This finding alone meant another triumph for Britain's most famous do- it-yourself lawyers, who took on the might of the giant fast food chain after it accused them of libel.
Supporters of the environmental campaigners, Dave Morris and Helen Steel, celebrated as the pair overturned some of the High Court rulings in their mammoth legal battle with McDonald's.
Lords Justices Pill, May and Keane handed down a 309-page judgment, which ruled that it was fair comment to say McDonald's employees worldwide "do badly in terms of pay and conditions".
They also said it was true that "if one eats enough McDonald's food, one's diet may well become high in fat etc, with the very real risk of heart disease".
The judges said their findings "must have a serious effect on their trading reputation since it goes to the very business in which they are engaged.
"In our judgment it must have a greater impact on the respondent's [McDonald's] reputation than any other of the charges that the trial judge had found to be true," they added.
The judges said they had "considerable sympathy" with the pair's argument that the leaflet meant "that there is a respectable (not cranky) body of medical opinion which links a junk food diet with a risk of cancer and heart disease".
But they said the allegations about cancer were not justified. They added that there was no truth in the charge about food poisoning and this was "especially serious" for a company in the restaurant business.
The judges also upheld that McDonald's was not responsible for starvation in the Third World or for the destruction of the rainforests and these allegations in the leaflet were "very harmful to a company's reputation".
The judgment says: "Nutrition and health risks always was and was bound to be an important element in the case, given McDonald's business.
"It is therefore highly significant that the allegation about the risk of heart disease has been justified.
"Moreover, even though the appellants failed to justify fully all the defamatory statements about the risks of cancer, it is well established that such facts as they did establish when seeking to prove justification of those statements may be relied upon in mitigation of damages."
They reduced McDonald's pounds 60,000 damages awarded in the High Court against Ms Steel and Mr Morris to pounds 40,000.
Mr Morris, 44, a former postman, and Ms Steel, 33, a former gardener, were ordered in June 1997 to pay the libel damages after a 314-day trial - the longest in English legal history - which is reckoned to have cost McDonald's pounds 10m.
After the trial, Mr Justice Bell ruled that the company had been libelled by most of the allegations in a London Greenpeace campaign leaflet entitled "What's Wrong with McDonald's?" But he found that the leaflet was true when it accused McDonald's of paying low wages to its workers, being responsible for cruelty to some of the animals used in its food products and exploiting children in advertising campaigns.
McDonald's attempt to suppress the leaflet resulted in more than 3 million being handed out in the UK alone as publicity over what became known as the McLibel Trial increased.
A website highlighting the case and containing more than 20,000 files about the trial and McDonald's has been accessed more than 65 million times.
Richard Rampton QC, for McDonald's, told the judges that his client would not be taking the matter to the House of Lords.
During the appeal hearing, Mr Morris told the court that McDonald's had never applied for its damages or costs or for an injunction banning further publication of the leaflet.
The use of libel laws by multinational corporations was a form of censorship, he said.
After the appeal hearing, Mr Morris said: "This is a complete vindication of our decision to appeal in that two further areas of McDonald's core business practices have effectively been slammed by the court, on top of those findings in our favour by Mr Justice Bell."