Carbohydrates have become a feared food group, to the extent that fruit avoidance is not uncommon. Is this justified? Can science help us make sense of it?
An interesting study was published in the American Journal of Physiology this month. The study compared effects of feeding female rats fructose or glucose, in addition to their usual diet. Fructose or glucose were dissolved in water and fed to rats. Rats fed fructose ate more, gained more weight, had higher plasma triglycerides and were on their way to developing fatty livers. In addition, their blood vessels became stiffer and less responsive to signals that induce relaxation - not a good thing! If, at this point, you are questioning your fruit intake please hold on. There is so much more to this story.
Another interesting contribution to the debate came from the “PURE" study. Data from this powerful study, involving 150 000 people from 17 different countries, clearly showed that diets high in carbohydrates were associated with higher cardiovascular risk. Risk really took off when percentage of carbohydrate in diet reached 50% or more. Diets higher in fat offered a degree of protection, particularly if they were rich in monounsaturated fats such as olive oil.
Things get even more interesting when data from the same study were used to tease out the effects of particular components of the diet. It turns out the fruit consumption is protective, vegetable and meat consumption is neither harmful nor protective. Eating 4 pieces of fruit of any kind appears to offer significant reduction in cardiovascular risk.
So, what is going on? On one hand, studies are telling us that carbohydrates and fructose in particular, are harmful, on the other, it appears that eating fruit is in fact beneficial. How do we reconcile this?
Let’s look at the study I mentioned in the beginning - the one where rats were fed a solution of either fructose or glucose. The rats received acellular sugar that is not bound inside plant cells. Eating acellular sugars can have serious metabolic consequences because gut bacteria are fed large amounts of sugar that is completely available to them. This is known to alter the balance of bacterial species in the gut leading to gut permeability and systemic inflammation which then contributes to development of obesity (this is a whole new topic).
What happens when carbohydrate intake is high, but it comes from whole plants? Kitavan Islanders of Melanesia are horticulturalists, eating a traditional diet that is up to 70% carbohydrate, originating from fruit and tubers. They also have high saturated fat intake. And yet, Kitavan Islanders have very low fasting insulin and blood glucose. Diabetes, obesity and atherosclerosis are virtually unknown. This is not explained by genetics - when individuals from these societies are exposed to Western diet they very quickly develop Western health issues.
Bottom line - it is the kind of carbohydrate, not the amount of carbohydrate that matters. When eaten as a part of a whole plant, carbohydrates are released very slowly and are much less available to the gut bacteria. This is what our gut microbiome has evolved with. In contrast, acellular carbohydrates, consumed in form of flours and sugars, deliver a damaging hit of highly concentrated and easily available carbohydrate. Fructose, delivered in the form of high fructose corn syrup or apple juice concentrate, can be particularly problematic and lead to increase in plasma triglycerides, development of fatty liver and weight gain.
Out of all of this, take home messages are:
- Carbohydrates are OK - as long as they come from whole plants
- Fruit is great as long as it eaten fresh and whole
- Read your labels - do not eat anything that is sweetened with various forms of acellular sugars. Examples include high fructose corn syrup, apple juice concentrate and glucose syrup