THE EFFECT OF PHYTIC ACID ON RATE OF STARCH DIGESTIBILITY IN VICIA FABA AND WHITE WHEAT BREAD IN VITRO

THE EFFECT OF PHYTIC ACID ON RATE OF STARCH DIGESTIBILITY IN VICIA FABA AND WHITE WHEAT BREAD IN VITRO

W. Khayata*, G. Bashor and M. Sadek

Department of Analytical and Food Chemistry, Faculty of Pharmacy, University of Aleppo, Syrian Arab Republic

ABSTRACT: The effect of the phytic acid on starch digestibility was studied in vitro. Starch digestibility was found to correlate negatively with the phytic acid content of the food tested.When sodium phytate was added (equivalent to 2% of phytic acid based on the starch portion), it was noticed that the rate of Viciafabastarch digestion by human saliva and at human body temperature (37°C) was decreased by 13%, while the rate of digestion of white wheat bread decreased by 20%. Furthermore, it was observed that mixing phytic acid with human saliva before adding the former to starch and following a pH adjustment reduced the rate of starch digestion by 6% when compared to mixing phytic acid with starch first and then adding human saliva to it.

Phytic acid, Vicia faba, White wheat bread, Human saliva, Starch digestibility

INTRODUCTION:All dietary carbohydrates, from starch to table sugar share a basic biological property: they can be digested or converted into glucose. Digestion rate, and in consequenceblood glucose response, is commonly thought to be determined by saccharide chain length ¹.

Throughout the past 25 years, however, the relevance of chain length in carbohydrate digestion rate has been questioned. Jenkins et al ^{2, 3} demonstrated different carbohydrate sources raise the blood glucose to different extents when fed in equivalent amounts.

On digestion in vitro, legumes release their carbohydrate products at a slower rate than cereals ².

This reduced rate of digestion has been related to the degree to which a food raises the blood glucose in vivo ⁴. For this reason, Jenkins et al ⁵ proposed the glycemic index as a system for classifying carbohydrate-contain foods according to glycemic response. It is defined as the incremental area under the blood glucose responsecurve elicited from a 50 g available carbohydrate portion of a food expressed as a percentage of that after 50g of carbohydrate from a reference food taken by the same subject.

Low GI diets have been demonstrated to improve the blood glucose control ⁶, increase insulin sensitivity ⁷, decrease serum cholesterol ⁸, decrease risk of developing type 2 diabetes ⁹, decrease risk of cardiovascular disease ^{10, 11} and improve weight control ¹². Dietary fiber has been suggested as the primary factor contributing to the slower rate of glucose release in foods ¹³, however, the rate of starch digestion cannot be explained by the amount of fiber alone since some legumes (e.g.lentils) have similar fiber contents as certain cereal products (e.g, whole meal bread) and yet are digested at very different rates in vitro. Nevertheless, the type of dietary fiber-component may contribute to the observeddisparity between foods. In addition starch type, food form, the highconcentration of anti-nutrients such as phyticacid, tannins, lectins and enzyme inhibitors may also play a role in starch digestibility ¹⁴.

Phytic acid (PA), myo-inositol hexaphosphoric acid- is the most important phosphate reserve compound in many plants. PA has long been considered an antinutrient ¹⁵, mainly due to its ability to bind with many divalent cations, proteins and starch and to consequently reduce their bioavailability ¹⁶.

To combat PA’s anti-nutritive effects, particularly in nutritionally compromised populations, many ways of removing it from foods have been suggested. However, it has also been suggested by many studies that consumption of PA may have some beneficial health effects ^17-20.

Although there is little available information on direct interaction of phytic acid and starch, they are structurally capable of combining through phosphate linkages ²¹. Phytic acid could also affect the starch digestibility through combination with proteins which are structurally closely associated with starch and/or through combination with the digestive enzymes which are themselves proteins.

In addition, phytic acid can bind minerals such as calcium whose are known to catalyze amylase enzyme activity ²². In this study, we attempt to determine the effect of sodium phytate on in vitro starch digestion rate in Vicia faba and white wheat bread.

MATERIALS AND METHODS: Vicia faba was obtained from Khan-Toman farms in Syria and cooked by boiling water. White wheat bread was obtained from Chamsine Bakery. Vicia faba and white wheat bread were analyzed for phytic acid content by the Latta and Eskin Method.²³ Starch content of food samples was determined by the Nielsen Method ²⁴.

For in vitro tests, 23.5 mg of sodium phytate (6% moisture, Sigma-Aldrich, and equivalent to 20 mg of phytic acid) was added to 1 g of potato starch and then mixed with 5 ml of fresh human saliva and 10 ml of distilled water. In one case, the sodium phytate was pre-incubated with saliva at 37°C for 15 minutes before mixing it with starch, once with adjusting the pH and another time without. The samples were placed in dialysis tubing (12000-14000 MWCO), and the slurry was massaged gently to mix. The dialysis tubing was suspended in a beakercontaining 800 ml distilled water maintained at 37°C with continuous agitation.

2 ml of dialysate was sampled every hour for 4 h, and pipetted into a 10-ml volumetric flask, diluted to volume with distilled water, and thoroughly shaken. Total carbohydrate content was analyzed using the phenol-sulfuric acid method (Dubois et al 1956).

The diluted solutions (2 ml) were pipetted into test tubes, and 1 ml of 5% phenol solution and 5 ml of concentrated H₂SO₄ were added. Each test tube was thoroughly mixed and left to stand for 25 min to permit color development. Absorbance was measured at 487 nm on a Jasco V650 Spectro-photometer.

One-gram starch portions from each Vicia faba and white wheat bread were placed instead of potato starch and prepared in the same manner to determine starch digestibility in these tested foods.

A blank for each test was prepared in the same manner, except that 5 ml of boiled saliva was used to eliminate the effects of naturally occurring starch digestion products. Each analysis was performed in duplicate.

A standard curve was prepared using solutions containing known concentration of maltose (0, 5, 10, 20, 30, 40, and 50 µg/ml), and used to calculate the concentration of released sugars in dialysate in test solutions. The addition of sodium phytate in vitro systems was at a level equivalent to 2% phytic acid based on starch content.

RESULTS: The phytic acid and starch concentration of cooked Vicia faba and white wheat bread are shown in Table 1. The results from the in vitro dialysis of potato starch are shown in Table 2. Starch plus saliva are served as the control.

The mixture of [phytic acid+ starch] + saliva at pH 7.35 decreased the sugar release by 33% at 1 h and 12% at 4 h in comparison with the control, while the mixture of [phytic acid + saliva] + starch at pH 8.05 decreased the sugar release by 46% at 1 h and 20% at 4 h and this was higher than the corresponding values of 37% and 17% obtained after the adjustment of pH at 7.35, yet the decrease in sugar release of [phytic acid + saliva] + starch at pH 7.35 was still higher than the corresponding values of [phytic acid + starch] + saliva at  pH 7.35.

TABLE 1: CONTENT OF PHYTIC ACID AND STARCH OF COOKED VICIA FABA AND WHITE WHEAT BREAD

* Dry basis. ^a Mean ± SD

TABLE 2: RESULT OF THE POTATO STARCH DIGESTION IN VITRO WITH OR WITHOUT PHYTIC ACID

The in vitro data of cooked Vicia faba starch digestion in Table 3 demonstrated that an addition of phytic acid to cooked Vicia faba reduced the starch digestibility by 21% at 1 h and 13% at 4 h in comparison with the control (cooked Vicia faba plus saliva).

In vitro results of white wheat bread in Table 4 showed a decrease of starch digestibility, in the presence of phytic acid, by 28% at 1 h and 20% at 4 h in comparison with the control (white wheat bread plus saliva).

TABLE 3: RATE OF STARCH DIGESTIBILITY IN COOKED VICIA FABA WITH OR WITHOUT PHYTIC ACID

TABLE 4: RATE OF STARCH DIGESTIBILITY IN WHITE WHEAT BREAD WITH OR WITHOUT PHYTIC ACID

DISCUSSION: Foods with a higher concentration of phytic acid related negatively with the sugar released from starch digestion, and furthermore with the glycemic response. To establish the effect of phytic acid on starch digestibility, 2% phytic acid (based on starch portion) was added to potato starch.

The in vitro system was used here since the sugar released from the dialysis tubing has been shown to relate well with the glycemic index ⁴.

The use of human saliva was justified by its ready availability and similarity in starch digestibility to results obtained with pancreatic juices ⁴.

Although the optimum pH for salivary amylase activity is between 6.8 – 7, adjustment of the pH at 7.35-when the phytic acid was pre-incubated  with saliva before mixing with starch-was chosen to match the starch digestion condition in the method of mixing phytic acid with starch then adding saliva. Thus, we could know the effect of phytic acid on amylase enzyme activity.

The results in Table 2 suggest that phytic acid may affect the starch digestibility through interaction with starch and/or interaction with amylase.

While phytic acid definitely affected the digestibility of pure starch, it is unknown whether it would also affect the digestibility of starch which is associated with other macronutrients such as proteins. Therefore, the digestibility of starch with or without phytic acid in cooked Vicia faba and white wheat bread was determined. Phytic acid significantly reduced the in vitro starch digestibility in cooked Vicia faba and white wheat bread Table 3 and Table 4.

In conclusion, the reduced rate of starch digestibility in vitro could, in part, be due to the presence of phytate. This, in turn, may explain the difference in rate of starch digestibility between cereals and legumes. The nature of the interaction remains uncertain.

ACKNOWLEDGEMENTS: The authors gratefully acknowledge the financial support received from the University of Aleppo.

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    How to cite this article:

    Khayataa W, Bashora G and Sadeka M: The effect of phytic acid on rate of starch digestibility in Vicia faba and white wheat bread in vitro. Int J Pharm Sci Res 2013; 5(1): 161-64.doi: 10.13040/IJPSR. 0975-8232.5(1).161-64

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