Should Carbohydrate Tolerance Determine Our Diet?

Forty-five years ago, the American nutritionist Ancel Keys launched the Seven Countries Study, the world’s first multi-country epidemiological investigation into the relationship...
14 March 2017


Forty-five years ago, the American nutritionist Ancel Keys launched the Seven Countries Study, the world’s first multi-country epidemiological investigation into the relationship between diet and heart disease. Keys had long been convinced that the high amounts of saturated fat in milk and meat were behind the relative high incidences of cardiovascular problems in the US, compared to Mediterranean countries.

His study concluded that saturated fat led to increased levels of cholesterol, believed to be the main dietary cause of heart disease, and, from 1977, this led to a drastic change in medical advice. Banishing fat from your diet was billed as the way to a healthier future. But, over the past three decades, the ‘war on fat’ has coincided with an increase in obesity and type 2 diabetes rates across the western world.

Replacing fat with sugar
Was Keys wrong? Some scientists have suggested that the main problem with his advice was that people simply replaced fat in their diet with refined carbohydrates and highly processed low fat products with a large sugar content. They believe that obesity and type 2 diabetes are hormonal regulatory disorders relating to insulin, the hormone which helps the body use or store the glucose it absorbs from food. Toronto-based clinician Jason Fung said that rather than fat being the enemy, he believes the human body is ill-equipped to handle modern diets with a high-carbohydrate intake throughout the day. This pushes the body to a state where it becomes resistant to its own insulin, because the levels of this hormone are constantly high.

“We know there’s an epidemic of insulin resistance out there,” says Jay Wortman, a clinician from Northern Alberta. “And it could be that in recent human history, dietary changes have occurred far too rapidly for our bodies to adapt.”

With our ever-growing reliance on processed foods, the consumption of one particular carbohydrate molecule has increased remarkably over the past few decades and the work of several research groups suggest it could be uniquely bad in driving insulin resistance.

“People have always consumed fructose,” Wortman says. “It’s the molecule which gives sugar its sweetness. But while there’s probably a safe level of fructose you can have, once you exceed that, the damage starts to accrue.”   

In one study, scientists from Professor Peter Havel’s group in the Department of Molecular Biosciences at UC Davis found they were able to induce an insulin resistant state and many features of metabolic syndrome such as dyslipidemia, in rhesus monkeys, simply by increasing fructose intake over a short period of time. A subset of the monkeys developed type 2 diabetes, a condition caused by an excessive insulin demand, which overloads and ultimately wipes out the beta cells in the pancreas that make this vital hormone.

It can take 12 years of being in an insulin-resistant state to develop type 2 diabetes. Insulin resistance is a progressive condition which induces weight gain, high blood pressure and other metabolic syndromes such as dyslipidemia (deranged fat levels in the bloodstream) and fatty liver, a condition where excessive amounts of fat accumulate in the liver cells. Through insulin resistance, chronic conditions like asthma are believed to occur at a greater rate. Oxidative stress levels also increase, and while a causal relationship between oxidative stress and mental health problems has not been confirmed yet, a number of studies have implicated oxidative stress in several psychiatric conditions, including depression, anxiety disorders, schizophrenia and bipolar disorder.

“We need to try and detect insulin resistance early on,” Wortman says. “There’s been rodent studies which show you can alter the course if you change your diet. We don’t have proof in humans yet but we believe this is the case.”

At present, detection of diabetes and pre-diabetes is done via a glucose tolerance test which compares your blood sugar levels with pre-specified international cut-offs. Surveys in Australia have found that one in four individuals over 25 either have diabetes or are at risk and some believe the real figure could be even higher. 

“Insulin resistance usually flies under the radar for a long time because the glucose tests only measures blood sugar, not insulin levels,” Wortman says. "Insulin resistance usually flies under the radar for a long time because the glucose test only measures blood sugar, not insulin levels,” cautions Wortman. “You’re able to maintain normal blood sugar for some time by hypersecreting insulin, which turns sugar into fat, so unless you check both together you may not realise there’s a chronic problem developing."

Some researchers believe that, to tackle insulin resistance, we need a greater understanding of our own carbohydrate tolerances.  The American Medical Association recommends a minimum carbohydrate intake of 130g a day but, in reality, most of us will consume 250-300g or more. How our bodies respond is down to a combination of genes and environmental effects. Some people can regularly consume a far greater carbohydrate intake with no ill-effects while others will quickly experience symptoms of metabolic disturbance such as weight gain.

The genetic element behind our food tolerance levels has inspired a fad of personalized diets over the past decade but with little hard evidence behind them. “Genetics and other factors do influence an individual’s ability to metabolise carbohydrates, but there are currently no specific genetic tests that can predict this,” says Ahmed El-Sohemy, a researcher at the University of Toronto who wrote a paper criticising the popular "eat right for your blood type" diet. “Any claims to the contrary are not likely to be grounded in scientific evidence.”

Family history
Instead, family history may provide more substantial clues regarding your own susceptibility to insulin resistance. If you have relatives who have suffered from type 2 diabetes or fatty liver then it is probably inadvisable to be exceeding 250g/day of carbohydrates on a regular basis. For those who have already developed insulin resistance, some scientists believe that a low carbohydrate diet is the way forward, but not all are convinced the story is quite so straightforward.

“It’s overly simplistic to suggest that insulin resistance can solved just by not eating carbs,” says Joanna McMillan, who has researched nutrition at the University of Sydney. “Insulin resistance can be caused by fat deposits in cells and there is some evidence that high saturated fat intakes increase insulin resistance. To me what is certainly clear is that replacing saturated fats with high carbohydrate, particularly refined carbohydrate, does not reduce cardiovascular disease risk and may make matters worse. But eliminating carbohydrate completely and eating a high fat diet may have health consequences as we don’t fully know the impact of different fats on brain function, cancer risk, body fat deposition and so on.”

Professor Jennie Brand-Miller, from the Charles Perkin Centre at the University of Sydney, is wary of dietary guidelines shifting towards a new war on carbohydrate as she believes a certain amount of carbohydrate is still vital for body health.

“If you completely avoid carbohydrate for too long you risk becoming increasingly carb-intolerant and your body will struggle to process it in future,” she says. “The brain is a greedy organ and glucose is its main source of fuel. It will use alternative fuels when it has to, but there’s evidence that it doesn’t work as well and makes inappropriate judgements when glucose intake is too low. People often complain of feeling aggressive and irritable when glucose-deprived.”  

As we grow older, the body’s metabolic machinery does wear out and some believe that we should adapt by perhaps aiming to get closer to the minimum limit of 130g of carbohydrate a day. However Brand-Miller believes it is wrong to focus attention entirely on this particular dietary element. 

“What the last 40 years has shown us is that we shouldn’t just concentrate on eliminating one macronutrient. You have to be careful about your protein and fat choices too as you get older. You can eat good carbs, good fats and good proteins while large amounts of highly refined carbohydrate and saturated fats are not so good. Any meal disturbs our body’s homeostatic balancing mechanism, and bringing things back to equilibrium requires a symphony of different hormones. So it’s all a juggling act between fats, protein and carbohydrate and their quality to get the best ways of re-establishing that equilibrium without doing harm to the body.”


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