Obesity is a global issue, with figures last year from the World Health Organisation showing that worldwide rates have more than doubled since 1980.
In England, 62.8% of all adults and 30.3% of all children were categorised as overweight or obese in 2011, according to the Health and Social Care Information Centre. Excess weight and obesity are associated with increased risk of disease development, including type II diabetes mellitus, cardiovascular disease and cancer. Therefore, it is important to develop strategies to reduce or prevent weight gain. Key influences on body weight have been identified as physical activity, diet, genetics, thermogenesis and population demographics.
Interventions designed to modify dietary intake have been shown to positively affect body weight, with subsequent improvements in measures of health. There is widespread interest in the development of foods that can contribute to weight-loss strategies. In particular, a number of research studies have been carried out to investigate the effects of various foods on satiety.
An appetite for alginates
Satiety is defined as the inhibition of appetite following an eating episode. Another important term is ‘satiation’, which refers to the satisfying of appetite during a meal, leading to meal termination. Many physiological and psychological factors may control satiation and satiety (see image 3).
The consumption of dietary fibre has been associated with increased feelings of satiety and a decrease in subsequent energy intake. In particular, soluble fibres such as alginates are thought to influence satiety in this way.
An alginate is a soluble fibre extracted from the cell walls of some species of brown seaweed, where it performs a structural role. Sodium alginates (formed during extraction of alginate from seaweed) are used as an ingredient in the food industry due to their gelling, viscous and stabilising properties.
Gelling of alginates can occur either when pH is below 3.5 (acidic gelation), or in the presence of multivalent cations, such as calcium (ionic gelation). The acidic conditions of the stomach may be sufficient to cause gelation, but this environment can be altered depending on the properties of the food bolus. For example, a large volume of water may inhibit complete gel formation by diluting the pH level of the stomach.
Instead, ionic gelling can be ensured by the addition of calcium salts (for example, calcium carbonate) to food preparations. It has also been reported that ionic gelation is preferable, rather than relying solely on acidic gelation, because stronger gels are formed.
Alginates contain guluronic acid (G) and mannuronic acid (M) in varying concentrations. These uronic acids may be grouped in G or M segments, or may form an alternating block of MG residues. The strength of the gel formed, and viscosity of the fibres, is dependent on the guluronic acid content of the alginate. Optimal hydration is also considered an important factor in gelation.
The ability of alginate to gel appears to be a key factor in controlling appetite and it is likely that the gel properties – volume, strength and viscosity – decide the impact on gastric emptying rate and gastric distention. Gastric distention is known to regulate food intake via signals sent from the stomach to the brain, which result in perceptions of fullness and satiety. Therefore an increase in gastric distention and slowing down of gastric emptying rate may enhance or prolong feelings of satiety.
But what is the evidence to support a role for alginates in satiety control? The potential of alginates to influence satiety, appetite and energy intake has been tested in a number of intervention studies. Key factors to consider when comparing the results are differences in the alginates used (quantity, viscosity and guluronic acid content) and the test food matrix (liquid, solid or capsule).
In terms of the food matrix, many studies have used a water-based test vehicle. Alternatives include a milk-based drink, cookies and a cereal bar. Another study used bread to deliver alginate, although in this case, whole seaweed was incorporated rather than solely alginate.
In a review of the efficacy of alginate supplementation on appetite, following acute (single meal) alginate intake, reported feelings of appetite appear reduced, although the effects on subsequent food intake are less clear. After short-term intake (less than a week) it appears that there is some evidence for suppression of food intake when alginate is provided in a hydrated form. This effect is not reproduced in solid foods or capsules, supporting the requirement for adequate hydration in order for gelation to occur. Food or capsule breakdown may also have been inadequate, thus reducing alginate, acid and/or calcium interactions.
Finally, in a 12-week study that assessed the influence on weight loss of longer-term alginate supplementation and an energy-restricted diet, positive changes were seen in body weight and body composition.
Additionally the effect of alginate in combination with another fibre (guar gum) has been assessed. No effect on appetite or energy intake was observed. A number of explanations are provided:
- fibres were incorporated into a solid food (cereal bar) which impaired the effect
- alginate content (1.1g) was lower than in other studies showing a positive effect (1.6-3.73g)
- the types of alginate and guar gum chosen were not effective.
Challenges and future work
It is apparent that the functionality of alginates in solid matrices requires further investigation. Current research indicates that the most consistent effect occurs when alginates are incorporated into a water-based beverage. The lack of effect in solid matrices may be due to impairment of gelling, perhaps due to inadequate hydration or reduced exposure of alginate within the stomach.
Despite these results, it is desirable to create alginate-containing foods, rather than beverages, as it has been reported that very viscous or solid foods are expected to result in greater satiation/satiety than a clear beverage. Such prior beliefs and associations can impact on satiety and satiation. Therefore, the potential of different matrices and product formulations should be investigated in order to create efficacious alginate-containing solid foods.
Another key challenge surrounds the production of a palatable product that provides optimal satiation. Highly viscous alginates are reported to have the greatest efficacy, but consumers may be intolerant of very viscous products. Improved palatability can be obtained by designing a product that gels on contact with the acidic stomach environment. This will induce acidic gelation and/or ionic gelation if an acid-soluble calcium salt is used.
However, there is a risk that the gastric juice will not reach a pH lower than 3.5 before the bolus leaves the stomach, which may result in inadequate gelling. Furthermore, a study has reported the potential for early gel formation in the mouth due to the presence of water and divalent cations in the saliva. Early gelation is also reported to be problematic in liquid products where calcium is available in the presence of alginate.
Sodium alginate is the most commonly used type of alginate. In the UK, the Department of Health has set targets for salt reduction, with the aim to reduce consumer intake of salt to no more than 6g a day. Therefore, the food industry is under pressure to minimise the sodium content of their products.
In a 12-week study involving daily supplementation with 45g of sodium alginate a day, weight loss was observed; however, there was no subsequent reduction in blood pressure. The authors speculate that sodium intake may have cancelled out the usual blood-pressure-lowering effect of weight loss. Therefore, the sodium content of alginate-containing foods should be considered during product development.
Alginates show promise as a satiety-enhancing functional ingredient. Further research is required and should focus on the type and quantity of alginate used, viability of different food and beverage matrices, improvement of palatability, sodium content of products containing sodium alginate and monitoring of gelation within the stomach.
