For years, there has been heavy debate as to whether sugar or fat is the culprit in the obesity epidemic. In the 1970s, this debate reached its peak and fat was pinned as the cause of our general decline in metabolic health. However more recently, sugar has come back into the limelight, with links to obesity, type II diabetes incidence and fatty liver disease.

With a general failure in halting the obesity and diabetes epidemics under the current guidelines and the lack of evidence that low-fat diets improve obesity, cardiovascular risk and other general health problems1-18, it is now more important than ever to take a good look at the research done on sugar and fat to understand how these wee molecules affect our health.

What’s the story with fats?

For years, fats have been the bad guy, with many products boasting fat free or low fat alternatives. But how did this all get started? What scientific evidence is there to really substantiate the anti-fat argument? In short it comes down to this – the type of fat matters. Let me elaborate a little.

The links between fat intake and metabolic disease are more to do with the “bad fats”, such as trans fats found in processed food, biscuits, salad dressings…and the list goes on. In particular, trans fats have been strongly linked to coronary heart disease9,19-22. The jury isn’t quite out on saturated fats. Some studies suggest, that saturated fats significantly worsen insulin resistance10,23-29 and increase blood pressure10,30,31, while others indicate that some forms of saturated fat (in particular those derived from plants) are not as detrimental to our health9,32.

“Good fats” (monunsaturated and polyunsaturated) are found in foods such as seafood, avocado, nuts and olive oil. These have been shown to reduce low density lipoprotein (LDL), sustain high density lipoprotein (HDL) and improve insulin sensitivity9,10,12,32-37. Some research does suggest that there isn’t a link between these ‘good fats’ and cholesterol levels, however most of these studies only sampled a small number of people for a short period of time10,38-42.

How about low fat diets? Overwhelmingly, the majority of research suggests that there is no benefit of a low fat diet vs. a high fat diet for weight loss1-18. Furthermore, there is little evidence to suggest adverse affects of a general high fat diet10,43-45. Interesting stuff! 

What’s the story with sugars?

There is a lot of evidence to suggest that consuming too much sugar can have a negative effect on our metabolic health. Regular sugar consumption produces a constant release of the hormone insulin. If this continues, it can lead to serious problems, such as the synthesis of tryglycerides46-50, insulin resistance51,52, fatty liver disease53,54, type II diabetes55-59, an increase in very low density lipoprotein (the bad kind of cholesterol) and the accumulation of fat on all tissues49,50,60-63. Studies have also shown that our brains respond differently to fructose containing sugars than to glucose containing sugars49,64.

Some research however does cast sugar in a sweeter light. Several studies conclude that there is no clear evidence that added sugar is worse than any other source of calories for the development of diabetes or obesity65-69. However, many of these studies were either industry funded or had a strong financial interest to maintain high levels of sugar consumption49, so should be taken with a grain of salt!

So how about low sugar diets? Much of the research suggests that this way of eating is beneficial for reducing blood glucose levels50,70-78, weight loss and serum triglyverides50,62,79-81, often more so than low fat or low calorie diets50,78,82-84.

The verdict

All in all, most of the current evidence suggests that unsaturated fats are good for our general health, but that those cheeky trans fats should be avoided! The jury isn’t quite out on saturated fats. High-sugar consumption can also have a negative impact on our health by increasing the risk of type II diabetes, fatty liver disease and obesity. However there is a lot more work required to tease out the differences between various fat and sugar content diets. Remember, the more we understand about the types of foods we’re eating, the better quipped we will be to make awesome food choices!*

(*Amended 19/01/2016)



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