Carbs earn their place at the table

I’m disgusting to be with at a sandwich lunch. I peel apart the focaccia, pick at the filling for scraps of protein to assuage my hunger and then discard a plate piled high with unwanted carbs. All in the cause of a flatter abdomen and better health.

This behaviour is part of a backlash.

In the 1980s when public health researchers demonised fat, they didn’t foresee that we’d trade fats for unlimited sugar and fructose and bowls of white rice or pasta. The result was bigger bellies and ultimately type 2 diabetes.

The pendulum then swung back to reducing carbohydrate intake and boosting protein – protein is good at satiating appetite while helping to shed weight. Cheerleaders for this strategy included US doctor Robert Atkins and more recently US science writer Gary Taubes (author of Good Calories, Bad Calories).

In effect we have been subjected to huge changes in our diets for decades, with poor evidence to support the reason for the changes.

But over the last year or so, a series of studies led by Steven Simpson, director of the Charles Perkins Centre at the University of Sydney, has questioned our see-sawing dietary fads. These studies could bring carbs – or at least some types – back into favour. Their work has been published in respected journals such as Cell Metabolism and Cell Reports.

The Sydney researchers designed 25 lifelong diets for mice, which varied their ratios of fat, protein and corn-based carbs. The mice were free to eat as much of their assigned food as they liked. Some of the diets packed more calories per gram than others, so their total calorie intake varied.

What the Sydney scientists found was surprising. How long the mice lived was not connected to their calorie intake, but to the ratio of protein to carbs consumed.

The longest-lived mice in the best metabolic health – their heart doctors would have been happy with their blood lipid profiles, sugar, and insulin levels – were the ones eating more carbohydrates and less fat and protein. A low ratio of protein to carbs appeared to be key. The shortest-lived mice were either on a low protein, low carb, high fat diet or a high protein, high fat, low-carb diet, similar to the original Atkins diet. The mice on the Atkins-type regime were leaner, but the plumper, higher carb eating mice lived 30% longer.

The best diet balance of them all turned out to be what the mice selected for themselves – namely around 55% of calories as carbohydrates, around 23% as protein and 22% as fat. That happens to be pretty close to what we call the Mediterranean diet, which many studies have found to be associated with lower rates of type 2 diabetes and heart disease and a longer life expectancy.

So how does this ideal macronutrient ratio compare to the effects of calorie restriction? Many animal studies show that if you restrict calories by 30-40%, the reward is a 30% increase in lifespan. That’s almost impossible to test in humans, although it is known that reducing calorie intake improves our cardio-metabolic health.

The Sydney group then decided to compare, head to head, the higher carbohydrate, lower fat and protein diet to calorie restriction, again in mice, and to test their longevity and metabolic health.

The results showed that the free eating, higher carb mice lived as long as those on 40% calorie restriction, and were presumably happier too, even if they were a bit chunkier.

The carbohydrates in the diet, by the way, were not highly processed and contained little or no sugar, and the fat was low by human standards at 20%.

Now these were mice, not humans, so there’s a limit to how far we can extrapolate. But the findings do suggest a high, relatively unprocessed carb diet is healthy.

The explanation? Simpson believes the ratio of protein to carbs is sensed by two yin-yang metabolic pathways known as MTOR and AMPK. At high protein ratios, the pathways optimise growth and reproduction at the expense of self-maintenance. At low protein ratios, the pathways flip. Complex carbs may also exert a benefit by maintaining a healthy microbial community in the lower bowel.

So have these findings made me a more pleasant companion at a sandwich lunch?

Not yet. But that’s more about my inability to stop myself going back to the table for seconds and thirds.