Lostridium histolyticum ... one of the microbes related to obesity. Credit:Kevin Kearney A popular theory as to why these "modern" diseases exist is that people simply do not move as much as their hunter-gatherer ancestors did, thanks to desk jobs and a culture of convenience. But a recent study questioned how much a sedentary lifestyle is to blame when it comes to obesity. Researchers from the United States and Britain recently put the hunter-gatherer theory to the test by measuring the total daily energy expenditure of a tribe living in a way that closely resembles the lifestyle of our foraging ancestors. The Hadza tribe in northern Tanzania hunt and gather on foot without the use of modern tools such as guns; and divide duties by sex, with women foraging in groups and men hunting game and gathering honey. Contrary to what they expected, researchers found total energy expenditure among Hadza adults was similar to people living more sedentary lifestyles in Europe and the US.

"The similarity in metabolic rates across a broad range of cultures challenges current models of obesity suggesting that Western lifestyles lead to decreased energy expenditure," the study authors wrote in their report, published online at PLoS. "We hypothesise that human daily energy expenditure may be an evolved physiological trait largely independent of cultural differences." The lead-on from this seems to be that people are simply eating much more than the Hadza. But "a calorie is not a calorie, and calories in minus calories expended does not always equal 'stored as fat'," Andrew Holmes, an associate professor of biochemistry at Sydney University's school of molecular bioscience, says. He has the rather unglamorous job of examining human faeces to figure out what the human body does with food and the calories in it. To do that he looks at what is left once food passes out of the body, with a particular interest in the bacteria that is in faeces.

Professor Holmes believes how the gut individually responds to various nutrients in food has more to do with why people get fat than the amount of calories in it. If two people do the same amount of exercise and eat the exact same calories per day, does it matter which foods they get those calories from? Professor Holmes says it does. Just as everyone has individual fingerprints, he says people also have individual microbiota – the system of microbes in the body which includes bacteria, most of which live in the gut. "About 30 years ago – and this area has only really exploded in the last five years – we developed [the] ability to look at what microbes are present in a system," he said. "Until then, we tended to assume microbes didn't matter, even though they are the most abundant things out there. That assumption was hopelessly wrong." Because there are more bacterial cells in the body than human cells, the net activity of microbiota has an enormous impact on metabolism and health, Professor Holmes says. "The gut is the interface between what we eat and what we absorb.

"It is where food crosses walls and what goes into our system and what goes into our systems differs according the microbiota we each have." Studies have shown that obese people have a characteristically different microbiota to those who are lean, and faeces are made up largely of gut microbes. "So if you look at the poo of lean versus obese people, the calories and bacteria present in the poo are different in those who are obese," he says. Studies have found that obese people have microbes that cause them to absorb kilojoules and store them as fat much more efficiently than someone who is slim, he says, which means they absorb and use the calories from a piece of chocolate much differently to people of a healthy weight. So what is causing some people to have microbiota that more readily stores calories as fat and uses macronutrients less effectively? Professor Holmes believes it is a combination of what microbes people are exposed to through the environment, diet and time span. To better understand the influence of microbes, scientists conducted a study where they kept mice delivered by caesarean in a germ-free environment which meant they also grew up in a microbe-free environment. When scientists tried to make those mice fat through diet, they found it was extremely difficult to do so. Scientists then exposed some of the germ-free mice to microbes from fat mice and some to the microbes of lean mice. Those who had been given the microbes from fat mice gained a lot more weight. It did not matter if both groups ate the same types and amounts of food. Those mice with an abundance of the "wrong" microbiota got fat. Other studies have shown that microbiota may also be implicated in diseases such as cancer and mood disorders.

These results exemplify why health can be much more complicated than diet and exercise. The mice experiment shows children of obese parents may be disadvantaged from birth, if they either inherit the gut microbiota of their parent or are fed the wrong types of food leading to changes to their microbiota. "If you continually over-eat, then you select for a microbiota that are better at harvesting energy," Professor Holmes said. "If parents get stuck into hamburgers and chocolate they've adapted their microbiota to a poor one and then when they give birth they give those to their child." Professor Holmes is now exploring how microbiota can be changed and all the different components of diet that may have an impact on gut microbiota. To do this, he is working with Stephen Simpson, the director of the Charles Perkins Centre for obesity, diabetes and cardiovascular disease. It may seem to be an odd collaboration, since Professor Simpson's previous research has focused on winged insects – specifically locusts. But one of Professor Simpson's findings could offer an important insight into why people are driven to eat too much.

He discovered that locusts will form swarms and eat everything in sight until they have satiated their need for protein. Once they have consumed enough, the swarm stops. Humans too have an in-built threshold to the amount of protein that can be absorbed and Professor Simpson suspects, like locusts, people eat their way through foods – often the wrong types of foods – until a protein target is met. But it could be that some people have microbes that more efficiently allow the absorption of protein and, therefore, a feeling of satiety, which is why experts in seemingly unrelated fields are working together to figure out why people are eating the foods they do and what happens as it passes through the body. It is also why health is more complex than the public health messages and guidelines that are useful in making health messages easy to understand, but not so useful in explaining why sometimes exercise and diet alone just do not work as well for some people as for others. Andrew Hoy, from the Bosch institute for medical research at the University of Sydney, says easy to understand health messages are appreciated by the broader population, but this sometimes means that complex processes can become oversimplified. "I think that it is all really complicated and there are no easy answers because the easy answers have been tried and haven't cut through for everyone,"Dr Hoy, a researcher of lipid metabolism, says.

"If it were easy, all of us would be lean. But, at the end of the day, public health messages are coming from evidence-based conclusions and just because they might not be working for everyone it doesn't mean they're not an overall good approach." How it happens Bacteria in the gut have both health promoting and toxic properties. If the number of pathogenic properties grows too high, gut microbiota can be thrown out of balance. Loading

Sugars, flours and processed foods can produce an inflammatory response in the microbiota. The ration of toxic to healthy microbes leads to an altered microbiota that promotes, for example, the extraction of more calories and sugar from food and can trigger an insulin response. This can lead to the development of diabetes, obesity and insulin resistance.