Metabolic syndrome and diabetes: how much blame does sugar deserve?

• NHMRC dietary guidelines recommend limiting intake of added sugars, but do not specify an amount.¹ WHO guidelines say added sugars should make up no more than 10% of a person's total energy intake.²
  
• While the evidence is weak that added sugar directly contributes to diabetes and heart disease, there is evidence that it is associated with weight gain and dental decay.  Added sugars can increase the energy content of a diet while diluting its nutrient density.¹
  
• Added fructose may be metabolised differently to other forms of sugar, particularly when combined with glucose, and may play a larger role in the development of metabolic syndrome and type 2 diabetes,³ but evidence is still emerging.
  

Last year, a review by cardiovascular researchers James DiNicolantonio and Sean Lucan in BMJ's Open Heart made the case that sugar could be a contributing factor in hypertension and cardiometabolic disease.⁴ Now the same researchers have proposed that added fructose is a driver of type 2 diabetes in another review published in Mayo Clinic Proceedings.³

They argue that added fructose, in particular, impedes metabolism of glucose in multiple ways.³ For example, they cite animal and human studies published in the early 1970s that show concentrated fructose could decrease adenosine triphosphate in the liver, potentially contributing to less cellular binding of insulin, a possible reduction of insulin receptors and insulin resistance.³

Although a growing body of research is looking into the role sugar plays in metabolic syndrome, diabetes5,6 and cardiovascular disease,⁷ many of the studies have substantial limitations.

Small, short-term studies have provided some additional evidence of a link between sweetened beverages containing fructose and metabolic risk⁸ and cardiovascular risk factors.⁷ A 2010 meta-analysis of prospective cohort studies with more than 300,000 participants also found that those who consumed the most sugar-sweetened beverages had 26% greater risk of developing type 2 diabetes than those who consumed the least – but there was significant heterogeneity between studies and the researchers only compared people who consumed the highest amounts with the lowest, with no standardised categories.⁹ A 2013 analysis of cross-sectional data found that every 150 kilocalorie increase in sugar per person per day was associated with 1.1% increased prevalence of diabetes.¹⁰

A meta-analysis of 10 trials in a systematic review commissioned by the WHO found that increased sugar intake (mostly sugar-sweetened beverages) was associated with increased weight.² The quality of available evidence for impact of changes to sugar intake on body weight was moderate in both adults and children.²

Like all previous versions, the 2013 NHMRC guidelines¹ recommend limiting added sugar. While noting recent research linking consumption of sugar-sweetened drinks to increased risk of type 2 diabetes, metabolic syndrome and weight gain, the NHMRC says there are insufficient studies to determine the relationship between sugar intake and type 2 diabetes.¹

An extensive review by the NHMRC found:

• No new evidence that sugars cause or moderate the development of cardiovascular risk factors.¹
  
• Reduced energy intake and body weight are likely to be responsible for earlier findings which showed that lowering dietary sucrose could lower triglycerides.¹
  
• The association between sugar and increased risk of weight gain, dental caries and reduced bone strength was sufficient to warrant recommendations to restrict added sugar.¹
  
• Insufficient evidence to recommend an appropriate level of intake of added sugar on a population level.
  

However, the World Health Organization (WHO) recommends a sugar intake of less than 10% of total daily calories,² far less than most Australians consume.¹ In guidelines released in March 2015,² the WHO also focused on evidence linking added sugar to weight gain and dental caries, rather than diabetes or cardiovascular disease.

Although there is insufficient evidence that sugar contributes directly to cardiovascular disease, metabolic syndrome and diabetes, the World Health Organization and National Health and Medical Research Council recommend restricting and/or reducing sugar because of its association with weight gain, dental decay and reduced bone strength.

Honey, sucrose, agave nectar and high fructose corn syrup all contain high amounts of fructose. Sucrose, or table sugar, contains a 50/50 mix of glucose/fructose, while high fructose corn syrup contains 55–65% fructose.³ The main source of sugar in the Australian diet is sucrose (from sugar cane) that is added to foods.¹

Whole fruits remain part of a healthy diet.¹ The fructose contained in a whole piece of fruit is far less concentrated than the fructose contained in most processed foods.³