New US research has found that depression and anxiety could be as bad for health as smoking and obesity, increasing the risk of conditions such as heart disease and arthritis.

Carried out by researchers at the University of California, San Francisco, the new large-scale study looked at health data taken from a government study of 15,418 retirees with an average age of 68.

The participants' depression and anxiety symptoms were assessed using interviews, and data was also recorded on participants' current smoking status, weight, medical diagnoses and somatic symptoms on two occasions over four years.

The findings, published in the journal Health Psychology, showed that anxiety and depression symptoms predicted a higher rate of nearly all medical illnesses and somatic symptoms.

Participants with high levels of anxiety and depression had a 65 percent increased chance of a heart condition, a 64 percent higher risk of stroke, a 50 percent higher risk of high blood pressure and an 87 percent higher risk of arthritis, compared to those without anxiety and depression.

An increase in somatic symptoms such as headache, stomach upset, back pain and shortness of breath also appeared to be linked with high stress and depression. The effect was also as strong as or stronger than those of obesity and smoking, with the risk of experiencing headaches 161 percent higher in this group, compared with no increased risk among the participants who were obese and smokers.

"These increased odds are similar to those of participants who are smokers or are obese," said senior author Aoife O'Donovan, Ph.D, "However, for arthritis, high anxiety and depression seem to confer higher risks than smoking and obesity."

However, the researchers found that high levels of depression and anxiety were not associated with risk of cancer, a finding in line with previous studies, despite many patients believing that there is a link, say the researchers.

"Our findings are in line with a lot of other studies showing that psychological distress is not a strong predictor of many types of cancer," O'Donovan said. "On top of highlighting that mental health matters for a whole host of medical illnesses, it is important that we promote these null findings. We need to stop attributing cancer diagnoses to histories of stress, depression and anxiety."

"To our knowledge this is the first study that directly compared anxiety and depression to obesity and smoking as prospective risk factors for disease onset in long-term studies," said first author Andrea Niles, Ph.D., who believes the findings highlight the need for health care practitioners to pay more attention to these mental health conditions.

"Anxiety and depression symptoms are strongly linked to poor physical health, yet these conditions continue to receive limited attention in primary care settings, compared to smoking and obesity," she added.