Regular/diet soft drinks and caffeine linked to metabolic syndrome?

Coffee's impact on sugar metabolism is currently unclear.
Coffee has been linked to a reduced risk of diabetes in the general population but it is not yet known whether this is due to the caffeine content or other compounds in coffee.
Furthermore, caffeine may behave differently in people with diabetes. The Canadian Diabetes Association says the following about coffee "Drinking caffeine in large amounts as coffee over a short period of time has been shown to raise blood sugar. Caffeine does this by enhancing the effect of two hormones (adrenaline and glucagon). These two hormones release stored sugar from the liver resulting in high blood sugar". This can be advantageous in very active people like athletes who need a surge of glucose (atheletes been known to take caffeine supplements for extra "energy") but may be undesirable in the inactive person.
Emerging studies are shedding more light on caffeine metabolism.
In the July 23, 2007 online issue of the journal Circulation, Dhingra et al linked the consumption of more than one soft drink per day to the risk of developing metabolic syndrome or high levels of blood insulin (a precursor to developing diabetes). The association was found to be true for diet soft drinks (i.e. zero calorie) as well as those packed with sugar. The investigators suggest that the culprit might not be the soft drinks themselves, but instead the eating habits that soft drink consumption reflect. This study has fueled speculation by suggesting that some unknown ingredient (like caffeine) in diet sodas might be contributing to metabolic syndrome in susceptible individuals. The sweet taste in the mouth (from sugar or artificial sweeteners) may trigger the pancreas to release insulin which could result in hyperinsulinaemia (leading to metabolic syndrome) if intake is chronic.
In 2006 Rush et al from the Auckland University of Technology in New Zealand published a study in Asia Pacific
Journal of Clinical Nutrition showing that an energy drink containing sugar and added caffeine caused the body to convert sugar into fat more rapidly than lemonade. The energy drink tested by Rush's team contained 28g sucrose and 81mg caffeine per 250ml can, which is similar to the amount of sugar in soft drinks and caffeine in a brewed cup of coffee. The author believes these results could have huge implications when you think about how much sugar and caffeine people consume these days, and the high rates of inactivity. For the study, the New Zealand team recruited 10 healthy women aged 18 to 22 from a range of ethnicities. The subjects fasted overnight and were randomly given either 250ml of an energy drink or lemonade on the first day and the alternative on the second day of testing. The sugar in both drinks was absorbed rapidly into the bloodstream - within a minute and when caffeine is also present the sugar is more quickly converted to fat.
Sugar is a simple carbohydrate and evidence from this study shows that, coupled with a large amount of caffeine, the body rapidly turns it into stored fat. Professor Rush highlights that simple carbohydrates and caffeine were not such a large part of our diet in the past. Inactive people have trouble burning off excess energy and this leads to weight gain.
The findings are supported by a recent UK study, which found that caffeine increased cyclists' absorption of carbohydrates from a sports drink. But for those consumers not doing much exercise, these carbohydrates will be turned into fat.
The study also raises questions about the effects of consuming high-sugar foods and highly caffeinated drinks together in a short period of time. Caffeine lasts in the body for four to six hours, so people who have caffeinated drinks, including sugar-free energy drinks or coffee, and then eat sugary food within this time frame, may experience similar effects. This study is limited by its small sample size and the fact that the subjects were all young women. But the results are important as this area has not been explored before. This area needs further research, as well as the long term effects of combined caffeine and sugar on sedentary people's health.

Caffeine containing soft drinks (but not coffee) found to raise blood pressure

Most people can safely consume up to 250 milligrams of caffeine (equivalent to about 3-4 cups of coffee) per day. Caffeine can cause small spikes in blood pressure in sensitive individuals, which could be dangerous for people with high blood pressure or heart disease. However, in a study of hypertension-free women, drinking 2-3 glasses of caffeine-containing colas (regular and 'diet') daily was associated with an increase in blood pressure and yet coffee consumption was not associated with hypertension risk in the study. This elevated blood pressure translated to an increased risk of hypertension by about 30%. Researchers speculate that it is not the caffeine but perhaps some other compound that is responsible.More research is needed to confirm the results, but if you are concerned about high blood pressure, limiting caffeine intake is a smart health move.

Habitual caffeine intake and the risk of hypertension in women. Winkelmayer, W. C., Stampfer, M. J., Willett, W. C., Curhan, G. C., The Journal of the American Medical Association 2005 Nov 9;294(18):2330-2335.

Caffeine in Soft Drinks

The article below addresses the health concerns of popular energy drink Red Bull being combined with alcohol after the death of a British man.

Additional articles about caffeine:
Caffeine, friend or foe?
Caffeine Report (by the Australian New Zealand Food Authority)