The bitter side of food: the harmful effects of a high-sugar diet

IN BRIEF

• Evidence indicates that high sugar consumption, particularly in the form of sugary drinks and "free sugars," is associated with weight gain, type 2 diabetes, and an increased risk of cardiovascular events in many observational studies.
• The risk profile depends on the dose, form of intake (liquid vs. solid), frequency, and overall dietary context; not all associations are necessarily causal.
• Some correlations with certain types of cancer and non-alcoholic fatty liver disease (NAFLD) have been reported, but the quality of evidence varies and requires cautious interpretation.
• For oral health, the relationship between sugars and cavities is well-established in longitudinal studies and systematic reviews.

Abstract: what does science say?

Sugar is not a single factor: it includes monosaccharides and disaccharides naturally present in fruit and dairy products, but also added to processed foods. The broadest epidemiological literature associates high consumption of free sugars and sugary drinks with a higher risk of overweight, type 2 diabetes, and, in some analyses, cardiovascular events and specific cancers. However, most of the available evidence is observational, with intrinsic limitations (confounding, dietary measurement, the role of energy excess). Biological plausibility exists for metabolic mechanisms (increased hepatic lipogenesis, changes in triglycerides, modulation of insulin resistance) and for local effects (dental damage), but the strength and generalizability of the associations depend on dose, form of consumption, and overall dietary context. Regulatory recommendations advise limiting "free sugars" in the diet as a public health measure.

Main section

Definition and sources of sugars

"Sugars" refer to simple molecules such as glucose, fructose, and sucrose, which can be intrinsic to foods (for example, in fruit) or added during processing and preparation. The distinction between "free sugars" (including added sugars and those present in juices/extracts) and intrinsic sugars is central to nutritional guidelines, because the former contribute more to uncompensated energy intake and are often associated with ultra-processed products. Total energy intake, fiber content, food matrix, and physical form (liquid or solid) influence the absorption and metabolic effects of sugars [4].

Evidence on the main associations with chronic diseases

Observational data and meta-analyses show a consistent relationship between regular consumption of sugary drinks and weight gain and risk of type 2 diabetes [1]. Global impact analyses estimated that sugary drinks contributed to a quantifiable proportion of deaths related to diabetes, cardiovascular diseases, and some cancers in 2010 [2]. Broad reviews (umbrella reviews) confirm associations between high sugar/SSB intake and various adverse outcomes, but highlight the low-to-moderate quality of some evidence and the need to evaluate context and confounders [3]. Recent large cohort studies have shown that the association with cardiovascular events is more evident for "free sugars" and for sugar in liquid form compared to complex carbohydrates [4].

Plausible biological mechanisms

There is biological plausibility for several mechanisms: caloric excess can promote an increase in fat mass; hepatic fructose can stimulate de novo lipogenesis, contributing to triglyceride accumulation and hepatic steatosis; alterations in plasma triglycerides and lipoprotein composition can link high sugar intakes to vascular risk [8][4]. Furthermore, repeated consumption of high-sugar foods can promote repeated episodes of post-prandial hyperglycemia and appetite fluctuations, although the long-term clinical relevance depends on many factors.

Practical section

What it means in practice

From a practical standpoint, evidence suggests that reducing the consumption of added sugars, particularly sugary drinks, can help reduce the overall risk of weight gain and metabolic diseases in the population. It's not about "blaming" individual foods, but about considering diet quality: replacing free sugars with foods higher in fiber and whole food matrix tends to improve metabolic profiles [4][5]. At the public prevention level, impact analyses and international guidelines recommend percentage limits of energy intake from free sugars as an operational reference; evidence suggests benefits from reducing intake, particularly when the reduction lowers overall caloric intake [9][5].

Some non-prescriptive practical tips

For those who want to reduce exposure: stick to a few practical rules (preferably drink water, limit sweetened beverages, read labels to discover "hidden" sugars, prefer whole fruit over fruit juice). It's important to evaluate the diet as a whole: even foods perceived as "healthy" can contain significant amounts of free sugars (for example, juices and some commercial beverages). For specific clinical conditions (diabetes, NAFLD, etc.), it is advisable to consult healthcare professionals for individual guidance [8][4].

Key points to remember

• High consumption of free sugars and sugary drinks is associated, in epidemiological observations, with a higher risk of weight gain and type 2 diabetes.
• The form of intake (liquid vs. solid) and the food matrix are relevant: sugary drinks are frequently responsible for uncompensated caloric intake.
• There is also evidence of an association with cardiovascular events and, in some analyses, with certain cancers, but the quality of the evidence varies.
• For dental health, the relationship between sugars and cavities is widely supported by longitudinal studies and systematic reviews.

Limitations of the Evidence

Most research is observational: it shows associations and cannot, by itself, prove definitive causal links. Common problems include residual confounding (e.g., lifestyle, socioeconomic status), errors in measuring dietary Intake, and variability in sugar classification systems. Systematic reviews and umbrella reviews highlight heterogeneity among studies and, in some cases, an overall low to moderate quality of evidence [3]. To assess causality, well-designed experimental studies and complementary approaches (e.g., interventional, randomized studies, and causal inference methods like Mendelian randomization) are required, but such evidence is not always available or direct for all clinical questions.

Finally, many observed effects may be mediated by overall energy excess: distinguishing the specific effects of sugar from those resulting from increased caloric Intake is methodologically complex and remains an area of research [1][3].

Editorial Conclusion

Recent literature indicates that reducing free sugar intake, and in particular reducing the consumption of sugary drinks, is a reasonable strategy for the population-level prevention of weight gain and associated metabolic conditions. The evidence does not support absolute formulations or simplifications: the risk depends on dose, frequency, and the overall dietary context. Public health policies (clear labeling, reduction of sugar content in foods, fiscal measures on sugary drinks) and informed food choices can reduce collective exposure. For individual clinical decisions, consultation with healthcare professionals remains crucial.

Editorial Note

This article has been updated to reflect the latest evidence and to improve clarity and transparency. The scientific sources cited have been verified and DOIs are provided in the Scientific Research section for direct consultation. The information provided is for informational purposes only and does not replace personalized medical advice.

SCIENTIFIC RESEARCH

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2. Singh GM, Micha R, Khatibzadeh S, et al. Estimated global, regional, and national disease burdens related to sugar-sweetened beverage consumption in 2010. Circulation. 2015;132(8):639–666. https://doi.org/10.1161/CIRCULATIONAHA.114.010636
3. Umbrella review: Dietary sugar consumption and health: umbrella review. BMJ. 2023;381:e071609. https://doi.org/10.1136/bmj-2022-071609
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7. Systematic review: Consumption of Sweet Beverages and Cancer Risk. A systematic review and meta-analysis of observational studies. Nutrients. (DOI) https://doi.org/10.3390/nu13020516
8. Jensen T, Abdelmalek MF, Sullivan S, et al. Fructose and sugar: a major mediator of non-alcoholic fatty liver disease. J Hepatol. 2018;68(5):1063–1075. https://doi.org/10.1016/j.jhep.2018.01.019
9. Systematic review update on caries: Moores CJ, Kelly SAM, Moynihan PJ. Systematic review of the effect on caries of sugars intake: Ten-year update. J Dent Res. 2022. https://doi.org/10.1177/00220345221082918
10. WHO guideline coverage and context: BMJ summary of WHO recommendations on sugar intake. BMJ. 2015. https://doi.org/10.1136/bmj.h1261