It's so frustrating - the latest weight-loss fad works for your best friend, and not you.
But before you blame a weak willpower and that occasional chocolate and glass of wine, some new research could spare your conscience.
A study published in the journal Cell links the bacteria that live in the human gut - the microbiome - with how the body manages food.
"Our research suggests that each person's microbiome is driving food effects on the body," said Eran Elinav, of the Weizmann Institute of Science in Israel, and lead immunologist on the study.
Dr Elinav presented a sneak-peek of his new findings at 'Bugs, Bowels and Beyond', the 2015 National Scientific Conference of the Australian Society for Medical Research held in Adelaide this week.
He showed world-first evidence that the gut microbiome could be the reason you can't shake those extra kilos despite eating the same meals as your skinny pal.
The study flies in the face of accepted nutritional dogma that each of us metabolise foods in the same way.
"Almost every diet that you can find is based one way or another on grading systems, such as glycaemic index ratings on food," said Dr Elinav.
The glycaemic index provides an indication of the human blood sugar response to a certain food: lower is thought to be better. For example, the glcyaemic index of white rice is stated to be around 90, whereas quinoa is 53. Almonds are close to zero.
However, because each figure has been calculated based on an average response in a small group of people, it does not reflect how much variation occurs across entire populations.
"In our study of 800 people, we found that each reacts very differently to the same foods," explained Dr Elinav.
"This brings into question the whole concept of the 'one size fits all' diet."
In some individuals, foods that are broadly viewed as being healthy - such as sushi or tomatoes - in fact resulted in spikes in blood sugar levels. In other people, treats such as croissants and red wine resulted in minimal change.
High and persistent spikes in blood sugar levels over the course of many years are associated with the development of Type II diabetes.
Dr Elinav reported that for all 800 participants in the main part of the study, a personalised food response profile was created based on data from health questionnaires, body measurements, blood tests, glucose monitoring, stool samples, and a mobile-app (www.personalnutrition.org) used to report lifestyle and food intake.
This information was then used to create an algorithm that could be applied to the broad population to successfully predict blood sugar responses to foods.
"We validated the predictive algorithm with a new group of 100 people, and then tested it in a group of individuals most of whom were pre-diabetic," said Dr Elinav.
"We saw an overwhelming improvement in blood sugar levels with individually-tailored 'good' diets," he said.
Because the researchers had collected stool samples from the study participants, they were able to pin down blood sugar responses of individuals to foods as being closely linked with the profile of bacteria living in the bowel.
"In all of these studies the microbiome was a major and important factor in the algorithm," said Dr Elinav.
Associate Professor Geraint Rogers is Director of Microbiome Research at the South Australian Health and Medical Research Institute (SAHMRI), and was in the audience when Dr Elinav presented these new findings at 'Bugs, Bowels and Beyond' in Adelaide.
"Dr Elinav's research really demonstrates the power that can come from combining large data sets to generate new insights into health," said Professor Rogers.
"The results show that a precision medicine approach - whereby algorithms are used to show connections between lots of different factors - is applicable to the personalisation of preventative health, and not just for treating cancer and other diseases."
However he cautions against blanket application of this approach.
"It doesn't mean we should suddenly turn around and overindulge in food stuffs we have traditionally shied away from," Professor Rogers said.
"We need to think about nutrition too."
Professor Rogers also believes public health impacts should stem from data-intensive studies.
"Research shouldn't just be about creating tools that are only available to people who can pay for it," he said.
"We want to improve the health of individuals and of the whole care system."
Professor Rogers's microbiome expertise is applied across many themes at SAHMRI, including Infection and Immunity, Mind and Brain, Healthy Mothers, Babies and Children and Aboriginal Health.
"We have access to really good research platforms, including metatranscriptomics, metabolomics and proteomics," he said.
"SAHMRI is also working within a network across Australia for advanced bioinformatics, including data handling and data processing."
Professor Rogers has recently published a review describing the role of the lung microbiome in respiratory health.