Being overweight and a smoker makes a person biologically older than slim non-smokers of the same birth age, UK and US researchers have found.
Smokers were biologically older
Smoking accelerated the ageing of key pieces of a person's DNA by about 4.6 years. For obesity it was nine years.
These genetic codes are important for regulating cell division and have been linked to age-related diseases.
The study in the Lancet was based on 1,122 twins from a database held by St Thomas' Hospital in London.
The researchers looked at telomeres - strips of DNA that cap the end of chromosomes and appear to protect and stabilise them.
Telomeres shorten each time a cell divides, until there is nothing left, making cell division less reliable and increasing the risk of disorders. This happens naturally with ageing.
Both smoking and obesity are important risk factors for many age-related diseases, therefore Professor Tim Spector and colleagues set out to see whether they might accelerate telomere shortening.
Among the study sample, all women aged 18-76, 119 were clinically obese, 203 were current smokers and 369 were ex-smokers.
By analysing blood samples for DNA the researchers found telomere length decreased steadily with age, as expected.
However, the telomeres of the obese women and smokers were far shorter than those of lean women and those who had never smoked of the same age.
Each pack year - the number of cigarette packs smoked per day multiplied by the number of years smoking - was equivalent to a loss of an additional 18% on top of the average annual shortening of telomeres.
A woman who had smoked a pack per day for 40 years accelerated her ageing by 7.4 years, according to telomere length.
Professor Spector, from the twin research unit at St Thomas' Hospital, said: "What you are seeing here is that the entire body is ageing from smoking, not just your heart or your lungs.
"So you are accelerating your whole chromosomal clock by this activity which is an important message for younger people to think about.
"People would probably think twice if they knew that at every age they were five or seven years older than their contemporaries biologically because that has influences on their skin, brain and bones."
Tobacco smoke contains poisons. The research suggests that these poisons may affect cells at one of the most fundamental levels.
Excess fat is believed to disrupt the chemical proposition of the body in a negative way.
Such stressors can damage the body.
Dr Lorna Layward, research manager at Help the Aged, said the work supported what we already know about smoking and obesity being extremely damaging to health.
"While the research is not conclusive, we should take heed of the alarm bells. Most over 65s are not getting enough exercise which has massive implications aside from obesity, such as declining strength and mobility. Giving up smoking is the biggest thing you can do reduce your chances of developing coronary heart disease.
"In today's fast-paced life, many of us say we don't have time to exercise or eat healthily, but unless we change our ways we will soon have to find time to cope with ill health."
Professor Thomas von Zglinicki from Newcastle University said: "Telomere length is related to age and might be one biomarker of ageing, but whether it really 'defines' biologiocal age is quite a different question."