To investigate the effects of cold storage and vinegar addition on glycaemic and insulinaemic responses to a potato meal in healthy subjects.
Subjects and setting:
A total of 13 healthy subjects volunteered for the study, and the tests were performed at Applied Nutrition and Food Chemistry, Lund University, Sweden.
Experimental design and test meals:
The study included four meals; freshly boiled potatoes, boiled and cold stored potatoes (8°C, 24 h), boiled and cold stored potatoes (8°C, 24 h) with addition of vinaigrette sauce (8 g olive oil and 28 g white vinegar (6% acetic acid)) and white wheat bread as reference. All meals contained 50 g available carbohydrates and were served as a breakfast in random order after an overnight fast. Capillary blood samples were collected at time intervals during 120 min for analysis of blood glucose and serum insulin. Glycaemic (GI) and insulinaemic indices (II) were calculated from the incremental areas using white bread as reference.
Cold storage of boiled potatoes increased resistant starch (RS) content significantly from 3.3 to 5.2% (starch basis). GI and II of cold potatoes added with vinegar (GI/II=96/128) were significantly reduced by 43 and 31%, respectively, compared with GI/II of freshly boiled potatoes (168/185). Furthermore, cold storage per se lowered II with 28% compared with the corresponding value for freshly boiled potatoes.
Cold storage of boiled potatoes generated appreciable amounts of RS. Cold storage and addition of vinegar reduced acute glycaemia and insulinaemia in healthy subjects after a potato meal. The results show that the high glycaemic and insulinaemic features commonly associated with potato meals can be reduced by use of vinegar dressing and/or by serving cold potato products.
The Swedish Agency for Innovation Systems (Project No P11900-3 A) and Öresund Starch Profiles (ÖSP).
Today we see a worldwide increase in the prevalence of diseases linked to insulin resistance. Although sedentary lifestyle play a major role, the characteristics of the diet is highly involved in the genesis of the insulin resistance syndrome. One quality parameter of the diet, which is increasingly implicated in relation to the insulin resistance syndrome is the glycaemic index (GI). The GI classifies carbohydrate-rich foods according to their blood glucose raising effect (Jenkins et al, 1981). Several studies have shown beneficial effects with low GI diets in the prevention and treatment of insulin resistance (McKeown et al, 2004) and related diseases, for example, cardiovascular disease (Liu et al, 2000) and type II diabetes (Salmeron et al, 1997a, 1997b).
The GI of carbohydrate foods can be optimised by the modulation of certain food factors affecting the rate of glucose delivery to the blood by, for example, reducing the gastric emptying rate and/or by delaying the digestion and absorption of carbohydrates in the small intestine. This may be achieved by choosing specific raw materials and/or processing conditions that influence the bioavailability of the carbohydrate substrate. Although not a prerequisite, food factors that reduce GI also frequently lead to formation of resistant starch (RS) (Granfeldt et al, 1995; Björck et al, 2000). RS is the starch fraction which is not absorbed in the small intestine of healthy individuals (Asp, 1992) and thus, provide the colonic microflora with a fermentable carbohydrate substrate, which promote colonic production of short-chain fatty acids (SCFAs). Studies indicate that especially butyrate is formed during colonic fermentation of RS (Scheppach et al, 1988). Butyrate is considered to be an important substrate for the colonocytes (Roediger, 1980). Furthermore it is not only associated with benefits in relation to colonic health in general, but also with favourable symptoms in patients with ulcerative colitis (Brouns et al, 2002; Hallert et al, 2003). Several studies have shown increased amounts of RS upon retrogradation of starchy foods, for example, in boiled potatoes after storage in a refrigerator (Englyst & Cummings, 1987; Kingman & Englyst, 1994; Åkerberg et al, 1998). Potato products are generally regarded as high GI foods (Soh & Brand-Miller, 1999; Foster-Powell et al, 2002) and the possibility to simultaneously lower the glycaemic response and increase the RS content offers an interesting way of improving the nutritional value of the potato.
Another factor that can be exploited to improve glycaemia and insulinaemia to starchy foods is addition of organic acids, such as acetic acid (Ebihara & Nakajima, 1988). Both Brighenti et al (1995) and Liljeberg and Björck (1998) found a significant lowering of the glycaemic responses in healthy subjects when adding acetic acid, in the form of vinegar, to starchy meals. Furthermore, vinegar addition to a test meal composed of a white bagel, butter and orange juice was shown to improve acute postprandial insulin sensitivity in insulin resistant subjects (Johnston et al, 2004), as measured using the procedure by Matsuda and DeFronzo (1999).
In the present study, the effect of cold storage of potatoes (8°C, 24 h) and/or addition of vinegar (28 g/292 g potatoes) on acute blood glucose and insulin responses to a potato meal was investigated in healthy subjects. Furthermore, the impact of cold storage on the yield of RS was measured using an in vitro method.
Subjects and methods
Potentially available starch was analysed in the test products according to Holm et al (1986). Total starch content was determined in freshly boiled and boiled and cold stored potatoes with the method by Björck and Siljeström (1992). Further, in vitro RS content was analysed in freshly boiled and boiled and cold stored potatoes according to Åkerberg et al (1998) with the exception that incubation was performed at 37°C instead of 40°C (Fredriksson et al, 2000).
Subjects and meals
In all, 13 healthy volunteers (10 women and three men) aged 19–32 y, with normal body mass indices (22.5±2.1 kg/m2; mean±s.d.) and without drug therapy participated in the study. Potatoes of the cultivar Sava (a firm winter potato variety) were bought from a local farmer. Tubers of approximately the same size (128±18 g, mean±s.d.) were chosen so that each meal consisted of three potatoes. The potatoes were peeled, weighed and boiled in 1 l of water for 21–30 min to reach the same eat consistency. Furthermore, they were cut into four pieces and either served freshly boiled or after cold storage at 8°C for 24 h. In addition, cold stored potatoes previously boiled were served with a vinaigrette sauce including 28 g white vinegar (6% acetic acid, Druvan, Eslöv, Sweden) and 8 g olive oil (Zeta, Di Luca & Di Luca AB, Stockholm, Sweden). In all, 28 g vinegar corresponded to 28 mmol acetic acid. White wheat bread, baked according to Liljeberg and Björck (1994) was used as a reference. All meals, containing 50 g available starch, were consumed over 12 min and served with 250 ml water and 150 ml coffee/tea. The meals were served in random order at the same time in the morning after an over-night fast. The tests were performed approximately one week apart and commenced at the same time in the morning.
Capillary blood samples were taken prior to the meal (0) and at 15, 30, 45, 60, 90 and 120 min after the meal for analysis of glucose and after 15, 30, 45, 90 and 120 min for analysis of serum insulin. Glucose concentrations in whole blood were determined with a glucose oxidase peroxidase reagent and serum insulin concentrations were determined on an integrated immunoassay analyzer, CODA™ Open Microplate System (Bio-Rad Laboratories, Hercules, CA, USA) using an enzyme immunoassay kit (Mercodia Insulin ELISA, Mercodia AB, Uppsala, Sweden). Approval of the study was given by the Ethics Committee of the Faculty of Medicine at Lund University.
Calculations and statistical methods
For each subject and test meal, the incremental postprandial areas under the curves for glucose and insulin were calculated (Graph Pad Prism, version 3.0; Graph Pad Software, San Diego, CA, USA). All areas below baseline were excluded. The glycaemic and insulinaemic indices were calculated from the 90 and 120 min incremental blood glucose and insulin areas, respectively, by using white wheat bread as a reference (GI/II=100/100). Statistical analyses were performed with incremental areas as well as for concentrations of blood glucose and insulin at each time point. Values are presented as mean±s.e.m. All statistical analyses were performed with Minitab Statistical Software (release 13 for Windows; Minitab Inc., State College, PA, USA). Significances were evaluated with the general linear model (analysis of variance) followed by Tukey's multiple comparisons test. Values of P<0.05 were considered significant.
The RS content was determined in vitro in freshly boiled, and in boiled and cold stored potatoes (8°C, 24 h). The amount of RS was significantly higher in the cold stored potatoes compared with the freshly boiled ones (Table 1).
No statistical difference was seen in GI or II values whether calculated from areas at 90 or 120 min postprandially (Tables 2 and 3). The GI (120 min) of freshly boiled potatoes was 168, which is very high (Table 2). The GI was significantly lower for cold potatoes with vinaigrette and fell within the same range as that of the white wheat bread reference. II values (120 min) were significantly lower for cold potatoes (134), and cold potatoes with addition of vinaigrette (128) compared with freshly boiled potatoes (185) (Table 3). The meal containing cold potatoes with addition of vinaigrette sauce resulted in significantly lower blood glucose responses at 15–45 min and at 90 min after the meal, compared with freshly boiled potatoes (Figure 1). Cold stored potatoes and white bread displayed significantly lower blood glucose responses at 15–30 min. White bread induced significantly lower insulin response than freshly boiled potatoes at 30–45 min postprandially (Figure 2).
The present study showed that cold storage of boiled potatoes generated appreciable amounts of RS. Further, the possibility to substantially decrease glycaemic and insulinaemic responses to a potato meal was explored and found to be practically viable. Boiled potatoes are generally known to cause high GI's and II's, independent of variety and/or cooking method (Soh & Brand-Miller, 1999; Foster-Powell et al, 2002). Since potatoes are one of the staple foods in western countries, it is of interest to find methods to lower the accessibility of starch, hence lowering the glycaemic impact after a meal. In this study, two methods to reduce the rate of starch delivery to the blood were tested; cold storage and vinegar addition. Several studies have shown a decrease in starch availability, and in most cases also a subsequent increase in RS, when boiled potatoes are stored in a refrigerator, as evaluated in vitro (Englyst & Cummings, 1987; Kingman & Englyst, 1994; Åkerberg et al, 1998; Garcia-Alonso & Goni, 2000). Cold storage of potatoes has also been demonstrated to affect starch bioavailability in vivo. Consequently, Englyst and Cummings (1987) reported a dramatic increase in starch malabsorption with up to 12% of the starch passing indigested to the ileostomy effluent. Further, studies by Kanan et al (1998) demonstrated a 69% reduction of the GI following cold storage of boiled potatoes as calculated from the GI-values given in the international GI-tables (Foster-Powell et al, 2002). The increase in RS in the studies mentioned above is within the range of 1–4 times amounting to up to 7% RS (starch basis) after cold storage. Mainly amylose has been suggested to be involved in the formation of RS in, for example, autoclaved cereal starches (Berry, 1986). However, in boiled potatoes, also retrograded amylopectin seems to play a role (Englyst & Cummings, 1987). Differences in thermal processing conditions may explain why either amylose or amylopectin are involved in the formation of RS, but also the botanical origin of the starch plays an important role. Amylopectin from potato retrograde to a higher extent than cereal amylopectin following storage at 6°C for 2 or 4 days, which has been attributed to the longer outer chains in the case of potato amylopectin (Fredriksson et al, 1998). For many starchy foods, a reduction in GI is accompanied by a higher RS content. However, the lowered available starch content per se does not explain the lowered GI (Jenkins et al, 1987; Wolever, 1990; Raben et al, 1994; Björck et al, 2000).
The mechanism by which organic acids reduce glycaemia and insulinaemia to, for example, white bread and rice (Liljeberg & Björck, 1998; Sugiyama et al, 2003) is not fully understood. In the case of acetic acid, the magnitude of reduction in glycaemia may be as much as 35%, depending on the level of acetic acid. Brighenti et al (1995) suggested that the acetic acid caused an inhibition of the amylases whereas others have found evidence of a delay in gastric emptying rate (Liljeberg & Björck, 1998). The studies mentioned above have been performed in healthy subjects. In a recent study, vinegar added to a meal with a bagel, butter and orange juice was shown to improve postprandial insulin sensitivity in insulin resistant subjects (Johnston et al, 2004). Furthermore, lactic acid has also been shown to induce metabolic benefits in humans (Liljeberg et al, 1995; Östman et al, 2002a). In a recent feeding study with obese hyperinsulinaemic rats, the insulin demand was reduced at an oral glucose tolerance test performed at the end of a dietary period with wheat bread baked in the presence of lactic acid (Östman et al, 2005). It seems to be a prerequisite that lactic acid is present during heat treatment of the starch in order to produce a lowered GI, possibly by introducing an amylolytic barrier (Östman et al, 2002b). Thus, different organic acids seem to have the capacity to modulate acute glycaemia although through different mechanisms. Several studies have been performed to evaluate the potential effects of acetic acid on glucose metabolism. Acetic acid has thus been shown to suppress disaccharidase activity in Caco-2 cells (human colonic carcinoma cells) (Ogawa et al, 2000) and to activate gluconeogenesis and induce glycogenesis in the rat liver after a fasting state (Fushimi et al, 2001). Although it has been established that the presence of acetic acid in a meal delays the gastric emptying rate and thus reduce postprandial glycaemia (Ebihara & Nakajima, 1988; Liljeberg & Björck, 1998), more studies are needed to elucidate to what extent other physiological mechanisms also may be involved and affect glucose metabolism.
In the present study, vinegar was mixed with olive oil to a vinaigrette sauce. In a study by Liljeberg & Björck (1998), white vinegar corresponding to 18 mmol/50 g available carbohydrates was served to white wheat bread together with 8 g olive oil. The GI and II of the vinegar supplemented white bread meal were calculated to 64 and 65, respectively. Adding vinegar alone (18 mmol/50 g available carbohydrates) appears to be less effective in modulating glycaemia to white bread (Östman et al, 2005), and the corresponding GI and II's were 89 and 103, respectively.
In the present study, the combination of cold storage and addition of a vinaigrette sauce substantially reduced GI for the boiled potatoes with 43% and II with 31%, respectively. Cold storage per se tended to reduce GI (26%) although the effect did not reach statistical significance. However, cold storage per se did significantly reduce II with 28%. As a comparison, in a study by Östman et al (2005), addition of 28 mmol white vinegar to white bread decreased GI significantly with 23% and II with 22% when calculated at 90 min.
It is concluded that cold storage of potatoes significantly increased RS intake from the potato meal, from 1.7 to 2.5 g. Since the estimated total daily intake of RS in 10 European countries has been estimated to 4.1 g (Dysseler & Hoffem, 1994) a meal consisting of three cold potatoes will provide approximately 60% of the daily intake, which may be advantageous from a health perspective. RS in the form of resistant potato starch has been shown to promote comparatively high yields of butyric acid upon colonic fermentation compared with other sources of indigestible carbohydrates, as judged from butyric acid analysis in human faeces and caecum and colon in pigs, respectively (Cummings et al, 1996; Martin et al, 1998). Cold storage and addition of vinegar also significantly lowered glycaemic and insulinaemic excursions to a potato meal. These effects are noteworthy and could be exploited to lower glycaemic impact of potatoes. However, it should be noted that the pickled potato salad still produced a glycaemic and insulinaemic response of a similar magnitude to that after the white wheat bread reference and it would be interesting to find other processing conditions and/or combinations to achieve a further lowering of the metabolic response.
We thank Lisbeth Persson for invaluable technical assistance.
About this article
Nutrition & Metabolism (2013)