A recent article in Science News, Personalized diets may be the future of nutrition. But the science isn't all there yet, got me thinking about obesity and diabetes. These diseases are often, but not always, linked.

Obesity is a worldwide epidemic affecting about one-quarter of the world's population. Obesity is associated with a wide range of chronic diseases, including cardiovascular disease, cancer, and diabetes. Diabetes is the leading cause of death worldwide, affecting 422 million people. In the US, in 2012, the total estimated cost of diabetes in the United States was $245 billion.

The "calories in-calories out" theory of obesity, the gospel in the weight loss literature for many decades, is wildly inaccurate and over-simplistic. The Science News article focuses primarily on the wide variability of physiologic responses of individuals to the same food. The piece also covers many other inconsistencies in the obesity literature.

Historically, we talk about someone being "blessed with good genes," those that could eat anything and not gain weight. However, genetic make-up accounts for less than half of the variability seen in obesity studies. What's more, in a recent study, the macronutrient content of food — the number of carbs, fat, and protein-accounted for less than a third of the variability. The remaining 20-30% is still a mystery. Potential factors include:

• the type and combinations of food you eat
• the time you eat
• how much stress you're under
• how much you exercise
• the integrity of your gut barrier
• the amount of inflammation in your body
• your environmental exposures
• your hormones, etc.

One area, in particular, seems to be integral: the microbiome. We don’t yet understand how or to what extent.

The original pre-clinical studies of the microbiome focused on the development of obesity. Turnbaugh in his classic paper found that one could induce obesity in skinny mice by transferring the gut flora (through feces) of fat mice. The skinny mice gained significant weight even with reduced food intake.

Other studies have demonstrated early exposure to antibiotics changes the composition of the gut flora and is associated with several chronic health conditions, including obesity. In a previous entry, I mentioned how antibiotics are administered to animals in industrial farming to stimulate "growth promotion."

The microbiome plays a role in obesity; we don't understand the mechanism(s) of action or their relative importance yet. The impact of gut flora on obesity has many potential avenues, including:

• the efficiency of calorie harvesting from food
• the maintenance of gut barrier function
• the production of substances such as SCFA
• the influence on hormone production
• the impact on systemic inflammation
• other?

Recent studies lay the groundwork for the future use of the microbiome as a therapeutic option for obesity. As an example, scientists demonstrated, in non-diabetics, the presence or absence of particular strains of bacteria in the gut was a more accurate predictor of human glycemic response than the amount of carbohydrate in ingested food.

We are in the infancy of understanding the mechanism of action the microbiome has on human diseases. As our understanding evolves, personalized diets, diets targeted to the unique genetics and microbiome of an individual, will play an expanding role in the treatment of obesity (and other disease). We do not know enough yet. But the time for personalized nutrition is not too far away.