The products are part of the growing market on which Britons will spend pounds 20m this year. But although nearly all of them are legal, some are banned under Olympic rules because they contain chemicals that produce steroids in the body. One product, made by the United States company EAS, has been banned from sale in the United Kingdom because it contains a chemical called yohimbe, which speeds the metabolism like an amphetamine.
Medical experts have raised doubts that the unbanned products would have any positive effect for users who are not athletes in training.
The Gladiators stars Hunter and Rhino appear in promotional literature for Maximuscle, a company based in north London that sells products it claims are "for advanced body builders". Some of them are banned under International Olympic Committee (IOC) rules.
In the US, EAS pays Theismann to appear in pamphlets for its range of food-derived products, trading on his huge reputation as a legendary - if ageing - quarterback.
Although it is legal to sell the products in the UK, some carry warnings that they should not be taken by people under 21 or those who have medical conditions.
Britons buy huge quantities of "food supplements" claimed to boost brainpower and muscle growth - although Olympic doctors say most of the products help only top athletes doing specialised training.
The new products, costing up to pounds 40 a packet, are aimed at the growing number of fitness-conscious people who exercise regularly but think that professional athletes eat or drink "secret formulas" that give them their edge.
Consumers are thus turning to powders or mixes promising ingredients such as "glutamine precursors", "cell-volumising components" and "nutrients which support the formation and function of excitatory neurotransmitters".
But doctors advising the British Olympic team say that even products such as creatine - a protein naturally found in meat that has been shown to have an effect on training - are useful in only a tiny number of situations. The others, they say, do not enhance athletic performance - and if they did, they would be banned by the IOC. None of them is.
Retailers of the products insist that ordinary people can benefit from using them. "Sports nutrition is an embryonic market which is highly specialised at the moment," said Arnold Ferrier, chief executive of The Sports Nutrition Company, which markets products made by EAS. "But there are possibilities for extending it beyond weight training, where people are looking for muscle development, and into the mainstream. Really, it's for anybody concerned about developing their physique who spends time at the gym or on runs."
EAS has a variety of products whose potential benefits are couched in a mixture of bewildering scientific language and careful caveats. Its Myoplex Lite line, for example, is described as containing "a proprietary protein blend" with "ion-exchanged whey protein, as well as soya protein isolates, which may support the metabolism as well as growth of lean muscle mass, especially during a low-calorie diet or intense training cycle".
The products are all concentrated or refined forms of chemicals that occur naturally in various foods: fatty acids, amino acids, proteins and trace metals such as selenium. The products do not have to pass toxicity tests because they are made from foods, and are not marketed as medicines or drugs. Thus EAS cannot make definite claims about their effects; that would call for expensive, and time-consuming, clinical trials.
Richard Budgett, director of medical services at the British Olympic Association, said: "For the vast majority of these products there is no evidence that using them makes athletes any better than a balanced diet tailored to their needs."
Creatine is an exception, he said, but for particular uses. "It has been shown to enhance recovery after repetitive sprints. If you do five or six 10-second sprints with less than one minute's rest in between each, you will be less fatigued when it comes to the last sprint. But it's that specific. For the vast majority of sports, it's irrelevant. I think a lot of athletes are using it under a misconception."Reuse content