Lately, the science has really been stacking up evidence against consuming sugars in excess.
In addition to being linked to conditions like obesity, diabetes, inflammatory diseases, eating high levels of sugar has been associated with mental illnesses like depression. In a study published July 27 in Scientific Reports that followed over 8,000 adults over 22 years, researchers from University College London found that men who reported consuming foods that contained 67 grams of sugar per day or more were 23% more likely to be diagnosed with clinical depression after five years from when the study began.
For their work, researchers followed the a cohort called the Whitehall Study II, which tracked health and stress data for civil servants aged 35 to 55 in London beginning in 1985. Every few years, participants filled out surveys about their diets and other markers of health—including whether or not they had been clinically diagnosed with mental health conditions like depression and anxiety. Participants didn’t have any mental illnesses diagnosed to start, and researchers used their food questionnaires to estimate how much sugar each person was eating per day.
After the first five-year follow up, men who ate the most sugar, which the authors categorize as 67 grams or more per day—almost twice the amount of sugar intake recommended by the American Heart Association, and roughly three and a half regular sized Snickers bars—had higher rates of mental health diagnoses than those who ate less sugar, regardless of whether or not they were overweight. Even during years when participants reported eating less sugar, levels of mental illness stayed the same, which suggests that previous sugar habits had led to depression or anxiety and not the other way around. In this study, the relationship between sugar and mental illness wasn’t well-defined among women.
Anika Knuppel, a doctoral candidate in epidemiology at the University College London and lead author of the current paper, cautions that these studies can’t prove added sugar causes mental illness. Studies that follow self-reported health data over time are inherently flawed because even when participants have honest intentions, they have poor memories about what they eat. The only thing that could would be a randomized controlled study, which would be unethical to perform knowing the links between sugar and other health consequences, Knuppel says.
But there are theories as to how excess sugar may affect mental health. James Gangwisch, a psychologist at Columbia University who found (paywall) a link between sugar and depression in postmenopausal women, has postulated that foods high in sugar that are easy to break down may cause our blood sugar to immediately rise, and then plummet. This crash puts a stress on the body, and it responds by releasing hormones like cortisol and adrenaline, which, over time may lead to anxiety or depression. Furthermore, animal research in rats has shown (paywall) that diets high in fat and sugar can lead the brain to produce less of a protein called BDNF, which has been associated with anxiety and depression in humans, Knuppel says.
All this is to say that there isn’t proof that sugar causes mental illness, but that there is a growing amount of evidence that suggest that eating a lot of extra sugar has consequences that go far beyond our waistlines. It’s worth considering how much added sugar is in your own diet beyond what’s found naturally in foods like fruits, which don’t give us the same blood sugar spike that foods like candy do. The US Food and Drug Administration has mandated that all food labels include added sugars by July 26, 2018.
Correction: This post has been updated to reflect that lower levels of BDNF have been associated with anxiety and depression.