April 12, 2024

How free sugars affect human health

In a recent review article published in Nutrientsresearchers summarize current evidence on the effect of free sugars on health outcomes in humans, including mood, cognition, cardiovascular disease, diabetes and obesity.

They concluded that excessive consumption of added sugars can negatively impact health and overall well-being, highlighting the need for further research into how different carbohydrate forms affect diverse populations.

Study: The impact of free sugar on human health: a narrative review. Image credits: qoppi/Shutterstock.com


Non-communicable diseases (NCDs), including chronic conditions such as heart disease and diabetes, are largely preventable but are responsible for a significant proportion of global deaths.

Although researchers have emphasized lifestyle modifications to prevent and treat NCDs, and there is evidence that improved diet provides significant benefits, there is debate about the specific role of sugar consumption.

In the late 20th century, decreased fat consumption led to increased intake of carbohydrates and added sugars, especially high-fructose corn syrup. This coincided with rising rates of obesity, diabetes and heart disease.

In recent years there has been a slight decrease in sugar intake, prompted by health guidelines that advocate a limited daily sugar intake to limit health risks.

Some studies have linked the excessive consumption of sugar to various health problems. At the same time, other research indicates that sugar is not inherently more harmful than other dietary energy sources.

Effects of sugar on health

Global obesity rates have increased significantly in recent decades, with obesity linked to several non-communicable diseases. The debate over the main cause of obesity – excess sugar, fat or total calorie intake – continues, with recent declines in sugar consumption alongside persistent obesity rates suggesting generational effects.

Studies comparing low-carb and low-fat diets revealed mixed results, highlighting the need for personalized nutritional interventions.

Sugar consumption, especially from fructose and sugary drinks, has been linked to an increased risk of type 2 diabetes (T2DM) in numerous studies. However, findings are inconsistent, with some shorter-term studies failing to establish clear relationships.

Long-term studies suggest a significant impact of fructose intake on insulin resistance and diabetes risk, especially in women. The protective effects of dietary fiber and certain fats on diabetes risk are also noted, highlighting the complexity of dietary factors influencing T2DM.

The role of refined carbohydrates and sugary drinks in heart disease is increasingly recognized, with studies pointing to their association with dyslipidemia and increased cardiovascular risk.

While some studies show a direct link between sugar consumption and heart disease, others find conflicting results, possibly due to differences in study duration and methodology.

Research suggests that sugars may play a detrimental role in cardiovascular health, although the specific types of carbohydrates and fats consumed may have different effects.

Chronic excessive sugar intake is thought to impair cognitive function, with studies in animals and humans showing neurological and cognitive impairments associated with high sugar consumption.

Maternal sugar consumption during pregnancy and lactation may also influence offspring cognition.

While some research suggests short-term cognitive benefits of sugar consumption, the long-term effects are less clear and may be influenced over time by factors such as glucose control and dietary habits.

The impact of sugar on mood and behavior is debated, with inconsistent findings from different studies. While short-term studies suggest potential mood benefits of sugar consumption, especially after fasting, longer-term studies point to correlations between high-sugar diets and depression, anxiety and other mood disorders.

Confounders and methodological challenges complicate research into the relationship between added sugars and psychological health, which warrants further research.

Underlying mechanisms

Chronic high sugar consumption is hypothesized to influence mood through neurological mechanisms. Western diets, high in sugar, are associated with inflammation, reduced BDNF in the hippocampus and changes in dopamine signaling, which resemble addictive behavior.

Sugar consumption can disrupt dopaminergic pathways, leading to an increase in sugar seeking and consumption, similar to addiction. Disruption of the microbiome, especially from high-sugar diets, worsens inflammation and contributes to obesity and neurodegeneration.

Sugar-induced dysbiosis can lead to intestinal permeability, causing systemic inflammation and neuroinflammation, potentially explaining the neurological and psychiatric disorders associated with sugar and obesity.


Minimal scientific evidence supports the claim that added dietary sugars provide health benefits; rather, a growing number of studies indicate that they have negative effects, especially if consumption is excessive, prolonged and high in fructose.

Glucose supplementation may be helpful under certain conditions, but can also be obtained from dietary sources, including whole grains, vegetables, and foods.

Although not all added sugars need to be eliminated, nutritionists recommend limiting their intake to no more than 10% of total energy expenditure.

The reviewers emphasized the need for further research into how different artificial sweeteners and macronutrients affect health outcomes and the challenges posed by sugar-related restrictions.

Although large-scale population studies may not be ideal for identifying individualized effects, cohort studies and randomized controlled trials in diverse populations can provide insights into the precise effects of macronutrients and how they interact to modify health outcomes.

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