Diet sodas are popular beverages marketed as sugar-free and low-calorie alternatives to regular soda. Almost every popular sugar-sweetened beverage on the market has a “light” or a “diet” version - Diet Coke, Coke Zero, Pepsi Max, Sprite Zero, etc. They were first introduced in the 1950s for people with diabetes, though they were later marketed to people trying to control their weight or reduce their sugar intake. But are these drinks truly a healthier choice? This article delves into the composition of diet soda, its potential effects on weight, diabetes, heart health, and other aspects of health, providing a comprehensive overview of the "Diet Coke Index."
What is Diet Soda?
Diet soda is essentially a mixture of carbonated water, artificial or natural sweetener, colors, flavors, and other food additives. It usually has very few to no calories and no significant nutrition. For example, one 12-ounce (354-mL) can of Diet Coke contains no calories, sugar, fat, or protein and 40 mg of sodium.
While recipes differ from brand to brand, some common ingredients in diet soda include:
- Carbonated water: While sparkling water can occur in nature, most sodas are made by dissolving carbon dioxide into water under pressure.
- Sweeteners: These include common artificial sweeteners, such as aspartame, saccharin, sucralose, or an herbal sweetener like stevia, which are 200-13,000 times sweeter than regular sugar.
- Acids: Certain acids, such as citric, malic, and phosphoric acid, are used to add tartness to soda drinks. They’re also linked to tooth enamel erosion.
- Colors: The most commonly used colors are carotenoids, anthocyanins, and caramels.
- Flavors: Many different kinds of natural juices or artificial flavors are used in diet soda, including fruits, berries, herbs, and cola.
- Preservatives: These help diet sodas last longer on the supermarket shelf. A commonly used preservative is potassium benzoate.
- Vitamins and minerals: Some diet soft drink manufacturers add vitamins and minerals to market their products as healthier no-calorie alternatives.
- Caffeine: Just like regular soda, many diet sodas contain caffeine. A can of Diet Coke contains 46 mg of caffeine, while Diet Pepsi contains 35 mg.
It's important to note that not all sodas that use artificial sweeteners are low in calories or sugar-free. Some use sugar and sweetener together. For example, one can of Coca-Cola Life, which contains the natural sweetener stevia, contains 90 calories and 24 grams of sugar.
Diet Soda and Weight Loss: A Complex Relationship
Because diet soda is usually calorie-free, it would be natural to assume it could aid weight loss. However, research suggests the association may not be so straightforward.
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Several observational studies have found that using artificial sweeteners and drinking high amounts of diet soda is associated with an increased risk of obesity and metabolic syndrome. Scientists have suggested that diet soda may increase appetite by stimulating hunger hormones, altering sweet taste receptors, and triggering dopamine responses in the brain. Given that diet soft drinks have no calories, these responses may cause a higher intake of sweet or calorie-dense foods, resulting in weight gain. However, evidence of this is not consistent in human studies.
Another theory suggests that diet soda’s correlation to weight gain may be explained by people with bad dietary habits drinking more of it. The weight gain they experience may be caused by their existing dietary habits - not diet soda.
Experimental studies do not support the claim that diet soda causes weight gain. In fact, these studies have found that replacing sugar-sweetened drinks with diet soda can result in weight loss. One study had overweight participants drink 24 ounces (710 mL) of diet soda or water per day for 1 year. At the end of the study, the diet soda group had experienced an average weight loss of 13.7 pounds (6.21 kg), compared with 5.5 pounds (2.5 kg) in the water group.
However, to add to the confusion, there’s evidence of bias in the scientific literature. Studies funded by the artificial sweetener industry have been found to have more favorable outcomes than non-industry studies, which may undermine the validity of their results. Overall, more high quality research is needed to determine the true effects of diet soda on weight loss.
Diet Soda, Diabetes, and Heart Disease: Emerging Concerns
Although diet soda has no calories, sugar, or fat, it has been linked to the development of type 2 diabetes and heart disease in several studies. Research has found that just one serving of an artificially sweetened drink per day is associated with an 8-13% higher risk of type 2 diabetes. A study in 64,850 women noted artificially sweetened drinks were associated with a 21% higher risk of developing type 2 diabetes. However, this was still half the risk associated with regular sugary drinks. Other studies have observed similar results. Conversely, a recent review found that diet soda is not associated with an increased risk of diabetes. Also, another study concluded that any association could be explained by the existing health status, weight changes, and body mass index of participants.
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Diet soda has also been linked to increased risks of high blood pressure and heart disease. A review of four studies including 227,254 people observed that for each serving of artificially sweetened beverage per day, there is a 9% increased risk of high blood pressure. Other studies have found similar results. Additionally, one study has linked diet soda to a small increase in the risk of stroke, but this was only based on observational data.
Because most of the studies were observational, it may be that the association could be explained another way. It’s possible that people who were already at risk of diabetes and high blood pressure chose to drink more diet soda. More direct experimental research is needed to determine whether there’s any true causal relationship between diet soda and increased blood sugar or blood pressure.
Diet Soda and Kidney Health: A Potential Risk
Drinking diet soda has been linked to an increased risk of chronic kidney disease. A recent study analyzed the diets of 15,368 people and found that the risk of developing end-stage kidney disease increased with the number of glasses of diet soda consumed per week. Compared with those who consumed less than one glass per week, people who drank more than seven glasses of diet soda per week had nearly double the risk of developing kidney disease.
A suggested cause for the kidney damage is the high phosphorus content of soda, which may increase the acid load on the kidneys. However, it has also been suggested that people consuming high amounts of diet soda may do so to compensate for other poor dietary and lifestyle factors that may independently contribute to the development of kidney disease.
Interestingly, studies investigating the effects of diet soda on the development of kidney stones have found mixed results. One observational study noted that diet soda drinkers have a slightly increased risk of kidney stone development, but the risk was much smaller than the risk associated with drinking regular soda. In addition, this study has not been supported by other research. Another study reported that the high citrate and malate content of some diet sodas may help treat kidney stones, particularly in people with low urine pH and uric acid stones. However, more research and human studies are needed.
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Diet Soda and Pregnancy: Considerations for Expectant Mothers
Drinking diet soda while pregnant has been linked to some negative outcomes, including preterm delivery and childhood obesity. A Norwegian study in 60,761 pregnant women found that intake of artificially sweetened and sugar-containing drinks was associated with an 11% higher risk of preterm delivery. Earlier Danish research supports these findings. A study in almost 60,000 women found that women who consumed one serving of diet soda per day were 1.4 times more likely to deliver preterm than those who did not.
However, recent research in 8,914 women in England did not find any association between diet cola and preterm delivery. However, the authors admitted that the study may not have been big enough and had been limited to diet cola. It’s important to note that these studies were only observational and offer no explanation of exactly how diet soda may contribute to preterm birth.
Furthermore, consuming artificially sweetened drinks while pregnant is significantly associated with an increased risk of childhood obesity. One study found that the daily consumption of diet drinks during pregnancy doubled the risk of a baby being overweight at 1 year of age. Further research is needed to analyze the potential biological causes and long-term health risks for children exposed to artificially sweetened sodas in the womb.
Other Potential Health Effects of Diet Soda
There are several other documented health effects of diet sodas, including:
- May reduce fatty liver: Some studies have shown that replacing regular soda with diet soda can reduce fat around the liver. Other studies have found no effect.
- No increase in reflux: Despite anecdotal reports, carbonated drinks have not been found to make reflux or heartburn worse. However, the research is mixed, and more experimental studies are needed.
- No strong links to cancer: Most of the research on artificial sweeteners and diet soda has found no evidence it causes cancer. A slight increase in lymphoma and multiple myeloma in men was reported, but the results were weak.
- Changes to the gut microbiome: Artificial sweeteners may alter the gut flora, leading to reduced blood sugar control and potentially increasing the risk of type 2 diabetes. One study found all of the six tested artificial sweeteners damaged the gut microbiome in various ways. Another found the way people’s gut flora reacted to artificial sweeteners was highly individualized.
- Increased risk of osteoporosis: Diet and regular cola is associated with bone mineral density loss in women, but not in men. The caffeine and phosphorus in cola might interfere with normal calcium absorption.
- Tooth decay: Like regular soda, diet soda is associated with dental erosion due to its acidic pH level. This comes from the addition of acids, such as malic, citric, or phosphoric acid, for flavor.
- Linked to depression: Observational studies have found higher rates of depression among those who drank four or more diet or regular sodas per day. However, experiments are needed to determine whether diet soda is a cause.
While some of these results are interesting, more experimental research is needed to determine whether diet soda causes these issues, or if the findings are due to chance or other factors.
Diet Soda and Diabetes: A Closer Look
The absence of sugar or calories does not necessarily make diet soda a suitable drink for people with diabetes. A 2018 study included 2,019 people who did not have diabetes at the start of the study. By the follow-up, 368 people had developed type 2 diabetes. The study found that there was a risk of type 2 diabetes among participants who consumed artificially sweetened sodas, as well as those who consumed sugar-sweetened sodas. The study showed that participants with a higher body mass index (BMI) were at an increased risk of diabetes and more likely to be drinking diet soda. Even when using statistical methods to remove the influence of BMI, daily diet soda consumption was a significant predictor of developing diabetes among those participants who had overweight or obesity at baseline.
Another 2018 study also found that consuming more than 4 cans of diet soda per week was associated with a more than twofold risk of proliferative diabetic retinopathy (PDR) in people with diabetes. PDR is an advanced diabetic eye disease that happens when new blood vessels grow in the retina.
Other factors that have come to light regarding the health risks of diet sodas include weight gain and metabolic syndrome, low levels of high-density lipoprotein (HDL), also known as “good” cholesterol, high levels of triglycerides, a type of fat in the blood, high blood sugar levels, larger waistline, high blood pressure. Older 2015 analysis suggests that people who drank at least one soft drink per day had a higher risk of metabolic syndrome compared with people who did not drink soda. This seemed to be particularly evident in consumers of diet soft drinks.
While these are observational results and do not prove a cause-and-effect relationship, diet soda is unlikely to be the best option for people looking to manage or prevent type 2 diabetes. A 2015 study looked at the long-term effects of diet soda on waist size, an indicator of visceral or belly fat. This type of fat increases the risk of chronic disease more than fat in other areas of the body. The study lasted for 9.4 years and included a total of 749 participants over 65 years of age. The waist circumference of participants increased when they drank diet soda for a long period. Participants who drank diet soda on a daily basis showed nearly quadruple the waist gain than those who did not drink it. This shows a long-term link between consuming diet soda and developing belly fat. This, in turn, may increase a person’s risk of type 2 diabetes.
Alternative Sweeteners in Diet Soda: A Potential Concern
With the rise in the popularity of diet soda comes a matching increase in the use of alternative sweeteners. People with diabetes sometimes view these sweeteners as viable alternatives to provide sweet flavoring, as they do not contain sugar. The artificial sweeteners in diet sodas may still promote health risks, though many do not directly raise blood sugar. These risks include affecting the balance of healthy bacteria in the intestines, which may indirectly affect insulin sensitivity and appetite hormones.
The most common alternative sweeteners, whether artificial or natural, in diet sodas are:
- Sucralose: A 2013 study showed this sweetener can raise blood sugar higher when carbohydrates are consumed later compared to those who did not consume any sucralose. Sucralose also causes peaks in insulin levels despite not containing sugar. The brand name is Splenda.
- Aspartame: This is a chemical sweetener found in everything from diet soda to chewing gum. Aspartame can increase body weight and fat mass, which may increase the risk of diabetes.
- Acesulfame Potassium (Ace-K): This is a sweetener commonly used in combination with other sweeteners in beverages and snack foods. Ace-K has been linked to body weight and fat gain in animal studies.
- Sorbitol: This is a sugar alcohol that is common in diet foods and drinks. A 2022 study in mice found that long-term consumption of sorbitol altered the gut microbiome and induced glucose intolerance.
- Stevia: A moderate amount of natural stevia leaf is generally a safe alternative to sugar for people with diabetes.
- Erythritol: This is a corn-based type of sugar alcohol with fewer calories than table sugar but retains a majority of the sweet flavor. Consuming a moderate amount should not affect blood sugar or insulin levels. In fact, erythritol has shown protective effects for people with diabetes, though more research is necessary.
Healthy Alternatives to Diet Soda
There are numerous alternatives that people might prefer to try instead of diet soda.
- Carbonated water with a splash of fruit juice: People who drink soda for the refreshing fizz can choose carbonated water instead. Add a splash of fruit juice, such as lime, lemon, or grapefruit, for that sweet kick. The combination is rich in nutrients and rehydrates the body.
- Unsweetened tea: Unsweetened black tea can be a tasty alternative for people who drink soda for the caffeine boost. Iced, unsweetened black tea is also available and provides the same level of refreshment as a soda. Some research suggests that drinking 3 to 4 cups of coffee per day may also help decrease the risk of type 2 diabetes. Many low caffeine or noncaffeinated herbal teas such as citrus green tea, peppermint, and hibiscus are readily available.
- Stevia leaves: People who crave the sweetness of soda might want to consider sweetening tea or carbonated water with whole stevia leaves.
The Beverage Hydration Index (BHI)
Not all beverages are created equal from a hydration standpoint. Absorption is affected by the amount of fluid ingested, electrolyte and carbohydrate content, and the presence of diuretic agents (substances that promote urine production). The rehydration process is affected both by the volume of fluid ingested as well as the sodium content. It has long been known that the presence of carbohydrates and electrolytes in a drink increase the rate of fluid absorption after drinking.
Similar to how the glycemic index is intended to define the blood-glucose response to the ingestion of foods compared to a white bread or glucose standard, a beverage hydration index (BHI) could serve to quantify water excretion from the kidneys in response to various beverages compared with still water. The cumulative volume of urine passed over a fixed period of time can be measured as a marker of fluid absorption and retention. The volume and amount of electrolytes and sugars of ingested fluids affect their absorption and retention, but various beverages have yet to be systematically compared.
A study examined the effects of 13 different beverages on urine output in order to establish a beverage hydration index. Urine output from ingestion of full-fat milk, skim milk and the oral rehydration solution was lower than that from ingestion of still water, translating to a higher hydration index. To establish a beverage hydration index (BHI), still water was used as a control and is thus assigned a value of 1.0. Because the water content of the drinks varied from 88 to 100 percent, the BHI was calculated with and without an adjustment for the amount of water ingested from each drink.
Using unadjusted values, a hydration index that was higher than water was observed for full-fat milk (1.50), skim milk (1.58), oral rehydration solution (1.54) and orange juice (1.39). However, after adjustment for water content only full-fat milk (1.32), skim milk (1.44) and oral rehydration solution (1.50) were significantly different from water. The electrolyte content of each beverage was also measured. Urine output over four hours was lower after drinking full-fat milk, skim milk or an oral rehydration solution, compared with drinking the same amount of water.
This study takes the important first step toward establishing a beverage hydration index, which can be used to compare the short-term hydration potential of different beverages. This can serve a similar purpose as the glycemic index and insulin index: a way to compare how different liquids get processed and absorbed by our bodies. The drinks with the highest BHI also had the highest electrolyte content. Acute ingestion of a high-sodium beverage has been shown to result in an increase in total body water. People often associate sports drinks with having a high electrolyte content, but in reality they contain less sodium than milk, and less potassium than seven of the other tested beverages, including tea and coffee, making the total electrolyte content of sports drinks quite low.
In contrast to the enhancement of fluid absorption by electrolytes are the known diuretic effects of caffeine and alcohol, which act by inhibiting arginine vasopressin (also called anti-diuretic hormone). No effect from the caffeine was observed from any of the drinks in this study, which ranged from 96 to 212 milligrams. A measurable effect on urine output would typically require more than 300 milligrams of caffeine, while the coffee used in this study was reported as having a caffeine content of 212 milligrams per liter. The alcohol content in one liter of lager beer also did not increase urine output in this study.
This study established a new method for quantifying the effect of various beverages on hydration. No differences were found compared to water after ingestion of cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water or a sports drink. Only full-fat milk, skim milk and an oral rehydration solution led to a reduced urine output after drinking one liter, compared with drinking the same amount of still water.
Diet Soda and Metabolic Syndrome: A Complex Association
Diet soda consumption was assessed by food frequency questionnaire at baseline (2000-2002). Incident type 2 diabetes was identified at three follow-up examinations (2002-2003, 2004-2005, and 2005-2007) as fasting glucose >126 mg/dl, self-reported type 2 diabetes, or use of diabetes medication. Metabolic syndrome (and components) was defined by National Cholesterol Education Program Adult Treatment Panel III criteria. At least daily consumption of diet soda was associated with a 36% greater relative risk of incident metabolic syndrome and a 67% greater relative risk of incident type 2 diabetes compared with nonconsumption (HR 1.36 [95% CI 1.11-1.66] for metabolic syndrome and 1.67 [1.27-2.20] for type 2 diabetes). Of metabolic syndrome components, only high waist circumference (men ≥102 cm and women ≥88 cm) and high fasting glucose (≥100 mg/dl) were prospectively associated with diet soda consumption.
Two longitudinal cohort studies have shown positive associations between diet soda consumption and incident metabolic syndrome independent of baseline measures of adiposity. Artificially sweetened beverages, such as diet soda, are commonly considered “benign” because they contribute no energy and few nutrients to the diet. Consequently, the previously observed diet soda-metabolic syndrome associations are generally speculated to be the result of residual confounding by other dietary behaviors, lifestyle factors, or demographic characteristics.
Biological mechanisms possibly explaining these associations are few and largely focus on artificial sweeteners in beverages/foods increasing the desire for (and consumption of) sugar-sweetened, energy-dense beverages/foods or disrupting consumers' ability to accurately estimate energy intake and remaining energy needs. Thus, diet soda consumption may result in overconsumption, increased body weight, and consequent metabolic dysfunction.
In MESA, diet soda consumption was positively associated with both incident metabolic syndrome and type 2 diabetes. Associations between diet soda and risk of type 2 diabetes were of greater magnitude than the associations observed between diet soda and metabolic syndrome. Consistent with these findings, diet soda was associated with development of high fasting glucose and high waist circumference during follow-up but not with other metabolic syndrome components, suggesting that in this analysis, metabolic syndrome associations were driven more by a pre-diabetic condition than the “syndrome” per se.
The possibility of confounding by other dietary and lifestyle/behavioral factors cannot be excluded from these observational studies. An association between diet soda consumption and subsequent weight gain is plausible. First, it has been hypothesized that artificial sweeteners may increase hedonistic desires for sweetness and more energy-dense foods. Second, overconsumption of other foods/beverages may also occur in conjunction with diet beverage consumption owing to overestimation of the number of calories saved by substituting diet beverages for sugar-sweetened beverages. Third, the association between diet beverages and weight gain may be biased by early awareness of energy imbalance, i.e., diet beverage consumption may serve as a proxy for early (failed) attempts to maintain weight.
In the current study, the associations between diet beverage consumption and risk of type 2 diabetes were attenuated, but remained significant, when adjusted for baseline BMI or waist circumference or changes in body weight or waist circumference across examinations. Therefore, data do not indicate that a change in body weight or fat distribution mediates the association between diet beverage consumption and risk of type 2 diabetes. However, associations between diet soda and metabolic syndrome were strongly attenuated when adjusted for these measures of adiposity. Consistent with these data, only the metabolic syndrome components high waist circumference and high fasting glucose were associated with prospectively reported diet soda consumption.