By Shiyi Liang
Throughout the world, the consumption of sugar gives people great pleasure. But this sense of pleasure makes people crave even more sugar and can build sugar addiction. Emotions associated with sugar withdrawal for those who are addicted include feeling depressed, inactive, and anxious. It has been suggested that sugar is more addictive than certain drugs – some experiments show that sugar is favoured by animals above even cocaine (Lenoir et al., 2007), which is a strongly addictive drug.
Common forms of added sugar, like high fructose corn syrup (HTCS), are made up of glucose and fructose. Many food processors would add these sugars to food to produce a sweet flavour in their products. People sense sweetness from T1R2 and T1R3 receptors on taste receptor cells. Taste information is transmitted to the gustatory nucleus in the medulla through cranial nerves and then diverged to the primary gustatory cortex (Bear, Connors & Paradiso, 2020). The hypothalamus will also receive gustatory information (Bear, Connors & Paradiso, 2020), and can control appetite and thirst (Jenkins, 1972).
Sugar intake produces rewards by two main pathways: nutrition and taste. In rodent studies, the nutritive path involves sugar activating melanin-concentrating hormone (MCH) neurons, and these neurons then transmit signals to dopamine neurons in the mid-brain that will further project to the dorsal striatum and increase dopamine release in the region. As for the sweet taste of sugar, it increases the secretion of dopamine in the ventral striatum (Freeman et al., 2018). Further mouse studies suggest that intermittent sugar access could induce sugar addiction.
Drug addiction is strongly related to dopamine signalling which starts from the dopamine neurons in the ventral tegmental area (VTA) and ends with dopamine release in the nucleus accumbens (NAc). The signalling mechanism of cocaine requires more than a single transport site, and both dopamine and serotonin transporters are required to produce a rewarding effect. Some other drugs will also need norepinephrine transporters (Hall et al., 2004). Sugar is found to have an opiate-like effect. One experiment treated rats with 12-hour food deprivation followed by 12-hour access to food and 10% sucrose. These rats showed reactions to opiate withdrawal when provided with naloxone, an opioid antagonist. The control group was treated with a similar protocol but without sucrose access. Brain dissection slices were made after 21 days of treatment (Spangler et al., 2004). Tissues from brain sections were taken out and mRNA expression of dopamine receptor genes (D1R, D2R, D3R) were measured (Spangler et al., 2003). The result showed the changes in mRNA levels in experimental rats are much the same compared with data from morphine-dependent rats in the previous experiment (Spangler et al., 2003; Spangler et al., 2004).
Different types of sugar produce a different hedonic effect on the brain. Glucose and fructose have different effects on regional cerebral blood flow (CBF). They both reduce regional CBF within the thalamus, but glucose also reduces regional CBF within the hypothalamus and striatum, which is strongly related to appetite and reward control. Functional magnetic resonance imaging can measure the effect of glucose and fructose after the subject drinks either a glucose or fructose drink. This can be combined with testing of the hormone response after the sugar consumption by taking blood samples. It was found that plasma glucagon-like-peptide 1 (GLP-1) concentration is higher in subjects that have taken a glucose drink and plasma PYY concentration is higher in subjects that have taken a fructose drink. GLP-1 is found to reduce appetite, and according to the behavioural rating test measuring hunger, fullness, satiety, the ratings of satiety and fullness is higher in the glucose group (Page et al., 2013).
There is hypothesis around sugar and mood. Sugar could be relevant to depression and anxiety. A group of scientists focused on sugar-sweetened beverages (SSB) trying to find out relationship between the risk of depression and sugar. According to their summarization involving 10 other studies, almost 40,000 cases of depression cases among 365,289 participants, they suggest a possible positive correlation between SSB intake and risk of depression (Hu, Cheng & Jiang, 2019). In a questionnaire evaluating anxiety symptoms and sugar craving (SC), 42.3% of participants experience SC and 81.1% them has anxiety symptoms. However, the study group found other contradicting views, so considering the complexity of SC, it suggests further studies required (Penaforte et al., 2019).
The sweetness preference appears to have a genetic difference (DiNicolantonio, O’Keefe and Wilson, 2017). An additional gut-to-brain system was found to report the presence of sugar in the brain. Neurons in the caudal Nucleus of the Solitary Tract (cNST) mediates sugar preference. The study silenced the synaptic transmission in the sugar-preference neurons in mice and they found the ability to form sugar preference is prohibited (Tan et al., 2020).
In brief, one of the reasons why sugar is addictive is the induced ‘reward’ felt on sugar consumption – which leads to further cravings. Additionally, it produces opiate-like effects in the brain, like some other addictive drugs do. Sugar over-consumption can lead to acceleration of skin aging, obesity and coronal diseases (Danby, 2010). Sugar addiction can be a type of food addiction. Diving into the nature of sugar addiction could help to inform the relationship between diet and obesity. Obesity can be described as abnormal fat accumulation, and food with massive amounts of added sugar could trigger this addictive effect and lead to over-consumption (Codella, Terruzzi & Luzi, 2017). However, research around sugar addiction is mostly centered on animals, with human models yet to be developed (Westwater, Fletcher & Ziauddeen, 2016). As such, there remains much to discover about the addictive nature of sugar.
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