The curious case of Tagatose...

It's quite an exciting thing on the face of it. 92% as sweet as sucrose with only 38% of the calories. An incredibly low glycemic index of 3 (sucrose has a GI of 65-68). Texture and stability so similar to that of refined sugar, allowing it to be used in a 1:1 ratio in baking with an almost indistinguishable taste profile. No tooth decaying properties. So why has no one heard of Tagatose?

Granulated tagose resembling common refined sugar

Where did it come from?

The discovery of Tagatose is equally as strange as its obscurity. Dr. Gilbert V. Levin, an American engineer, was the principal investigator of the Viking mission 'Labeled Release' exobiological experiment on the surface of Mars in 1976 - an attempt to discover Martian microbial life.

Dr. Gilbert V Levin

Dr. Levin invented and utilised a novel technique he termed radiorespirometry in his search for microbial life. Simply put, he'd add tiny amounts of radioactive material into nutrient solution, and measure for radioactive gas production (a byproduct of microbes metabolising the radioactive nutrients) using a Geiger Counter.

Dr. Levin, however, realised that there would be a potential problem with the application of this method on Mars. We know that microbial life on Earth is able to metabolise glucose. More specifically - D-glucose. How could he possibly know that the same is true of life on Mars? In attempting to counter this issue, Dr. Levin prepared both L and D enantiomers of various sugars. It was in this process that he concluded that if humans were unable to digest L-glucose, perhaps it had  application as a dietary sweetener. Due to the prohibitive costing involved in the production of L-glucose, however, he moved on to alternatives and closely examined D-tagatose, a fructose isomer. Subsequently, he founded and patented an inexpensive method of synthesising the sugar and began licensing its use.

Great! So why isn't it in my food?

Well... that's where things become less clear. Despite being licensed to the likes of Arla and Pepsi, there has never really been an upsurge in its use in the food we eat every day. Pepsi gave tagatose its mainstream debut in the USA in the form of the Diet Slurpee back in 2003.

The 'diet' Slurpee

That's about it though. Whilst is does appear in a very small range of products these days, it has nowhere close to the prevalence of its rival sweeteners: sucralose, aspartame, xylitol, stevia etc. So we're left contemplating why, given the obvious advantages it yields.

Cost

Despite being exponentially cheaper to produce when compared with L-glucose, it still hasn't reached the cost efficiency achieved through the sheer level of manufacturing that mainstream sweeteners have. At a purely consumer level, as of this date it costs around $12-13 per 454g with a typical production cost of around $3-6 per kilogram with a limited range of supply chains. This is over 20x the cost of granulated sugar alone.

Gastric issues

Only 20% of tagatose is digested in the small intestine, where it will enter glycolysis in an identical manner to fructose. The remaining 80% passes to the large intestine where it is fermented by intestinal bacteria. This isn't necessarily a bad thing - prebiotic agents such as this help stimulate the growth of this 'good' bacteria. The problem goes back to Dr. Levin's search for life on Mars - microbial fermentation produces gas. In a particularly large single dose (15g or above), tagatose is likely to cause noticeably increased levels of flatulence. 

And therein lies the problem. In small doses, other sweeteners are far cheaper to produce and have an adequate enough taste profile (think of the typical tea/coffee sweeteners). In large doses where tagatose would be an ideal replacement for sugar (confectionary), we find that there is a limit before there will be less than ideal side effects.

Diabetes

So whilst tagatose may not quite be set to replace regular sugars in our diet, scientists are finding other novel uses for it. Phase II and phase III studies of effectiveness found tagatose was able show a statistically significant reduction in HbA1c amongst diabetic patients (Fujisawa et al, 1991). The results of these studies and numerous others in recent years show great promise in the use of tagatose as an antihyperglycemic aide and as production costs decrease, I suspect we'll begin to see it become more popular as an ingredient of products aimed at diabetic consumers in the coming years.