Exercise may hold the key to youth, according to a study published on Monday which showed people who keep fit are up to nine years biologically younger than those who do not.

The findings are the first to show in humans how keeping fit affects the ageing process.

The study of 2,401 twins found that a sedentary lifestyle raises the risk of a range of problems from heart disease to cancer and appears to play a key role in the ageing process.

It all appears to boil down to the length of structures called telomeres -- which protect the DNA on the chromosomes, the researchers from King's College London wrote in the Archives of Internal Medicine.

Many studies have shown telomeres get shorter over time, suggesting the cells are ageing or dying. The study, which extracted a DNA sample from their volunteers, found people who exercised more each week had longer telomeres.

Exercise lowers the risk of a range of problems such as heart disease, diabetes and cancer, the researchers said.

"It is not just walking around the block. It is really working up a sweat," said Tim Spector, a genetic epidemiologist who led the study, in a telephone interview.

The study found people who exercised vigorously 3 hours each week had longer telomeres and they were 9 years biologically younger than people who did under 15 minutes.

Spector's team, who also adjusted for body weight, smoking, economic status and physical activity at work, also said moderate exercise for 1-1/2 hours each week provided a four-year advantage.

Studying twins also provided a unique opportunity to gauge the effects of exercise on people with the same or similar genetic make-up, the researchers said.

The reason why exercise has this effect is not clear but the researchers said they believe physical activity somehow defends against the natural process called oxidative stress, which damages and kills cells.

本周一公布的一项研究表明，锻炼身体是永葆青春的秘诀。研究发现，健身的人比不怎么锻炼的人在生理上要年轻九岁。

这个研究结果首次表明健身对于人衰老进程的影响。

这项对2401对双胞胎开展的研究发现，久坐的生活习惯会增大患心脏病和癌症等一系列疾病的风险，而且是影响衰老进程的关键因素。

伦敦国王学院的研究人员在《内科医学档案》中写道，所有这些问题似乎都归结于一种叫做“端粒”的特殊结构的长度，端粒能够保护染色体上的DNA。

很多研究已表明，随着年龄的增长，端粒会变短，这意味着人体的细胞正在老化或死亡。研究人员从志愿者体内提取的DNA样本中发现，每周锻炼较多的人端粒较长。

研究人员称，锻炼身体能够降低患心脏病、糖尿病和癌症等一系列疾病的风险。

该研究负责人、遗传流行病学家蒂姆?斯派科特在接受电话采访时说：“这里所说的锻炼并不只是简单的散散步，而是要运动到出汗。”

研究发现，每周进行3小时高强度锻炼的人端粒较长，而且比每周锻炼不到15分钟的人在生理上年轻九岁。

在排除体重、吸烟、经济状况和工作中的身体活动量等因素的影响后，斯派科特领导的研究小组发现，每周进行一至一个半小时的适度锻炼仍可以让你年轻四岁。

此外，研究人员称，对于双胞胎的研究为衡量运动对于基因组成相同或相似的人所产生的影响提供了独特的机会。

虽然锻炼能延缓衰老的原因目前还不清楚，但研究人员认为，运动在某种程度上抵抗了氧化应激的自然过程，这一过程会损坏或杀死细胞。