We all know that chronic stress is bad for health. Whether you are a “stress eater” or simply at the mercy of the physiological processes that accompany stress – namely increased cortisol secretion and insulin levels – both of which promote fat storage, stress can make sustainable weight loss extremely difficult.
Heart rate variability (HRV) has been around for decades. Initially used in fetal distress and acute trauma patients, HRV is now recognized as a reliable predictor of mortality risk after a heart attack. HRV is a measure of autonomic nervous system function and has recently been used as a health indicator in weight loss studies. But what exactly is HRV? And what does the “fight or flight” response have to do with diet and weight loss?
In addition to predicting prognosis after heart attack, reduced HRV is also related to cancer cachexia and cancer-related fatigue; fibromyalgia outcome; and may even be linked to cognitive impairment! What is HRV? We are by *no means* experts in neurophysiology, but in the most basic terms HRV measures the beat-to-beat variation in your heartbeat; essentially, the heartbeat-within-a-heartbeat (1). HRV (of which there are several different indices that are too complex for us to explain!) provides a marker largely of parasympathetic nervous system function – the parasympathetic nervous system controls body function at rest (think “rest and digest”). The role of sympathetic nervous system function within HRV remains controversial; however, the “fight or flight” response is often indirectly inferred from HRV. For a great summary on HRV assessment and use: click here.
Although not completely understood, reduced HRV (where desirable fluctuations in rhythm are blunted), is related to poorer prognosis for many clinical conditions – heart disease, cancer, diabetes and even chronic pain such as fibromyalgia. Just what does the beat-within-beat tell us? It may indirectly indicate poorly regulated resting parasympathetic function or a chronically stimulated sympathetic nervous system, where the “fight or flight response” is dysfunctional. In fact, HRV has been shown to be reduced in individuals who report a greater frequency and duration of daily worry and anxiety (2).
What does this have to do with nutrition? More and more as cardiac autonomic function is included as a measure in nutrition research; HRV is an metric dietitians and nutrition researchers should become familiar with. While the relationship between cardiovascular disease outcomes and HRV has been well documented, weight loss and impact of diet on HRV seem to be untapped areas for research. A blunted sympathetic nervous system response (which was measured by muscle sympathetic nervous system activity) has recently been associated with “weight loss resistance” and unsuccessful attempts to achieve weight loss (3). Clinically, this makes absolute sense – we have all come across clients or colleagues who are actively trying to lose weight but seem to “fail” for no other reason than stressful environments, be they work or home, and an accompanying challenge in regulating their own stress response.
How do you improve HRV? While there is research interest in targeting drug therapies to improve HRV, there are plenty of good-for-you-anyway tools to improve your “rest and digest” systems, including dietary strategies:
- Weight loss. Sjoberg et al. (4) found that a modest 10% reduction in body weight improved HRV in overweight and obese adults with type 2 diabetes.
- Physical activity, particularly aerobic training. This doesn’t have to mean high intensity though. Earnest et al. (5) found that even modest increases in physical activity improved HRV in older women who were previously sedentary, though there was a dose-dependent response were more activity showed greater improvement.
- Yoga and meditation. While research is limited at this time, meditation has been shown to be effective in increasing parasympathetic and, by inference, reducing sympathetic nervous system components of HRV to hypothetically reduce cardiovascular disease risk (6). In fact, RCTs of yoga interventions to improve HRV are currently being conducted (7).
- Mediterranean diet. In a twin heart study, researchers found that greater conformity to a Mediterranean diet (more fish, veggies and fruit, whole grains) was linked with better HRV, even when energy intake was adjusted for (8). By using twins, Dai et al. were able to indirectly “control” for genetic determinants of HRV, which did play a significant role as well. There is some new research looking at the potential benefits of DHA and EPA supplementation on HRV, though it has all been done in dogs!
While understanding nervous system function may seem like a bit of a stretch in the world of nutrition, HRV provides a new-to-nutrition measure of chronic disease risk. Markers of cardiac autonomic system imbalance such as HRV provide worthwhile insight into physiologic “sabotagers” of weight loss and may serve as important indicators of success in diet interventions to reduce chronic disease.
1. Billman GE. Heart rate variability – a historical perspective. Front Physiol. 2011; 2:86.
2. Brosschot JF, Van Dijk E, Thayer JF. Daily worry is related to low heart rate variability during waking and the subsequent nocturnal sleep period. Int J Psychophysiol 2007; 63:39–47.
3. Straznicky NE, Eikelis N, Nestel PJ, Dixon JB, Dawood T, Grima MT, Sari CI, Schlaich MP, Esler MD, Tilbrook AJ, Lambert GW, Lambert EA. Baseline sympathetic nervous system activity predicts dietary weight loss in obese metabolic syndrome subjects. J Clin Endocrinol Metab 2011 [epub ahead of print]
4. Sjoberg N, Brinkworth GD, Wycherley TP, Noakes M, Saint DA. Moderate weight loss improves heart rate variability in overweight and obese adults with type 2 diabetes. J Appl Physiol 2011; 110:1060-4.
5. Earnest CP, Lavie CJ, Blair SN, Church TS. Heart rate variability characteristics in sedentary postmenopausal women following six months of exercise training: the DREW study. PLoS One 2008;3:e2288.
6. Nesvold A, Fagerland MW, Davanger S, Ellingsen O, Solberg EE, Holen A, Svre K, Atar D. Increased heart rate variability during nondirective meditation. Eur J Cardiovasc Prev Rehabil. 2011 [epub ahead of print].
7. Cheema BS, Marshall PW, Chang D, Colagiuri B, Machliss B. Effect of an office worksite-based yoga program on heart rate variability: a randomized controlled trial. BMC Public Health. 2011;11:578.
8. Dai J, Lampert R, Wilson PW, Goldberg J, Ziegler TR, Vaccarino V. Mediterranean dietary pattern is associated with improved cardiac autonomic function among middle-aged men: a twin study. Circ Cardiovasc Qual Outcomes. 2010;3:366-73.
Acknowledgement: We would like to thank Michael Mackenzie, MSc, PhD Candidate at the University of Calgary for reviewing this post.