Tests revealed that their blood had a higher level of chemicals that cause an inflammatory response that can damage the brain.
Gum disease affects 80 per cent of Britons aged 55 and over. Around half of those aged 65-plus have also lost more than ten teeth - a sign of severe gum disease. Mark Ide, of King's College London, said treating gum disease in those who already have Alzheimer's might slow their decline.
And regular tooth-brushing may help prevent the disease in healthy adults. But he said 'most people are rubbish' at cleaning their teeth.
"We think it is better to do it really thoroughly once a day, having been instructed how by your dentist, than do it badly once a day," he said.
Previous research found that those who brushed their teeth once a day are 65 per cent more likely to develop dementia than those who brushed three times a day.
Dr Doug Brown, of the Alzheimer's Society, said: "This study adds evidence to the idea that gum disease could potentially be a contributing factor to Alzheimer's but we would need to see clinical trials to provide more solid evidence."