A new review explains how they are metabolised
Low calorie sweeteners are well known for their most important benefit – providing sweet taste to food and drinks with no, or virtually no, calories. While this feature is common to all low calorie sweeteners, they are all different substances and, as such, differences exist in how they are metabolised. A new review paper, by Magnuson et al.1, published in Nutrition Reviews describes the way in which the human body handles each of these sweeteners.
But why is it important? Why should health professionals know what happens to low calorie sweeteners when they are consumed? Magnuson and her co-authors explain: “Understanding the biological fate of the different low and no-calorie sweeteners is helpful in evaluating whether reports of biological effects in animal studies or in humans are indicative of possible safety concerns“. This information can be helpful to health professionals when patients have questions about the safety and usefulness of low calorie sweeteners during consultations, and to people engaged in research to understand the metabolism of these ingredients.
In the article below, we have summarised the information presented in the new review.
Setting the stage: How low calorie sweeteners’ metabolism fits into regulatory safety assessments
The authors describe how, prior to the regulatory approval of low calorie sweeteners, extensive studies are undertaken to understand their biological fate in the body (this is called toxicokinetics). These include studies on their absorption, distribution, metabolism, and excretion (ADME) after ingestion. Understanding the toxicokinetics of the individual low calorie sweeteners is a prerequisite for their safety assessment and subsequent approval by the food safety authorities around the world, including the European Food Safety Authority (EFSA).
Understanding the metabolic route of the individual low calorie sweeteners is the key to extrapolating results of pre-clinical safety studies conducted in animals to human risk assessment, and the establishment of an Acceptable Daily Intake (ADI). The ADI is an important, but often misinterpreted value. It is defined as the amount of a food additive, expressed on a body weight basis, that can be consumed daily over a lifetime without appreciable health risk2. The ADI is not a threshold between safe and unsafe; rather, it is a calculated value, that is markedly, and in the case of low calorie sweeteners, 100 times lower than the No Observed Adverse Effect Level (NOAEL) in animal model studies. Setting the ADI at a level 100 times less than the NOAEL in animal studies allows for variations such as interspecies differences and sensitive populations. Regulatory authorities around the world have used this approach for many decades, and it has proven to be reliably protective of public health (Rulis and Levitt).
How are low calorie sweeteners metabolised in humans?
The review examines the metabolic fate of five of the most commonly used low calorie sweeteners: acesulfame-K, aspartame, saccharin, sucralose and steviol glycosides.
The authors note that the five sweeteners are very diverse in their chemical structure, which is why each is metabolised differently. That said, there are commonalities in how some of the sweeteners are treated by the human digestive system.
- Saccharin, acesulfame-K (Ace-K), and sucralose do not undergo digestion in the gastrointestinal tract following consumption. All are quickly eliminated from the body with no accumulation.
- Aspartame and steviol glycosides (stevia) both undergo changes within the digestive tract, and the resulting products are absorbed. Aspartame is completely digested to its component parts, all of which occur in the diet, and so utilised by the body in exactly the same way as the same components in other foods. Steviol glycosides are partly digested by the gut microflora, to stevia which is absorbed, metabolised by the liver, and excreted in the urine.
There is more, however, to understanding the full picture with regard to the metabolism of each of these sweeteners.
Acesulfame-K (Ace-K): Once ingested, acesulfame potassium (Ace-K) is rapidly and almost completely absorbed unchanged into the systemic circulation and then excreted, primarily by the kidneys into urine. It is not metabolised in humans or animals. Click here for more information about acesulfame-K.
Aspartame: Aspartame is made of aspartic acid and phenylalanine, two amino acids used in protein synthesis. Once ingested, it is completely broken down in the gastrointestinal tract to these amino acids and methanol. These are then absorbed into the body in exactly the same way as from common foods like meat, fish and eggs, which are sources of protein and therefore amino acids, and fruit or vegetables which are sources of methanol. The amount of any of these dietary components is trivial compared to that consumed from other food and beverage sources. Because aspartame is digested just like other food, it is caloric. However, because it is so sweet it is used in tiny quantities, so the calories contributed are negligible. Click here for more information about aspartame.
Saccharin: 85% to 95% of ingested saccharin is absorbed and eliminated in the urine, with the remainder excreted in the faeces. Saccharin undergoes no digestion or other metabolic change either in the gastrointestinal tract or following absorption. Click here for more information about saccharin.
Sucralose: Following consumption most sucralose is unabsorbed and excreted unchanged in the stool. A small amount is absorbed which is not metabolized for energy. About 2% of what is consumed undergoes metabolism, to products of no safety significance, and after absorption there is no breakdown of sucralose for energy. Click here for more information about sucralose.
Steviol glycosides (stevia): Stevia leaf extract is a low calorie sweetener derived from the plant Stevia rebaudiana Bertoni, which contains one or more sweet-tasting compounds called steviol glycosides. Steviol glycosides are poorly absorbed in the body and pass through the upper gastrointestinal tract, including the stomach and small intestines, fully intact. Once the glycosides reach the colon, gut bacteria hydrolyse them to steviol. Steviol is then absorbed into the bloodstream and subsequently metabolised by the liver to steviol glucoronides, which are then excreted intact in the urine. Click here for more information about steviol glycosides.
This review underlines how much is understood about how we metabolise the most widely used low calorie sweeteners and underlines why we can have confidence not only in their safety but also in their innocuity. To quote the authors: “As LNCSs [low and no calorie sweeteners] have the potential to be useful tools in the management of diabetes and excessive caloric intake, it is critical to use the existing knowledge of the absorption, metabolism, and excretion of these compounds to address the controversies surrounding their use. In many cases, safety concerns about existing LNCSs can be addressed with a basic understanding of the differences in various LNCSs, the metabolism of LNCSs, and the low systemic exposure to these compounds after their ingestion in foods“.
The publication of this paper has been supported with an unrestricted grant by the Calorie Control Council.