Obesity leads to a shortened life span and is strongly associated with type-2 diabetes, cardiovascular disease, metabolic syndrome, certain cancers, kidney disease, obstructive sleep apnoea, gout, osteoarthritis and hepatobiliary disease [1,2]. Importantly, reductions in disease risk occur proportionally with the degree of weight loss [1].

Data from NHS England [3] indicate that among adults aged 16 years and over, sixty-eight percent of men and sixty percent of women were overweight or obese in 2019. Among children, eighteen percent of boys and thirteen percent of girls were obese. Globally, the picture is similar. According to the World Health Organization [4], thirty-nine percent of adults aged 18 years were overweight in 2016, with thirteen percent classified as obese.

Excess body weight also impacts negatively on the development of diabetes. While type-1 diabetes is largely autoimmune in origin, body mass influences disease trajectory; type-2 diabetes is more directly associated with excess weight and adiposity [5].

Together, these findings emphasise the urgent need to develop sustainable strategies for weight management.

Although tackling obesity is a multi-stranded issue including dietary habits and exercise provision, from the food sector it includes reducing the calorie density of foods, portion sizes and frequency of eating. However, until now, there has been little focus on controlling the rate at which carbohydrate calories in the food may be digested with the associated physical and cognitive impacts. Sugar reduction in foods with associated sugar taxes in drinks (such as in the UK) do have an impact on calorie provision. Less work is undertaken in terms of product development on reducing the digestibility profile of starch in foods to make it in effect more like unprocessed (native) starch. Starch that is slowly digested (SDS) has a great potential to support calorie reduction strategies as although it provides calories in the diet it does so relatively slowly with associated physiological benefits. Including obesity and associated health issues.

Foods relatively rich in SDS are those in which there is much of the native structure in place and/or where processing induced modification is limited. These include legumes (lentils, chickpeas, beans), whole grains (oats, barley), some pasta, some nuts and seeds and green banana. Note that there is a blur between when starch is described as SDS or resistant starch (RS). Any SDS can be digested in the human small intestine but slowly over many hours whereas RS is transported to the colon and fermented.

The concept of weight loss has been described succinctly by Higgins [6] and Aaseth et al. [2] where they report that for weight loss to be successful it is necessary to adopt a diet that creates a permanently negative and yet acceptable energy balance; where prolonged dietary adherence is crucial. Others have described how reduced energy input needs to be associated with increased energy output to be most effective [1]. Weight loss improves blood glucose profile abnormalities in obese patients with type-2 diabetes [7].

Although some evidence suggests that a low glycaemic load but relatively high-fat diet may facilitate weight-loss maintenance [8], intervention ultimately centres on calorie control, as all macronutrients (and ethanol) provide energy that must be regulated [9].

In adults, reduced intake of dietary sugars is associated with decreased body weight and vice versa [10]. However, where sugars are replaced isoenergetically with other carbohydrates, no comparable change in body weight is observed. This supports the view that excess calories - including those from dietary sugars - promote obesity, rather than any specific molecular effect of sugars themselves [11,12]. Country-specific weight-control registries similarly recognise the need to reduce calories from digestible carbohydrates to manage body weight [13].

Thus, while calorie excess is central to obesity, the metabolic handling of those calories warrants closer examination.

Although energy balance reflects calories from all dietary sources - digestible carbohydrates 3.75 kcal/g, dietary fibre (classified often now in the literature as providing the equivalent of 2.00 kcal/g through colonic, fermentation), protein 4.00 kcal/g, alcohol 7.00 kcal/g and fat 9.00 kcal/g - many diets focus on the carbohydrate portion (restriction) as the major approach to reduce calorie intake and ultimately body weight.

There is increasing emphasis on plant-based eating patterns while limiting calories from digestible carbohydrates - including sugars, amorphous starch and starch derivatives. The evolution of man has led to several factors which increase the availability of digestible carbohydrates in the diet. This, often at the detriment of carbohydrates which function as dietary fibre (with the associated gut health and systemic impact). Contributing factors include food availability and affordability, pre-prepared foods, fast food consumption, sugary drinks, snacking between meals, excess calorie intake and insufficient physical activity.

Blüher [14] has captured the issues associated with obesity very succinctly in the recent review with discussion around the clear doubly impacting imbalance (albeit with regional variation) between (i) excess food supply and (ii) increased sedentary behaviour. Comparable recent obesity/diabetes related reviews identify similar cause and effect on body weight [15,16,17].

Digestible sugars and amorphous starch derivatives provide approximately 3.75 kcal/g energy for the body. In the UK, added and free sugars contribute between seven and thirteen percent of total energy intake [18]. The World Health Organization recommends limiting free sugars to less than ten percent of total energy intake, ideally below five percent [19]. The UK Scientific Advisory Committee on Nutrition similarly recommends no more than five percent of total energy intake should come from free sugars [20].

Definitions of free and added sugar [20] being:

The role of dietary sugars - including their relative glycaemic index and origin (such as glucose derived from starch digestion) - in health and disease has been reviewed in several recent publications [21,22,23]. The links between digestible carbohydrates and health are largely defined by their effects on blood glucose concentrations, obesity, and related metabolic disorders.

Sugars are rapidly absorbed from the intestine whereas less rapidly digested carbohydrates provide slow and sustained release of blood glucose along with the overall health benefits resulting from low glycaemic and insulinemic response [24]. A dysregulation in energy provision (includes amount and timing)/metabolism is intricately associated with the pathogenesis of a range of disorders including neurological, cardiovascular, metabolic syndromes, autoimmune disorders and cancer [25]. At the cellular level this (disfunction) reflects [25]:

Substantial evidence indicates that dietary fibre supports a favourable gut microbiota composition, conferring benefits to the host through a bidirectional symbiotic relationship that underpins both physical and psychological health. Conversely, diets high in sugars and refined carbohydrates have been linked to the development and progression of metabolic syndrome and related chronic diseases, as well as detrimental effects on mood and mental well-being [26,27]. Furthermore, indigestible carbohydrates are of considerable physiological importance, enhancing gastrointestinal transit and acting as prebiotic substrates for beneficial microbial populations. Their fermentation by the gut microbiota generates short-chain fatty acids (SCFAs) and other bioactive metabolites that play key roles in maintaining intestinal and systemic health [28].

The regulation of body weight and the inter-individual variability in weight-related outcomes are determined by a complex interplay between genetic polymorphisms, epigenetic modifications, and alterations in hormonal signalling pathways and metabolic processes. These factors are further modulated by lifestyle behaviours, including nutritional intake, physical activity, sleep quality, and stress exposure, which collectively influence energy homeostasis and long-term body weight regulation [29].

Blood glucose responses following oral glucose tolerance tests differ markedly between metabolically healthy individuals and those with insulin dysregulation [30,31]. Several nutrition-related disease states have been associated with excessive consumption of digestible starch, particularly in the context of obesity and its contribution to the development of type 2 diabetes and cardiovascular disease [32]. As starch constitutes a major source of dietary energy and generally exerts limited effects on satiety, attention to the quantity consumed is important in preventing excessive energy intake and subsequent weight gain [32].

However, the relationship between dietary carbohydrates and metabolic disease is complex. Veit et al. [33] reported that current evidence does not support a direct association between dietary sugar intake and the incidence of type 2 diabetes. Notably, certain long-chain α-glucans may elicit a greater glycaemic response than sucrose. The authors concluded that excess energy intake and the consequent accumulation of body fat are more important determinants of diabetes risk than the consumption of specific carbohydrate types alone [33].

Importantly, digestible carbohydrates do not exert uniform metabolic effects. Their physiological impact is influenced by structural characteristics that determine the rate and extent of digestion, absorption, and subsequent glycaemic responses. Consequently, carbohydrate quality, in addition to quantity, is an important consideration in the prevention and management of metabolic disorders.

Starch digestibility is largely determined by its amorphous–crystalline ratio and its accessibility to digestive enzymes. Amorphous starch is digested rapidly and almost completely and can generate a high glycaemic index [34,35]. Any SDS also contributes to the overall glycaemic response; however, by convention, the GI is determined from blood glucose measurements collected over a two-hour period following ingestion. As a result, a proportion of the glycaemic response attributable to more slowly digested starch fractions may not be fully captured within the standard assessment period. Consequently, GI values may not completely reflect the total glycaemic potential of SDS. Although SDS may ultimately produce a similar cumulative glycaemic exposure, expressed as the area under the blood glucose response curve (AUC), to an equivalent anhydrous glucose load (adjusted for starch moisture content and water of hydration), this response is typically distributed over a longer time frame than the conventional two-hour measurement period. In contrast, non-amorphous starch exhibits varying capacities to resist digestion and is defined collectively as “resistant starch”, of which five forms have been described [35,36,37,38] (Table 1).

Three principal fractions of starch were defined by Englyst et al. [39] (Table 2):

The in vitro classification system developed by Englyst and colleagues over many years has, for the most part, correlated successfully with in vivo trials [42]. Within this framework: RDS corresponds to digestion within twenty minutes (reflecting residence time in the mouth and small intestine residence); SDS is digested between twenty to one hundred and twenty minutes (reflecting small intestinal digestion); and RS resists digestion for more than one hundred and twenty minutes and passes into the colon.

Thorburn et al. [43] reported that there was a good correlation between starch digestibility and blood glucose concentration, linking starch structure directly to metabolic response. The Body Mass index (BMI) is often considered an indicator of adiposity [44]. Several studies have identified a positive correlation between fasting blood glucose levels and BMI [45,46,47], including in elderly populations [48]. Consistent with the metabolic relevance of glycaemic response, BMI has also been reported to correlate positively with dietary glycaemic index [49]. Furthermore, reducing dietary energy derived from carbohydrates, or lowering the glycaemic index of the diet, has been shown to promote fat loss and improve cardiovascular risk factors [50].

However, some findings appear contradictory. In cases of iatrogenic (investigation- or procedure-induced) hypoglycaemia among individuals with type 2 diabetes, patients with lower BMIs exhibited lower plasma glucose concentrations [51]. Similarly, Plečko et al. [52] reported that a BMI above normal was an independent predictive factor for reduced risk of hypoglycaemia in both diabetic and non-diabetic patients, with the converse also observed. These observations highlight the complexity of interactions between starch digestibility, glycaemic response, BMI, and metabolic regulation.

The balance between starch fractions that are readily digested and those that resist digestion is illustrated in Figure 1. Dextrins represent hydrolysed starches that are predominantly amorphous and therefore readily digested, although some may be modified to restrict digestion.

Importantly, some dietary intervention trials comparing starch and sugars do not clearly distinguish between starch forms or account for differences in digestibility, which complicates interpretation of body weight and metabolic outcomes [10]. These features are highlighted in Table 3.

Indigestible carbohydrate refers to carbohydrate that resists digestion in the human small intestine. However, some forms, such as non-starch polysaccharides and RS, are fermented in the large intestine to produce short-chain fatty acids, which provide approximately 2 kcal/g of energy to the body. Common types of indigestible carbohydrates are presented in Table 4. These carbohydrates are more commonly referred to as dietary fibre.

Although the molecules classified as dietary fibre differ chemically in their sugar residue structures, residue modifications, bonding patterns, and molecular size, they are traditionally sub-classified nutritionally according to their solubility. More recently, however, colonic fermentability has been proposed as a more meaningful primary classification criterion than solubility [54,55].

Both dietary fibre and RS play important roles in body weight regulation and weight loss, as well as in the prevention and management of type 2 diabetes [56,57].

Chew and Brownlee [58] reviewed studies on the impact of dietary fibre - and fibre supplementation especially - on body weight/weight loss. Overall, they found that higher dietary fibre intake was associated with improved long term weight management, although the effectiveness of supplementation varied according to fibre type. Evidence of benefit was reported for glucomannan, chitosan, fibre complexes derived from Opuntia ficus-indica (prickly pear cactus; often enriched with acacia fibre), and soluble fibre blends containing konjac, sodium alginate, and xanthan gum. However, the authors noted that methodological differences among studies may have influenced the reported outcomes.

Several mechanisms have been proposed to explain the effects of dietary fibre on weight management [58]. These include: (i) increased satiety and satiation through bulking and intragastric gelation with potential regulation of cephalic and peripheral satiation-signalling hormones; (ii) decreased calorie uptake resulting from the indigestibility of fibre and its potential interference with macronutrient digestion and absorption; (iii) stimulation of the excretion of bile acids driving metabolic flux; (iv) increased energy expenditure during digestion itself and; (v) providing a substrate that alters the gut microbiota or the profile of fermentation by-products.

As an energy source, starch - particularly its slowly digestible and resistant forms - is considered significantly healthier than sugar when comparing their respective effects on physiological parameters, especially body weight [6,40,41,59,60,61,62,63,64].

Consumption of SDS has been associated with improved calorie control through attenuation of the postprandial glycaemic response and prolongation of satiety [65]. SDS-rich foods are digested more slowly than conventional starches, resulting in more stable blood glucose concentrations and a sustained feeling of fullness. By extending satiety and reducing subsequent food intake, SDS may contribute to lower overall energy consumption and reduced weight gain [65].

The literature, including the detailed review by Raghunathan et al. [66], suggests that the effects of SDS on weight management are mediated through three interrelated physiological mechanisms:

Thus, overall, in theory, low glycaemic index (GI) foods, such as those containing SDS, may support body weight management by promoting satiety and enhancing fat oxidation [24]. SDS has also been suggested to exert a protective effect against diabetes [43].

When considering the relationship between SDS and the overall GI, it is important to recognise that the two are not synonymous. The SDS does not necessarily result in a low GI, as GI is influenced not only by the rate of starch digestion but also by the rate of blood glucose clearance [67].

The influence of RS, a dietary fibre fraction, on body weight management has been discussed above (Section 5). Furthermore, RS supplementation has been associated with weight loss and improved insulin sensitivity in overweight individuals, effects that appear to be mediated, in part, through beneficial modulation of the gut microbiota [68].

Both SDS and RS have been described by some authors as nutraceuticals because of their associated health benefits [69]. However, the utilisation of these starch fractions - particularly SDS - must take into account the impact of processing on their functionality and, consequently, their digestibility.

The SDS form of starch may blunt the postprandial rise and subsequent decline in plasma glucose and insulin concentrations, leading to prolonged energy availability and enhanced satiety [70]. As discussed in some detail in Section 6. This is therefore a mechanism that works after digestion, rather than through the control of digestive enzymes or subsequent retardation of glucose transfer through the gut into the blood stream. Similar conclusions have been drawn by Goux et al. [71] in terms of the two factors which they consider the digestive impact of SDS on blood glucose concentrations:

The relative contributions of starch fractions to glycaemic responses have been investigated by determining the glycaemic index (GI) of a range of cereal-based food products. Meynier et al. [72] demonstrated that glucose availability was strongly correlated with both GI and glycaemic response. Approximately 53% of the variability in GI could be explained by the digestibility profiles of RDS and SDS, together with dietary fat and fibre content. Among these factors, SDS was identified as the principal determinant of glycaemic response, accounting for 17% of the observed variance, compared with only 6% for RDS.

Human intervention studies have further demonstrated the physiological effects of SDS-rich foods. Compared with extruded cereal products, SDS-rich biscuits slowed glucose release and its subsequent appearance in the peripheral circulation, attenuated postprandial glucose excursions, and promoted a more sustained distribution of glucose absorption following breakfast [73]. These effects were accompanied by lower postprandial concentrations of glucose-dependent insulinotropic peptide (GIP) and insulin [73].

The influence of SDS on appetite regulation has also been examined. Vinoy et al. [74] reported that cereal products with a high SDS content reduced postprandial glucose and insulin responses while enhancing subjective feelings of satiety following breakfast.

The SDS form of starch has been shown to activate the gut - brain axis, thereby contributing to the regulation of energy intake [75]. In rodent models, SDS consumption alters feeding behaviour, characterised by reductions in both meal size and meal frequency. These behavioural changes are accompanied by decreased hypothalamic gene expression of orexigenic neuropeptides and a concomitant increase in the expression of anorexigenic neuropeptides, suggesting a central mechanism through which SDS modulates appetite and satiety.

Hypoglycaemia, or low blood glucose, poses a significant risk to the body, which relies on glucose as its primary energy source. It is most commonly associated with insulin therapy [76] although inborn errors of metabolism, such as certain forms of glycogen storage disease (GSD), can also induce hypoglycaemia [35,77]. Other conditions that can cause hypoglycaemia include hormone deficiencies and kidney failure [78]. In addition, prolonged or intense exercise may also induce hypoglycaemia [77]. Slowly digestible starch (SDS) can help restrict hypoglycaemia over extended periods [35]. In addition to its physiological effects, consumption of SDS has also been associated with improvements in cognitive performance ([79,80], see below).

Hyperglycaemia can arise from several factors, including peripheral and hepatic insulin resistance, increased hepatic and renal glucose production, and high glucose loads from enteral feeds or intravenous infusions [81]. It is an independent predictor of mortality where the condition may induce decreased cerebral blood flow, intracellular acidosis and reduced adenosine tri-phosphate concentrations which may be evident in diabetes [81]. High intakes of digestible carbohydrate may accentuate hyperglycaemia in type-2 diabetics [82].

Evidence from animal studies supports a potential role for SDS in moderating these effects. Using streptozocin induced diabetic mice, Chen et al. [83] demonstrated that SDS feeding can improve the status with respect to hyperglycaemia and hyperlipidaemia. Similarly, human studies have shown that a diet containing a high proportion of SDS can improve glycaemic variability and reduce postprandial glycaemic excursions in patients with type-2 diabetes [84].

Unlike physiological health outcomes, where the relationship between blood glucose concentration and health parameters is increasingly well understood, the impact of glucose on cognition remains less clearly defined, although understanding is developing [85]. Both hypoglycaemia and hyperglycaemia impair cognitive performance, whereas maintaining a balanced and regulated blood glucose concentration, such as that provided by SDS, optimises cognition function [79,80,86,87,88,89,90,91]. The mechanisms linking blood glucose concentration to cognition and subsequent calorie consumption are not fully understood. However, it is reasonable to hypothesise that sustaining moderate blood glucose concentrations, as achieved with SDS, supports both brain and body functionality, including body weight regulation. The concept of the brain acting as a “glucostat” has been the subject of debate for a very long time [85,92,93].

Studies in obese rats have indicated that SDS consumption can reduce daily food intake and in parallel suppress the expression of appetite-stimulating neuropeptide genes associated with the gut - brain axis [75]. These findings suggest that SDS-containing foods could be developed strategically to support obesity management, representing an intriguing opportunity for food producers, nutritionists, and consumers.

In their recent review of the role of carbohydrates in cognitive function, Arshad et al. [94] discussed the potential cognitive benefits of slowly digestible carbohydrate sources, although without explicitly referring to slowly digestible starch (SDS). The authors noted that “Whole grains and legumes are complex carbs [sic] that gradually release glucose, supporting long- term cognitive function and reducing fatigue when performing cognitively demanding tasks.” This perspective is consistent with the broader literature, which suggests that carbohydrate sources providing a sustained release of glucose may support cognitive performance by promoting greater glycaemic stability. However, while the proposed relationship between SDS-rich foods and cognitive function is biologically plausible, further well-controlled studies are required to better define the magnitude, mechanisms, and scope of these effects.

The obesity epidemic is driving the food and drink sector to develop products with reduced caloric content through modifications to ingredients and portion sizes, alongside efforts to lower sugar content and alter carbohydrate digestion profiles more generally, including increasing dietary fibre content. By incorporating greater amounts of SDS into food products at the expense of sugars and RDS, energy remains available as glucose in the diet but is delivered in a more measured fashion, with the associated health benefits of reducing sugars and RDS.

The challenge is to ensure that pre-prepared foods remain sensorially desirable while making carbohydrate less readily and rapidly available during digestion. Interest in this area is growing within the food sector, with baked products, particularly biscuits, increasingly being developed with a high SDS content in mind. At present, however, it is not possible in most jurisdictions to make health claims relating to the rate of carbohydrate digestion. This reflects a number of factors, including inter-individual variation, differences in the amount and timing of consumption, interactions with other dietary components, and consumer-induced changes resulting from further processing or preparation.

Nevertheless, there is little doubt that the food industry is having to move beyond the post-war mindset of simply providing calories and protein towards a greater emphasis on controlling nutrient delivery and, in effect, helping to manage weight-related issues. This is occurring in parallel with the global uptake of medications designed to manage body weight by promoting rapid feelings of satiety and thereby reducing calorie intake. These include agonists of the incretin gut-derived hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), such as tirzepatide, which are already having a significant impact on diabetes management and the demand for bariatric surgery.

Controlling the rate at which glucose is released from the digestive tract into the bloodstream promotes a more stable state of glucose homeostasis. When glucose enters the circulation at a moderated pace, the body can balance more effectively its utilisation for immediate energy with its storage as glycogen. This improved metabolic regulation can positively influence body weight, BMI, and other physiological markers linked to conditions such as diabetes and cardiovascular disease. In addition, more stable glucose availability may also confer benefits for cognitive function.

From a food production perspective, enhancing the SDS content of food requires careful control of water to starch ratios and cooking times to restrict starch gelatinisation. Although native starches are not usually desirable to eat, they are relatively abundant in certain food categories, such as some breakfast cereals and biscuits. Expanding the range of such products could support improvements in health, particularly in relation to weight management. Taxation has already impacted on the sugar content of soft drinks. There may also be an argument to tax foods (including ultra-processed) which are high in glycaemic response. While this is not correlated directly or inversely with SDS content, it reflects the broader need to control calorie intake (amount, frequency etc.) and the form in which carbohydrate derived calories are delivered within the diet.