A 30-year study published in Scientific Reports suggests that people who consume high amounts of sugar intake may be more likely to experience mental health problems, especially men. Further, the study reveals a link between sugar and depression, which should be intriguing to everyone given the amount of sugar Americans consume. 
Nitty Gritty Study Details
More than 10,000 British civil servants between the ages of 35 and 55 were recruited for the University College of London’s Whitehall II study, which launched in 1985. In the study, researchers monitored the participants’ health and behavior, conducting surveys with each volunteer over 10 phases, 3 years apart.
Of the participants, 66.9% were men and 33.1% were women.
The researchers surveyed the participants’ diets at the 3rd, 5th, 7th, and 9th phases of the study. The team looked at 15 kinds of foods placed in the “sweet food and beverage” category, including cakes, cookies, and sugar-sweetened coffees, and teas.
The food intake surveys were then compared with a general health questionnaire designed to measure symptoms of depression and other common mental disorders (CMDs), such anxiety and insomnia.
The researchers wrote:
“The present long-term prospective study is the first to investigate the association of sugar consumption from sweet food/beverages with prevalent, incident and recurrent mood disorders, while also examining the effect these disorders might have on subsequent sugar intake.”
“Further, we found an increased likelihood for incident CMD in men and some evidence of recurrent depression in both sexes with higher intakes of sugar from sweet food/beverages.”
To see if there might be other underlying reasons for the participants’ depression, researchers also accounted for other factors besides sugar intake, including marital status, age, ethnicity, and smoking.
The Not-so-Sweet Findings
The study found that men who consumed more than 67 grams of sugar a day were 23% more likely to suffer from depression, anxiety, or other CMDs 5 years later, compared with men who consumed less than 40 grams. 
Additionally, both men and women with a mood disorder and a high intake of sugar were found to have a higher risk of developing depression again 5 years later, compared with participants who consumed less sugar. This association, however, was partly explained by their overall diet.
What’s more, the researchers found no evidence that participants changed their sugar intake after suffering mood disorders. In other words, if a person ate more sugar during a particularly low point in their lives, they didn’t cut their sugar intake once they were feeling better.
So What Might be the Link Between Sugar and Depression?
Anika Knüppel, a PhD candidate in epidemiology and public health at University College London, and the study’s lead author, explained:
“We are still not sure what causes depression, but some researchers believe that biological changes are at the root of it. Some of these changes could be influenced by sugar and sweet taste.”
She uses the example of a study in rats which found that diets high in sugar and fat can reduce BDNF, a protein which influences the growth and development of nerve cells in the brain. This protein is believed to play a role in the development of depression and anxiety.
In addition, high-sugar diets can increase inflammation. While inflammation protects the body against microorganisms and foreign substances, it can also zap people of energy and their ability to concentrate. Ongoing research suggests that some mood disorders may be linked to inflammation.
Knüppel said another possible culprit is the feel-good hormone dopamine. This theory also comes from a study of rats, which found that sweet foods are as addictive as cocaine.
In fact, several studies have shown that sugar is addictive. Processed foods – one of the sources, if not the source of added sugars – has been shown to be addictive. Some studies are very specific, such as this one from 2013, which showed that Oreo cookies are addictive.
Dopamine is a brain chemical involved in the reward system, and it is believed to affect mood. Many people become addicts due to mental disorders because they use substances to self-medicate themselves. However, addiction itself is linked to a greater likelihood of developing a mood disorder.
Lastly, Knüppel said the link between sugar and depression could also be explained due to an association with obesity – a condition related to mood – and other conditions.
“But these associations could also reflect a reverse phenomenon: Low mood could make people change their diet. Sweet foods could be used to soothe bad feelings by providing a short-term mood boost. And low mood and anxiety could make simple tasks, such as grocery shopping or cooking, so difficult and exhausting for the sufferer that they might start to avoid them. Instead, they might opt for junk food, takeout and ready meals – all of which have a high sugar content.”
The link between sugar and depression (actually, mental health overall) is far from clear-cut, but it goes to show, yet again, that there is no redeeming health value to sugar.
We should also point out that it’s possible more men than women were found to suffer mood disorders simply because there were more men than women in the study.