People following the Atkins diet should not gorge themselves on fatty foods, says the company marketing it.
The diet is followed by millions
Original advice suggested that foods such as butter, meat, cheese and eggs could be eaten "liberally".
Now, however, Atkins Nutritionals suggests that intake of saturated fats should be limited to 20% of total calories.
The change of tack will come as a relief to nutrition experts who say that a high-fat diet poses a health risk to some.
In the UK, the website of the Food Standards Agency expressed concern about the makeup of the diet, which heavily restricts certain food groups such as carbohydrates - for example bread and pasta.
High levels of protein intake might also pose a threat to patients with certain medical conditions, it is claimed.
However, millions in the UK have tried the diet, and many say it has helped them lose weight with minimal or no unpleasant side-effects.
They say that the emphasis on fresh food - and bans on high-sugar processed food - has in itself led to healthier eating.
Collette Heimowitz, director of research and education for Atkins Nutritionals, said that the new advice was intended to clarify the correct approach for dieters.
Collette Heimowitz: "Healthy balance"
She said: "The media and opponents of Atkins often sensationalise and simplify the diet as the all-the-steak-you-can-eat diet. This has never been true."
A spokesman for the UK arm of the company said: "The whole idea is to reduce your carbohydrate, but it is not all about red meat.
"Not all fats are bad for you, but like anything, they should be taken in moderation."
However, a spokesman for the US arm of the company said that the advice did not represent a change of tack.
She said: "Saturated fat remains a valuable part of
the Atkins nutritional approach.
"And there is absolutely no science to support any claims that eating red meat and saturated fat as part of your Atkins programme is anything other then beneficial."
A spokesman for the Food Standards Agency said: "Dieters should not be cutting any food group from their diet - it is about maintaining a balance - that is the key to a healthy diet.
"That means fruit and veg and bread and cereals, and less fats, salt and sugar."
The agency recommends that 11% of calorie intake should come from fats.