteria of pre-study questionnaires and screens,
• Have a regular sleep cycle with a sleep duration between 6 and 8 hours, do not exhibit extreme morning or evening preference (Horne and Ostberg, 1976)
• Agree to keep a constant sleep/wake cycle with a self-selected 8 hour sleep duration (from which it cannot be deviated by more than 30 minutes) for one week prior to the lab study,
• Obtain 15 minutes of sunlight within 1.5 hours of waking up and agree to nap only within a 4 h designated nap window for one week prior to the lab study,
• Allow confirmation of compliance to these instructions by wearing ActiHeart monitors continuously and complete daily sleep and event diaries for one week before the study session,
• Agree to refrain from alcohol, caffeine, heavy exercise and certain food components TWO days before the study session,
• Agree to weigh and record daily meals (based on individual energy requirements) for TWO days prior to the study,
• Agree to refrain from prescribed and 'over the counter' medication and food/vitamin supplements for a wash-out period three weeks before and during the study.

Exclusion criteria

• Are taking regular medication (also non-prescribed) or food supplements (e.g vitamins, minerals, fish oil, antioxidant tablets) from which it is not possible to refrain, known to influence: Sleep/alertness/the circadian timing system (e.g beta-blockers, barbituates, antidepressants, benzodiazepines, melatonin, ritalin, modafinil, soporifics, St John's Wort), any of the metabolic functions (e.g. affecting thyroid, kidney, liver or gastrointestinal function)
• Any of the inflammatory markers (e.g. aspirin, ibuprofen, antiobiotics, hay fever medication, medication for sore throats and colds)
• And/or any of the endothelial markers (e.g. ACE inhibitors and angiotensin (receptor) blockers, diuretics, Beta-blockers, anti-thrombosis medication),
• Have a history of any circadian or sleep disorder or metabolic, cardiovascular or chronic infectious / inflammatory disease as confirmed by the GP or the pre-study questionnaires (e.g. a Pittsburgh Sleep Quality Index > 5 will result in exclusion),
• Have a history of psychiatric or neurological disease or drug and alcohol abuse,
• Have donated over 400 ml of blood in the three months preceding the study,
• Have participated in shift work or have travelled across more than two time zones within three weeks of the study,
• Do not keep a regular sleep-wake cycle,
• Do not refrain from alcohol, caffeine containing drinks (e.g. coffee, coke, tea, redbull), heavy exercise and certain foods (e.g. those high in fat and green vegetables) TWO days before and during the laboratory session,
• Normally consume more than 4 cups of caffeinated beverages (e.g. tea, coffee, cola) daily
• Smokers