Loneliness raises levels of circulating stress hormones and levels of blood pressure. It can gradually undermine regulation of the circulatory system so that the heart muscle works harder and the blood vessels are subject to damage by blood flow turbulence. Hawkley and associates found that loneliness was associated with elevated systolic blood pressure independent of age, gender, race or ethnicity, cardiovascular risk factors, medications, health conditions, and the effects of depressive symptoms, social support, perceived stress, and hostility.
The Mayo Clinic found that a low social support network is equivalent to many of the classic risk factors of one-year mortality from a heart attack (i.e. elevated cholesterol levels, tobacco use, and hypertension). In other words, a lack of friendships increases the risk of dying from a heart attack within one year, and the value of meaningful relationships is as important as lowering cholesterol, controlling high blood pressure, and smoking cessation in preventing a heart attack. Another study showed that after a heart attack, even having a perception of low social support predicted death or another heart attack. In Japan, low social support was associated with a higher risk of stroke mortality in men.
References
Hawkley, L.C., et al., Loneliness and pathways to disease.Brain Behav Immun. 2003 Feb; 17 Suppl 1:S98-105
Hawkley, L.C., et al, Loneliness predicts increased blood pressure: 5-year cross-lagged analyses in middle-aged and older adults. Psychosom Med. 2009 Oct; 71(8):836-42
Hall, E. J., Keys to Optimal Health and Happiness ebook, College of Health Evangelsim, 2009