Discussion
This is the first study to objectively measure sedentary behaviour and physical activity levels in adults with clinically defined NAFLD, and to use this data to investigate the relationship between physical activity, liver fat and metabolic control. The data reveals that people with NAFLD achieved lower levels of physical activity and spent more time sedentary than healthy controls. Levels of physical activity or sedentary behaviour were not associated with the severity of liver fat or glucose control in this small well-characterised group.
The present data highlights that people with NAFLD undertake less daily physical activity, by TEE, AEE and steps, than their healthy counterparts. Previous reports from self-report questionnaire reports also reveal that NAFLD is associated with lower levels of physical activity. However, questionnaires have significant limitations and are subject to recall and social desirability bias, and are inaccurate in determining frequency, duration and intensity of physical activity. The poor associations between objective and subjective reports of physical activity in the present study highlight the importance of objectively assessing physical activity. The link between physical activity and liver fat in previous research highlights the positive effects of a physically active lifestyle upon IR, impaired glucose tolerance and T2DM. Physical activity should, theoretically, aid the prevention and/or progression of NAFLD through its reciprocal relationship with glucose control, and has been shown to improve liver histology when used as part of a lifestyle intervention in conjunction with diet and weight loss.
People with NAFLD also undertake less moderate and vigorous activity than healthy controls. The lower levels of these higher-intensity activities may have implications, as the intensity of the activity may also play a key role in improving metabolic control. However, the reports demonstrating that higher intensity activities/exercises are linked to improvements in metabolic control are not unequivocal. One meta-analysis found exercise intensity was not associated with a difference in HbA1c in people with T2DM. However, when using resistance training independently, moderate high-intensities were associated with greater improvements in muscle bulk and overall glucose control, and high-intensity interval training was shown to improve hyperglycaemia in patients with T2DM. Harrison and Day speculate that moderate exercise, performed 3–4 times per week, expending about 400 kcal each time appears adequate to augment improvement in the metabolic profiles of patients with NAFLD. However, although useful, the evidence underlying these clinical guidelines is lacking. There is no clear evidence on which exercise approach is best in improving metabolic control with recent data also suggesting benefit from resistance rather than cardiovascular exercise.
A novel observation in the present dataset is that adults with NAFLD spend more time being sedentary than those without fatty liver. Sedentary behaviour or physical inactivity is a growing health problem, silently putting people at heightened risk from a host of chronic diseases. This increase in physical inactivity may compound the detrimental health effects caused by lack of physical activity. In the present study, adults with NAFLD accumulated 22 h per day of sedentary activity. Sedentary behaviours involving sitting or lying down are characterised by a low MET value of less than 3, and are related adversely to metabolic biomarkers and to poorer health outcomes. Adults with NAFLD also had fewer sedentary to active transitions (ie, breaks in sedentary time) than the healthy controls. Sitting for prolonged periods reduces the opportunity for cumulative energy expenditure produced by muscle contractions as we move around throughout the day, and impairs the exercise/muscle contraction-stimulated uptake of glucose from the circulation and lipoprotein lipase activity thus hampering fat handling. Even if adults meet the public health guideline for leisure-time physical activity, they may have a high risk of becoming overweight or developing metabolic disorders if they spend a large amount of time in sedentary behaviours during the rest of the day. Combined, these results demonstrate for the first time that sedentary behaviour is prominent in NAFLD—targeting these periods of inactivity may constitute an effective means of improving liver fat.
In patients with NAFLD, BMI was negatively correlated with objectively measured markers of increasing physical activity and positively associated with sedentary time. In obesity, studies have shown similar findings whereby the more overweight/obese people are, the less physical activity they undertake, which drives the vicious cycle of increasing weight gain. Given the strong link between BMI and NAFLD, it is possible that the main effect of physical activity on NAFLD is through its relationship with body weight. This observation also highlights the potential for reducing sedentary behaviour and increasing physical activity in the prevention of weight gain, a major driver for the development of NAFLD and poor metabolic control. It should be noted, however, that the relative small sample size may not be sufficient to see other relationships.
The data produced by the multisensor array provides useful insights into free-living daily activity patterns in people with NAFLD. The MET levels provided also act as a guide as to the intensity of activity undertaken which allows clinicians to tailor advice to this. Volunteers found the monitors easy to use and unobtrusive, with little impact on daily activity. Adherence to wearing the monitor was high, as demonstrated by a mean percentage wear time of >96%. Limitations of these monitors are that they are not waterproof and thus need to be removed for any water-based activity. It should also be noted that the present study may be limited by the relatively small cohort sizes, the cross-sectional design which removes the ability to assign causality, and the absence of liver fat and blood sample measurements for control subjects.
Clinical Implications
The use of objective measures of physical activity and sedentary behaviour in the clinical environment may provide clinicians with a way to engage patients in discussion about activity/exercise. Data recorded can be used as a baseline measure from which to tailor subsequent physical activity counselling and build appropriate exercise programmes. Their use offers the opportunity to provide immediate feedback to patients when they return to clinic, by providing a short report or a more in-depth daily analysis of activity, from which discussions about lifestyle change and weight loss can materialise. Since the visual data being presented by the clinician represents the patient's actual day-to-day life, this may act as a valuable tool to aid in improving adherence, patient motivation and clinical outcomes.
In conclusion, people with NAFLD spent more time sedentary and less time physically active on a daily basis than people without fatty liver. Given the established relationship between sedentary behaviour and physical activity with metabolic regulation, weight gain and cardiovascular disease risk, high levels of sedentary behaviour and low levels of physical activity may represent effective therapeutic targets in the management of NAFLD. Often, patients are not aware how much physical activity or sedentary behaviour they actually engage in, so an objective measure will provide this feedback, and thus allow personal activity goals to be established in order to achieve their individual health targets. Combined, these data suggest that clinical care teams should consider the use of objective monitoring and targeting of sedentary behaviour and low levels of physical activity as a means to improve metabolism, prevent weight gain and delay disease progression in people with NAFLD.