Loren Cordain, a professor at Colorado State University and the author of “The Paleo Diet,” links the movement to a 1985 New England Journal of Medicine article, which proclaimed that the "diet of our remote ancestors may be a reference standard for modern human nutrition."

Another source of paleo converts is CrossFit, a fitness program known for grueling workouts combining weightlifting and gymnastics. CrossFit trainers, who teach at more than 1,200 gyms and other affiliates across the country, generally encourage clients to follow either a caveman diet or the Zone diet, which requires tracking calories. "Some of the gyms have hardcore paleo folks, and if you're a member of that gym then you're paleo, while other gyms are hardcore Zone," said Anthony Budding, who manages the content on CrossFit.com.

Experts in early humans dispute some of the tenets of latter-day paleos, including the belief that fasting is beneficial and that the body is unequipped to handle an agriculture-based diet.

Still, there is a "sharp contrast" between the strength and fitness of our distant ancestors and us, said Clark Larsen, a physical anthropologist at Ohio State University. "The male or female of 12,000 to 15,000 years ago will be considerably stronger and in better shape," he said. Unfortunately, life was short: If you made it to age 30 or so, you had done well.

But, it turns out, a small group of researchers has studied the question of exercise performance and time of day, even doing studies of heart rates. And not only are performances better in the late afternoon and early evening, but, contrary to what exercise physiologists would predict, heart rates are also higher for the same effort.

One recent study, by the late Thomas Reilly and his colleagues at the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University in England, found that people's maximum heart rates and sub-maximal heart rates were lower in the morning but that their perception of how hard they were working was the same in the morning as it was later in the day. Dr. Reilly and his colleague Jim Waterhouse, in a review published this year, also noted that athletes' best performances, including world records, were typically set in the late afternoon or early evening.

Greg Atkinson, also at Liverpool John Moores University, said that some researchers, noticing that heart rates during exercise were lower in the morning, reasoned the way I did -- that people must be more efficient in the morning. It would mean that exercise was easier in the morning. Of course, it seemed harder to me, but I could have been deluding myself. Not really, Dr. Atkinson said. It actually is harder to exercise in the morning.

"Most components (strength, power, speed) of athletic performance are worst in the early hours of the morning," he wrote in an e-mail message. “Ratings of perceived exertion during exercise have generally been found to be highest in the early morning." If you exercise later in the day, your muscles are more flexible and stronger and your heart and lungs are more efficient, said Michael H. Smolensky, an expert in chronobiology, the study of the body clock.

Tyler Florence is among the chefs who told me they had massively cut down on their meat intake. Even Michael Lomonaco, the chef of the Manhattan steak joint Porter House New York who recently knocked off 10 lb., eats a lot of simply cooked proteins surrounded by vegetables. "I do a slow-roasted salmon--there's no oil, no added fats--roasted on cedarwood in an oven. It's served with big white beans, fresh tomatoes and tarragon," he says. "What a great steak house this is: the chef is telling us to eat salmon."

But Jacques Torres, the New York City--based chocolatier, still eats his chocolate. Through Weight Watchers, he knocked off 20 lb. and then another 12 lb. in September in a charity competition for chefs that was sponsored by the weight-loss program. He stocks up on 70%-cocoa chocolate bars, with the goal of always having a low-sugar option on hand. Because when a craving hits, not even a Weight Watchers--trained, insanely talented pastry chef with a refined dessert palate can get in its way. "Last Sunday I was craving so much for sweets, I went to buy a cheesecake." Even worse, he confesses, "I bought it at the supermarket. I was in Jersey."

When more than 40 former contestants from The Biggest Loser gather Wednesday for a reunion television special, the winner of the program's first season, Ryan C. Benson, who lost 122 of his 330-pound starting weight, will be absent. Mr. Benson is now back above 300 pounds but he thinks he has been shunned by the show because he publicly admitted that he dropped some of the weight by fasting and dehydrating himself to the point that he was urinating blood.

*****

The series also highlights the difference between the pursuit of engaging television and the sometimes frenzied efforts of contestants to win, perhaps at the risk of their own health. Doctors, nutritionists and physiologists not affiliated with The Biggest Loser express doubt about the program's regimen of severe caloric restriction and up to six hours a day of strenuous exercise, which cause contestants to sometimes lose more than 15 pounds a week.

At least one other contestant has confessed to using dangerous weight-loss techniques, including self-induced dehydration. On the first episode of the current season, two contestants were sent to the hospital, one by airlift after collapsing from heat stroke during a one-mile race.

New contestants are entering the show more out of shape. Each of the last two seasons has broken the record for the heaviest contestant ever, at 454 and 476 pounds. Medical professionals generally advise against losing more than about two pounds a week. Rapid weight loss can cause many medical problems, including a weakening of the heart muscle, irregular heartbeat and dangerous reductions in potassium and electrolytes. "I'm waiting for the first person to have a heart attack," said Dr. Charles Burant, a professor of internal medicine at the University of Michigan Health System director of the Michigan Metabolomics and Obesity Center.

I wanted to win so bad that the last ten days before the final weigh--in I didn't eat one piece of solid food! If you've heard of "The Master Cleanse" that's what I did. Its basically drinking lemonade made with water, fresh squeezed lemon juice, pure maple syrup, and cayenne pepper. The rules of the show said we couldn't use any weight-loss drugs, well I didn't take any drugs, I just starved myself! Twenty-four hours before the final weigh-in I stopped putting ANYTHING in my body, liquid or solid, then I started using some old high school wrestling tricks. I wore a rubber suit while jogging on the treadmill, and then spent a lot of time in the steam room. In the final 24 hours I probably dropped 10-13 lbs in just pure water weight. By the time of the final weigh-in I was peeing blood.

"Our brains require 0.1 calories every minute simply to survive," he said.

"When we do something challenging such as a puzzle or a quiz we burn through 1.5 calories every minute."

The brain is made up of millions of neurons which communicate with other neurons, transmitting messages to the body.

Neurons produce chemicals called neurotransmitters to relay their signals. These neurons extract 75 per cent of the sugar glucose, available calories and 20 per cent of the oxygen from the blood to create these neurotransmitters.

Politicians and industry experts described the practice as "shocking" and said it would put further pressure on parents who wanted their babies to be perfect. Magazines have been heavily criticised in the past for airbrushing images of women to make them look thinner – a practice which eating disorder campaigners claim can help push impressionable young girls into becoming anorexic and bulimic. But this is the first time the industry has admitted using the technique to alter images of babies.

Practical Parenting and Pregnancy, a monthly magazine, has said it has retouched photographs of babies to "put them across in the best light". The practice came to light in a BBC documentary, My Supermodel Baby. In footage of a photo shoot for the magazine, the casting director explained how the photograph of baby model Hadley Corbett, five months, was airbrushed: "We lightened his eyes and his general skin tone, smoothed out any blotches and the creases on his arms," he said. "But we want it to look natural."

Daniella Delaney, the editor of the magazine which sells nearly 40,000 copies, told The Sunday Telegraph that photographs were airbrushed but that it was kept to a minimum. "We'll remove things and even-up skin tone, that sort of thing. But very little is done, in fact, because obviously babies are beautiful the way they are and that is what we went to get across."

*****

"Babies are not like adults you can't stop them from dribbling, so you might remove that bit of dribble from the chin. Or if the baby has just been crying, and their eyes are red, we might lighten the eyes. Or if they have just woken up because they have had a nap on the way in and we photograph them, we might remove a little bit of sleep. It is just those kind of things, very little really."