Does sugar cause depression?

The road to depression is paved with glucose. But you can turn back.

Maria Cross MSc
Dec 7, 2019 · 9 min read

Sugar makes you fat, which is bad enough. But can it really make you depressed? Clinically depressed, even? The evidence is compelling: it’s a journey that starts with a mild blood sugar imbalance and can end with serious mental illness.

Along this baited road are landmarks that signal the next level of downward progression, each step fuelled by glucose.

Blood sugar imbalance (aka impaired glucose regulation) is a common health problem, one that may be familiar to you. Ask yourself: How do you feel if you eat no food for three or more hours? Does the mere thought induce a sense of mild panic? Perhaps you are familiar with the tremors, headaches, fatigue, light-headedness, irritability and anxiety that characterise an inability to maintain blood sugar levels on an even keel.

Perhaps you snack regularly in order to avoid crashing. Frequent snacks are a common and indeed effective first-response solution.

In reality, you are only masking the problem temporarily: there’s something fundamentally wrong with your metabolic response if you cannot handle an unremarkable period of abstinence from eating.

The issue is neatly illustrated by this particular case history. In 2016, the journal Case Reports in Psychiatry published the story of a 15-year-old girl (“AB”) suffering from generalised anxiety disorder and symptoms of hypoglycaemia, or low blood sugar. These symptoms included fatigue, headaches, shakiness, and heart palpitations. AB described her anxiety as impacting her daily functioning, leading her to be absent from school on several occasions.

AB ate mainly refined carbohydrates. In response to her anxiety symptoms, she would eat chocolate, chips and fruit. Her breakfast consisted of smoothies, fruit juice and water. Mid-morning, she snacked on a bagel with margarine. Lunch was pasta or white rice with vegetables, followed by an afternoon granola bar or cookie. In the evening she ate much the same way, sometimes with the addition of meat. More cookies along with toast in the evening rounded up the day.

AB reported that sweet foods improved her symptoms. No surprises there: sweet food causes a sharp rise in blood glucose, so in AB’s case were just the tonic when her blood sugar crashed, as it did regularly throughout the day.

What these carbohydrate foods have in common is their ability to create a spike in blood glucose levels; they have what is called a high glycaemic index.

The glycaemic index (GI) is a system which measures the rate at which the carbohydrate component of a food item enters the bloodstream and raises blood glucose, on a scale of 0 to 100. Carbohydrates are categorised as having either a low, medium or high GI.

“Observational evidence” suggests a relationship between food with a high glycaemic index and the development of depression. Clinical trials in both children and adults exploring this relationship have also found that high GI diets result in a deterioration in mood.

Protein (meat or beans), fat and fibre were added to AB’s lunch and dinner and she was given protein powder with breakfast.

“The addition of protein, fat, and fiber to her diet resulted in a substantial decrease in anxiety symptoms as well as a decrease in the frequency and severity of hypoglycemia symptoms.” (Aucoin & Bhardwaj 2016)

Following these changes, AB reported improved anxiety, energy and other symptoms. She also reported that she had less inclination to snack on sweet snacks between meals.

In an unplanned yet telling experiment, when she briefly returned to her old eating patterns, she immediately experienced a return of her mood symptoms — which improved when she went back to the high fat and protein diet.

The researchers concluded:

“Dietary carbohydrates may be significantly related to emotional and cognitive symptoms such as anxiety and difficulty concentrating.”

Further down the rabbit hole: Insulin resistance

As you continue eating refined carbohydrates in order to deal with the damage created by eating refined carbohydrates, the chances are you will arrive at the next landmark on your journey: insulin resistance.

“There is substantial evidence both that depression can promote insulin resistance and that insulin resistance is a risk factor for the worsening of existing depressive symptoms.” (Singh et al 2018)

Each time your blood sugar spikes, insulin rushes in. Blood sugar drops. Eat and repeat.

And this is where it can all start to go more horribly wrong.

Insulin is released by the pancreas when food enters the stomach, and nothing stimulates this hormone quite like carbohydrate. Carbohydrates include biscuits, cakes, cereals and other starchy grains, bread, pasta, pastries, potatoes, and sweet and savoury snacks.

Blood sugar levels must be tightly regulated. Too much, or too little, are both potentially dangerous situations.

The body can only take so much of this sugar saturation. It wasn’t designed to manage endless peaks and troughs, and they eventually take their toll.

Insulin resistance arises when insulin starts to lose its effect — the body’s cells no longer respond as they should, and glucose remains in the blood. Sensing this, the pancreas continues to pump out more insulin, but to little or no effect.

In the brain, it becomes brain insulin resistance.

“Similarly, brain insulin resistance can be defined as the failure of brain cells to respond to insulin.” (Arnold et al 2018)

If you don’t turn back now, you could reach the next landmark, when brain insulin resistance meets metabolic syndrome and your mental health could take a turn for the worse.


Metabolic syndrome (MetS) is a cluster of symptoms — insulin resistance, high blood pressure and an expanded waistline — that increase your risk of developing type 2 diabetes, stroke, heart disease.

It doesn’t stop there. There is a high rate of metabolic syndrome among people with conditions that include depression, bipolar disorder and schizophrenia.

“MetS is more prevalent in patients with bipolar disorder or schizophrenia than in the general population. Individuals with bipolar disorder have the highest rates of MetS” (Nousen et al 2013).

Studies have consistently shown a relationship between depression and metabolic syndrome, even if the exact mechanism has not been established. One link is clear: both conditions induce a “low grade chronic inflammatory state.”

Scientists increasingly acknowledge that depression is an inflammatory disorder of the brain. It is already established that both metabolic syndrome and its successor, type 2 diabetes, are the product of “chronic, low-grade inflammation”.

Glucose is the common denominator. “Glucose is pro-inflammatory… A total of 75 g glucose intake causes acute oxidative and inflammatory stress” (Sun et al 2014).

Metabolic syndrome is a short step away from type 2 diabetes, described as a “rampant epidemic worldwide”.

“..The prevalence of depression among patients with type 2 diabetes mellitus is twice that of the general population.” (Garcia Rizo et al 2013)

Approximately 10% of people with major depressive disorder also have type 2 diabetes, a figure believed to be an underestimate.

It’s not just depression that teams up with diabetes. A 2016 study published in International Journal of Bipolar Disorders describes the high rate of diabetes in patients with bipolar disorder as “striking”. There is also high prevalence of diabetes in patients with schizophrenia.

Diabetes is the endpoint, as far as impaired glucose regulation goes. Dietary sugar and refined carbohydrates are right there at the heart of the matter.

It is known that there is a “strong positive correlation” between the global prevalence of diabetes mellitus and per capita consumption of sugar. The degree and duration of sugar consumption correlates with diabetes in a dose-dependent way. Conversely, low sugar consumption correlates with declines in rates of diabetes.

In 2017, the journal Scientific Reports published a study that examined the link between dietary sugar and depression. The researchers found that men who consumed 67 grams or more of sugar every day were 23% more likely to suffer from depression,over the following 5 years, than men who consumed less than 40 grams of sugar each day.

Just one can of sugar-sweetened soda may contain around 10 teaspoons — 40 grams — of added sugar.

Personally, I think 40 grams of sugar a day is quite eye-popping. In 2015, the World Health Organization issued guidelines that sugar should make up no more than 10% of total daily calorie intake. The WHO added that, better still, aim for no more than 5%, or around 25 grams (6 teaspoons), maximum per day.

But having just read that Starbuck’s “Signature Caramel Hot Chocolate” with oat milk contains 93.7 grams of sugar, I realise the trouble humanity is in.

By 2016, the US had the highest per capita consumption of sugar in the world, at roughly 126 grams a day. We do comparatively well in the UK, with an average per capita consumption of 93.2 grams a day. Still.

Also in the US, anxiety disorders affect 40 million adults. That’s 18.1% of the population. Furthermore, Over 7% of the population has had “at least one major depressive episode”.

We have always had a sweet tooth. We made do with fruit, and very occasionally honey, throughout the Palaeolithic era — almost all of our history. Then, when we became civilised, we worked out how to cultivate sugar and add it to as many processed foods as humanly possible.

We could probably all become diabetic if we tried hard enough. That is why, unlike type 1 diabetes, which is an autoimmune condition whose cause is unknown, type 2 diabetes is often described as a lifestyle condition.

If all this sounds a tad depressing (sorry), take heart. Many people turn around and head back up the road, reversing their insulin resistance, metabolic syndrome and even type 2 diabetes, and returning to an even keel.

The evidence suggests that there are three ways to reverse diabetes type 2 (and therefore all types of blood sugar disorders). The first is bariatric surgery. That’s a bit drastic. The second is calorie restriction, but that only works in the short term (the same applies as a weight loss strategy, something I wrote about in this article).

The third and most effective long-term solution is a carbohydrate-restricted diet. It’s also the most logical.

“In response to the new evidence on the efficacy of carbohydrate restriction, low-carbohydrate has recently been endorsed as an eating pattern by the ADA (American Diabetes Association) and the European Association for the Study of Diabetes (EASD)” (Hallberg et al 2019)

Before the discovery of insulin in 1921, carbohydrate restriction was the standard treatment for diabetes. Today, however, most health professionals are trained to treat the disease with drugs only. Achieving reversal “is not commonly encouraged by our healthcare system.”

So what about depression and other mental health disorders?

Eighty-two adults took part in a study that tested low and high glycaemic load diets, and were assessed for various mood states, including anxiety, depression, anger, hostility and confusion. They followed either a low or high glycaemic diet for 28 days, and then switched diets. Being on a high glycaemic load diet resulted in a 38% higher score for depressive symptoms than being on a low-glycaemic load diet.

“In conclusion, a high-glycemic load diet was associated with higher depression symptoms, total mood disturbance, and fatigue compared to a low-glycemic load diet especially in overweight/obese, but otherwise healthy, adults.” (Breymeyer et al 2016)

This was, however, just one study. A recent review that looked at the results of 11 studies, including clinical trials and involving over 80,000 individuals, found “a significant positive association” between the GI of a diet and the level of depression reported.

It is an unfortunate reality that craving for high glycemic carbohdyrates is a “common phenomenon” in people who are stressed and/or depressed.

Overeating on these carbohydrates is a form of self-treatment that can lead ultimately to diabetes and yet more depression.

Cutting out sugar and all those refined carbohydrates can be hard (at first), but the ends may well justify the means.

I write a lot about diet and mental health. Usually, the focus is on the nutrients in food that are required for healthy brain function, including mood and memory (see my publication Feed Your Brain).

What strikes me from my research is that it’s not only what you eat that matters — just as important is what you leave off the menu.

Feed Your Brain

Exploring the link between diet and mental health

Maria Cross MSc

Written by

Nutritionist and nutrition science writer, specialising in diet and mental health. Subscribe to for free brain food guide. @MariaXCross

Feed Your Brain

Exploring the link between diet and mental health

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