We’ve all been warned of the many negative effects the consumption of added sugars can have on our health. Most, however, focus on the effects on our physical health and the potential for weight gain but overlook the potential impact on our mental health.
Research into the effects of added sugar on our psychological well-being is ongoing. It has not yet been able to identify a precise cause and effect relationship between the consumption of added sugar and the development of depression. However, most studies indicate a definite link between the consumption of added sugar and the development of depression-related symptoms which researchers continue to investigate.
For those prone to depression, being aware of this fact and steering clear of excess added sugar intake might help to prevent the worsening of depression symptoms.
Natural sugar versus added sugar
Many foods that we eat, including natural ones like fruit, vegetables and grains, contain carbohydrates which are a form of natural sugars. Consuming whole foods that contain natural sugar is an essential part of a healthy diet.
Our bodies digest whole foods slowly, as they are broken down, they release sugar in a steady supply that is then used to supply various cells throughout the body with energy. Diets rich in fruit, vegetables and whole grains are associated with lowering the risk of chronic disease development, protecting us from heart disease, diabetes and even certain cancers.
The problem, however, comes in with added sugars, which food manufacturers add to food to improve its flavour and extend its shelf life.
Research shows that several possible reasons why habitual added sugar intake may potentially trigger symptoms of depression. These include1:
1. Added sugars alter the gut-brain axis
Researchers have recently discovered that the human gastrointestinal system (i.e. the stomach and bowels) and the central nervous system (i.e. the brain and spinal cord) are connected both physically and biochemically2. This connection is referred to as the gut-brain axis. What’s particularly interesting is that this link between the brain and gut has significant implications in the development of psychiatric disorders, including major depressive disorder (MDD)3.
Chemical messengers referred to as neurotransmitters (like serotonin and dopamine) which are produced in the brain play a role in controlling our feelings and emotions. Interestingly, cells within the gut, as well as the naturally occurring microorganisms (knowns as microbes or microbiota), that reside in it also produce these neurotransmitters. They also produce other chemicals that affect how the brain functions. Simply put, what’s going on in our ‘tummies’ can affect how we feel.
The gut-brain axis is also linked through the immune system. The gut and its microbes play a vital role in the function of the immune system and inflammation by governing what is allowed into the body and what is removed4.
Research suggests that diets high in sugar can disrupt the delicate balance of microbes in the body, which causes dysfunction. It is believed that this may contribute to the development of depression symptoms by promoting inflammatory processes throughout the body.
Inflammation is the body’s natural way of dealing with infections and illness and is a natural part of the immune system function. However, when the immune system is active for extended periods, the inflammatory toxin (LPS) produced by certain bacteria can cause chronic inflammation when too much of it passes from the gut into the blood. When this occurs, instead of healing the body, the inflammatory response begins to harm it, leading to various health conditions and diseases. LPS, in particular, has been linked to the development of several brain disorders, including depression.
2. Added sugars affect ‘feel-good hormone’ (dopamine) signalling
On a biological level, sugary foods provide energy to our bodies without them needing to work too hard to break the food down into glucose. For this reason, when we eat sugary treats, the brain’s reward system is activated. The primary chemical messenger (neurotransmitter) involved in this system is dopamine which signals when something we’re doing, or eating is positive.
When this happens, our behaviour is reinforced, increasing the likelihood that we’ll repeat it in future. When it comes to sugar, the dopamine ‘hit’ we get when eating it, rapidly teaches us that we should eat more of it in future. However, because sugar is so readily available in modern society, this presents a problem.
The brain can rewire itself; this is known as neuroplasticity. Rewiring can take place in all areas of it, including the reward system. When the reward pathway is repeatedly activated by eating large amounts of sugary foods, the brain gets used to the frequent stimulation and builds up a kind of tolerance to it. This means that more significant volumes of sugar are required to attain the same reward feeling, which some liken to addiction.
According to the KU associate professor of clinical psychology and one of the study’s authors, the consumption of sweets acts like a drug. While they immediately boost mood, in high doses, they can have the opposite effect and lead to longer-term consequences that worsen mood, reduce feelings of wellbeing, increase inflammation and cause weight gain.
3. Added sugars contribute towards oxidative stress
Simply defined, oxidative stress is the result of an imbalance between reactive oxygen species (free radicals) and the antioxidants that combat these. When free radicals outnumber antioxidants in the body, several toxic effects may follow, which lead to the destruction of cells, inflammation and accelerated ageing.
The brain is highly sensitive to oxidative stress which disrupts its natural function of chemicals (neurotransmitters) within it. Elevated levels of oxidative stress have been linked to the development of various mental disorders including anxiety, bipolar disorder, schizophrenia and depression. However, a precise cause and effect relationship is yet to be established5,6.
A growing body of research indicates that added sugar consumption can cause oxidative stress. When this occurs in conjunction with chronic inflammation, depression may arise.
4. Added sugars and insulin resistance
Insulin is a hormone that allows glucose (sugar) to enter our cells for conversion into energy. Think of it as the key to the ‘locks’ or receptors of these cells. Insulin resistance occurs when the cell’s receptors become less sensitive to insulin and don’t ‘open’ as quickly. As a result, the cells are deprived of the fuel they need to produce energy.
The brain is full of insulin receptors which control its use of glucose for energy. Emerging evidence shows that depression can arise from the brain’s insulin resistance and the resulting disruption of energy supply.
Excessive refined sugar consumption has been linked with the development of insulin resistance7. Oxidative stress and inflammation, both of which are connected to the development of depression8,9, also play a role in the development of insulin resistance. When the flow of insulin is disrupted as a result, it’s anti-inflammatory and antioxidant properties are compromised, making inflammation and oxidative stress, and thus depression, worse.
5. Toxins
Advanced glycation end-products (AGEs) are toxic molecules that are formed when sugar molecules interact with proteins, fats and other substances in the body. Increasingly research is showing that excessive added sugar intake promotes the formation of AGEs which can contribute to oxidative stress, inflammation and even impaired cognitive function. In some research, scientists have found a correlation between AGE levels and the severity of depression symptoms in people10.
Sugars occur naturally in a variety of fruits and vegetables. Eating sugar in moderation is generally not an issue for most people, but it is advisable to always pay attention to the amount of sugar you eat. Don’t forget that most prepared and processed foods, sauces, candy and even many drinks all have added sugars of which you may not be aware.
If you suffer from depression, switching to a lower-sugar diet by simply restricting added sugars may help. It’s important to remember that this doesn’t mean cutting out all sugar by going totally carb-free, you can still enjoy all other natural carbohydrates like fruit and vegetables.
References
1. Reis D, Ilardi S, Namekata M, Wing E, Fowler C. The depressogenic potential of added dietary sugars. Med Hypotheses. 2020;134:109421. doi:10.1016/j.mehy.2019.109421
2. Kelly J, Kennedy P, Cryan J, Dinan T, Clarke G, Hyland N. Breaking down the barriers: the gut microbiome, intestinal permeability and stress-related psychiatric disorders. Front Cell Neurosci. 2015;9. doi:3389/fncel.2015.00392
3. Cheung S, Goldenthal A, Uhlemann A, Mann J, Miller J, Sublette M. Systematic Review of Gut Microbiota and Major Depression. Front Psychiatry. 2019;10. doi:3389/fpsyt.2019.00034
4. Rooks M, Garrett W. Gut microbiota, metabolites and host immunity. Nature Reviews Immunology. 2016;16(6):341-352. doi:1038/nri.2016.42
5. Salim S. Oxidative Stress and Psychological Disorders. Curr Neuropharmacol. 2014;12(2):140-147. doi:10.2174/1570159x11666131120230309
6. Bajpai A. Oxidative Stress and Major Depression. JOURNAL OF CLINICAL AND DIAGNOSTIC RESEARCH. 2014. doi:7860/jcdr/2014/10258.5292
7. Macdonald I. A review of recent evidence relating to sugars, insulin resistance and diabetes. Eur J Nutr. 2016;55(S2):17-23. doi:1007/s00394-016-1340-8
8. Lindqvist D, Dhabhar F, James S et al. Oxidative stress, inflammation and treatment response in major depression. Psychoneuroendocrinology. 2017;76:197-205. doi:1016/j.psyneuen.2016.11.031
9. Lin D, Wang L, Yan S, Zhang Q, Zhang J, Shao A. The Role of Oxidative Stress in Common Risk Factors and Mechanisms of Cardio-Cerebrovascular Ischemia and Depression. Oxid Med Cell Longev. 2019;2019:1-13. doi:1155/2019/2491927
10. van Dooren F, Pouwer F, Schalkwijk C et al. Advanced Glycation End Product (AGE) Accumulation in the Skin is Associated with Depression: The Maastricht Study. Depress Anxiety. 2016;34(1):59-67. doi:1002/da.22527
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