Glycaemic index values for commercially available potatoes in Great

Glycaemic index values for commercially available potatoes in Great Britain

C. Jeya K. Henry

*, Helen J. Lightowler

, Caroline M. Strik

and Michael Storey

Nutrition and Food Science Group, School of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane Campus,

Headington, Oxford OX3 0BP, UK

British Potato Council, 4300 Nash Court, John Smith Drive, Oxford Business Park South, Oxford OX4 2RT, UK

(Received 18 February 2005 – Revised 5 July 2005 – Accepted 11 July 2005)

The glycaemic response to eight potato varieties commercially available in Great Britain was compared against a glucose standard in a non-blind, random-

ised, repeated measure, crossover design trial. Seventeen healthy subjects (three males, fouteen females), mean age 32 (

SD 13) years and mean BMI 22·3 (SD

3·6) kg/m

, were recruited to the study. Subjects were served portions of eight potato varieties and a standard food (glucose), on separate occasions, each

containing 50 g carbohydrate. Capillary blood glucose was measured from ﬁnger-prick samples in fasted subjects (0 min) and at 15, 30, 45, 60, 90 and

120 min after the consumption of each test food. For each potato variety, the glycaemic index (GI) value was calculated geometrically by expressing

the incremental area under the blood glucose curve (IAUC) as a percentage of each subject’s average IAUC for the standard food. The eight potato varieties

exhibited a wide range in GI values from 56 to 94. A trend was seen whereby potatoes with waxy textures produced medium GI values, whilst ﬂoury pota-

toes had high GI values. Considering the widespread consumption of potatoes in Great Britain (933–1086 g per person per week), this information could be

used to help lower the overall GI and glycaemic load of the diets of the British population.

Glycaemic index: Potato variety: Great Britain: Glycaemic load

The glycaemic index (GI), ﬁrst introduced in 1981 (Jenkins et al.

1981), is a classiﬁcation of the blood glucose-raising potential of

the carbohydrates in foods. It is deﬁned as the incremental area

under the blood glucose curve (IAUC) of a 50 g carbohydrate

portion of a test food expressed as a percentage of the response

to 50 g carbohydrate of a standard (reference) food taken by

the same subject, on a different day (Food and Agriculture

Organization/World Health Organization, 1998).

Carbohydrate foods consumed in isoglucidic amounts produce

different glycaemic responses depending on many factors, such

as particle size, cooking and food processing, other food

components (e.g. fat, protein, dietary ﬁbre) and starch structure

(Bjorck et al. 1994). The principle is that the slower the rate of

carbohydrate absorption, the lower the rise of blood glucose

level and the lower the GI value (Brand et al. 1991). Indeed,

high GI foods are characterised by fast-release carbohydrate and

higher blood glucose levels. A GI value $ 70 is considered

high, a GI value 56–69 inclusive is medium and a GI value

# 55 is low, where glucose ¼ 100 (Brand-Miller et al. 2003).

The GI of foods may have important implications for the

prevention and treatment of the major causes of morbidity and mor-

tality in Western countries, including type 2 diabetes, CHD and

obesity. Today, there is an important body of evidence to support

the therapeutic potential of low-GI diets, not only in diabetes

(Bjorck et al. 1994; Frost et al. 1994; Gilbertson et al. 2001; Augus-

tin et al. 2002) but also in subjects with hyperlipidaemia (Jenkins

et al. 1987a). In addition, low-GI foods have been associated with

prolonged endurance during physical activity (Thomas et al.

1991), improved insulin sensitivity (Frost et al. 1998), reduced

food intake (Holt et al. 1995; Warren et al. 2003) and increased

colonic fermentation (Jenkins et al. 1987b; Wolever et al. 1992).

More recent data support the preventive potential of a low-GI diet

against the development of type 2 diabetes and CVD (Salmeron

et al. 1997a,b; Frost et al. 1999; Meyer et al. 2000).

Potatoes generally have one of the highest GI values of any food,

although some varieties appear to be lower than others. Published GI

values are highly variable (Foster-Powell et al. 2002); however, it

can be difﬁcult to compare values as the variety of potato is rarely

speciﬁed. It is necessary to clarify the GI of different varieties of

potatoes as they are a major staple in Great Britain, contributing

approximately 9 % of total carbohydrate intake (National Statistics,

2004). In addition, there is widespread consumption of potatoes.

Current ﬁgures show that average potato consumption (fresh and

processed) in Great Britain ranges between 933–1086 g per

person per week (National Statistics, 2004).

Previous research has shown no difference in GI values

between three varieties of potato (Sebago, Pontiac and Desiree;

Soh & Brand-Miller, 1999). However, it was demonstrated that

young or ‘new’ potatoes have a lower GI than more mature pota-

toes, which may be attributed to differences in starch structure. If

a low-GI potato variety can be identiﬁed, it could be used to lower

the overall GI and glycaemic load of the Western diet and thus

decrease the risk of type 2 diabetes, CHD and obesity (Soh et al.

1999). There are limited data on the GI values of different var-

ieties of commercially available potatoes within Great Britain.

The aim of the current work is to investigate the glycaemic

* Corresponding author: Professor C. J. K. Henry, fax þ 44 (0)1865 483242, email [email protected]

Abbreviations: GI, glycaemic index; IAUC, incremental area under the blood glucose curve.

British Journal of Nutrition (2005), 94, 917–921 DOI: 10.1079/BJN20051571

q The Authors 2005

https://doi.org/10.1079/BJN20051571 Published online by Cambridge University Press

response of eight varieties of potato commercially available in

Great Britain.

Methods

Subjects

Seventeen healthy subjects (three male, fourteen female) were

recruited through posters distributed throughout Oxford Brookes

University in addition to announcements in lectures and through

personal networks. Subjects were staff and students (undergraduate

and postgraduate) from Oxford Brookes University and were

moderately active, non-smoking men and women. Exclusion

criteria were as follows: age , 18 or . 55 years; BMI $ 30 kg/m

;

fasting blood glucose value . 6·1 mmol/l. Mean age was 32 (

SD 13)

years and mean BMI was 22·3 (

SD 3·6) kg/m

. Ethical approval for

the study was obtained from the University Research Ethics Commit-

tee at Oxford Brookes University. Subjects were given full details of

the study protocol and the opportunity to ask questions. All subjects

gave written informed consent prior to participation.

Study protocol

The protocol used was adapted from that described by Wolever

et al. (1991) and is in line with procedures recommended by

the Food and Agriculture Organization/World Health Organiz-

ation (1998). To determine the GI of a food, tests should be

repeated in six or more subjects, thus in the present study, for

each test food, ten of the seventeen subjects recruited to the

study were randomly selected. On the day prior to a test, subjects

were asked to restrict their intake of alcohol and caffeine-contain-

ing drinks and to restrict their participation in intense physical

activity (e.g. long periods at the gym, excessive swimming, run-

ning, aerobics). Subjects were also told not to eat or drink after

21.00 hours the night before a test, although water was allowed,

in moderation.

Test foods

Eight potato varieties were tested: Maris Peer; Maris Piper;

Desiree; Estima; Charlotte; Marfona; Nicola; King Edward.

These varieties, all sourced from the British Potato Council,

represent a diverse range of commercial varieties commonly

consumed as fresh, rather than processed, potatoes in the UK.

The eight varieties were compared with a standard food (glucose)

and were tested in equivalent carbohydrate amounts (50 g). The

total solid and carbohydrate content of each potato variety was

obtained from published values (Storey & Davies, 1992; NIAB,

2004). In accordance with Food and Agriculture Organization/

World Health Organization (1998) recommendations, subjects

tested each potato variety once and the standard food three times

in random order on separate days, with at least a 1 d gap between

measurements to minimise carryover effects. Subjects were studied

in the morning after a 12 h overnight fast. To avoid starch retrogra-

dation, potatoes were freshly prepared and cooked each morning.

Potatoes were peeled, cut into quarters and then boiled in 1·5

litres unsalted water for 15 min until tender. Subjects consumed

the standard food/test product at a comfortable pace, within

15 min. The test products and the standard food were served with

200 ml water. A further 200 ml water was given during the sub-

sequent 2 h. Subjects were encouraged to keep physical activity to

a minimum during the testing.

Blood glucose measurements

A qualiﬁed technician performed blood glucose measurements.

A fasting blood sample was taken at 0 min and the standard

food/test product was consumed immediately after this. Further

blood samples were taken at 15, 30, 45, 60, 90 and 120 min

after starting to eat.

y = 0·9912x + 0·3143

0·9562

Fig. 1. Pearson regression and Bland –Altman analyses of blood glucose measurements between the Ascensia Contour (ASC) and HemoCue 201 þ analyser

(HEM). For details of procedures, see this page.

Table 1. Incremental area under the blood glucose

curves (IAUC) and glycaemic index (GI) values for

each potato variety (Mean values with their standard

errors)

IAUC GI value

Potato variety Mean

SEM Mean SEM

Maris Peer 182 33 94 16

Maris Piper 167 17 85 4

Desiree 133 18 77 17

Estima 143 13 66 5

Charlotte 134 30 66 5

Marfona 93 21 56 3

King Edward 166 10 75 10

Nicola 130 10 59 7

For details of procedures, see this page.

C. J. K. Henry et al.918

https://doi.org/10.1079/BJN20051571 Published online by Cambridge University Press

Blood was obtained by ﬁnger-prick using the Glucolet 2 multi-

patient lancing system (Bayer HealthCare, Newbury, Berks.).

Recent reports suggest that capillary blood sampling is preferred

for reliable GI testing (Food and Agriculture Organization/World

Health Organization, 1998; Wolever, 2003). Prior to a ﬁnger-

prick, subjects were encouraged to warm their hand to increase

blood ﬂow. Fingers were not squeezed to extract blood from the ﬁn-

gertip as this may dilute with plasma. Blood glucose was measured

using Ascensia Contour

automatic blood glucose meters (Bayer

HealthCare). The blood glucose meters were calibrated daily

using control solutions from the manufacturer and were also

regularly calibrated against a clinical dry chemistry analyser

(Reﬂotron

Plus; Roche, Lewes, Sussex) and the HemoCue

Glucose 201 þ analyser (HemoCue

Ltd, Angelholm, Sweden).

Figure 1 shows the Pearson regression and Bland –Altman ana-

lyses (Bland & Altman, 1986) for a random selection of 106

blood samples simultaneously measured using the Ascensia Con-

tour

and the HemoCue Glucose 201 þ analyser. There was a

very strong correlation (r 0·978, P# 0·001) and good agreement

(mean difference 2 0.3 mmol; 95 % CI 2 0·3, 2 0·2; limits of

agreement 2 0·75, 0·21) between blood glucose measurements

using the automatic analyser and the HemoCue analyser.

Calculation of glycaemic index

The IAUC, ignoring the area beneath the baseline, was calculated

geometrically for each food (Food and Agriculture Organization/

World Health Organization, 1998). The mean, standard deviation

Fig. 2. Incremental area under the blood glucose curves (IAUC) for eight potato varieties ((X), (a) Maris Peer, (b) Maris Piper, (c) Desiree, (d) Estima, (e) Char-

lotte, (f) Marfona, (g) King Edward, (h) Nicola). D, Glucose. For details of procedures, see p. 918. Values are the mean change in blood glucose (BG) with their

standard errors represented by vertical bars (n 10).

Glycaemic index and potatoes 919

https://doi.org/10.1079/BJN20051571 Published online by Cambridge University Press

and CV of the IAUC of each subject’s repeated standard food

were calculated. The IAUC for each test product eaten by each

subject was expressed as a percentage of the mean IAUC for

the standard food eaten by the same subject. The GI of each

potato variety was taken as the mean for the whole group.

Statistical analysis

Statistical analysis was performed using the Statistical Product

and Service Solutions software version 11.0.1 (SPSS, Chicago,

IL, USA). To examine the correlation and agreement between

the automatic analyser and the HemoCue Glucose 201 þ analy-

ser, Pearson’s correlation coefﬁcient and the method of Bland &

Altman (1986) were used. Levels of inter- and intra-individual

variation of the three standard (glucose) tests were assessed by

determining the CV. One-way ANOVA was used to compare

IAUC values and GI values between the eight potato varieties.

Kendall’s rank correlation coefﬁcient (Kendall’s t) was used to

assess the relationship between GI value and potato texture

rating. Statistical signiﬁcance was set at P, 0·05.

Results

The mean CV of glycaemic responses to the three standard tests

for the seventeen subjects was 23 %. The inter-individual

variation in glycaemic response to the standard test for the seven-

teen subjects was 30 % CV. These values are consistent with

previously reported data (Wolever, 1990).

Mean IAUC and GI values for each test product are presented in

Table 1 and Fig. 2. There was no signiﬁcant difference in IAUC

between the different potato varieties (P¼ 0 ·188). The potatoes

exhibited a large variation in GI values ranging from 56 (Marfona)

to 94 (Maris Piper), although the differences in GI were not

statistically signiﬁcant (P¼ 0·330).

For practical measures, GI values are often grouped into cat-

egories as producing either a low, medium or high glycaemic

response. The cut-off values are as follows: low # 55; medium

56–69 inclusive; high $ 70 (Brand-Miller et al. 2003). The GI

ratings for the tested potato varieties are given in Table 2 along-

side characteristics of the potato varieties. There was a strong

positive correlation (Kendall’s t-b 0·656; P¼ 0·034) between the

GI value and texture rating. In general, potatoes with ﬂoury

textures (low in moisture, low in sugar with high starch) were

in the high-GI category, whilst those with ﬁrm to waxy textures

(high in moisture, low starch) were in the medium-GI category.

Generally, the earlier crop varieties of potato (second early) and

the salad potatoes tend to have waxy textures while the main

crop varieties tend to have ﬂoury textures.

Discussion

The GI values reported here are comparable to those given for

boiled potatoes in the recent international GI tables

(Foster-Powell et al. 2002). A GI value range of 56 – 101 has

been reported for boiled potatoes, where the type of potato has

been speciﬁed, and a GI range 23–76 for unspeciﬁed potato

type (Foster-Powell et al. 2002). More recently, Fernandes et al.

(2005) reported a GI value of 89 for boiled red potatoes. Small

differences of , 10 –15 units are within the error associated

with the measurement of GI (Wolever et al. 1991; Foster-

Powell et al. 2002). Discrepancies may be due in part to cooking

methods used, which may change the structure of the starch

(Englyst & Cummings, 1987). In particular, it has been reported

that precooking and reheating potatoes before consumption will

elicit a lower glycaemic response compared with potatoes con-

sumed immediately after cooking (Fernandes et al. 2005).

The lower GI rating of ﬁrm/waxy potato varieties agrees with

previous ﬁndings (Soh & Brand-Miller, 1999). However Maris

Peer, which was originally bred as a low-DM potato for use by

the canning industry, is the one variety in the trial that appears

to differ markedly in its GI rating (high) relative to its commercial

usage as an early potato and as such may warrant further study.

Changes in the nature of the starch, predominantly an increase

in the amount of amylopectin (Soh & Brand-Miller, 1999), may

be partly responsible for the difference in glycaemic response

seen with ﬂoury, more mature potatoes. Amylopectin has a

branched structure that makes it more readily digestible than

linear chain starch leading to a higher glycaemic response

(Van Amelsvoort & Westrate, 1992).

In conclusion, these results provide a comparison of the glycae-

mic responses of several commercially available varieties of

potato. Considering the widespread consumption of potatoes in

Great Britain (933–1086 g per person per week), it is useful to

have information regarding the glycaemic response of common

varieties. The identiﬁcation of potato varieties with lower glycae-

mic responses may help lower the GI of the British diet and

potentially have beneﬁt in the management and prevention of

some chronic diseases.

Acknowledgements

This study was supported by the British Potato Council Project

R256.

References

Augustin LS, Franceschi S, Jenkins DJA, Kendall CWC & La Vecchia C

(2002) Glycemic index in chronic disease: a review. Eur J Clin Nutr

56, 1049 – 1071.

Bjorck I, Granfeldt Y, Liljeberg H, Tovar J & Asp NG (1994) Food prop-

erties affecting the digestion and absorption of carbohydrates. Am J

Clin Nutr 59, S699–S705.

Bland JM & Altman DG (1986) Statistical methods for assessing agreement

between two methods of clinical measurement. Lancet I, 307–310.

Brand JC, Colagiuri S, Crossman S, Allen A, Roberts DCK & Truswell

AS (1991) Low-glycemic index foods improve long-term glycemic

control in NIDDM. Diabetes Care 14, 95 – 101.

Table 2. Glycaemic index (GI) classiﬁcation and characteristics of each

potato variety

Potato variety GI classiﬁcation Crop type Texture†

Texture

rating*

Maris Peer High Second early Firm 5

Maris Piper High Main crop Floury 5

Desiree High Main crop Firm 5

Estima Medium Second early Firm, moist 4

Charlotte Medium Salad Firm, waxy 4

Marfona Medium Second early Waxy 3

King Edward High Main crop Floury 6

Nicola Medium Salad Firm 4

* Information obtained from the British Potato Council website www.potato.org.uk. Texture

ratings are measured on a scale from 1 to 9 (1 being most waxy and 9 being the most

ﬂoury).

For details of procedures, see p. 918.

C. J. K. Henry et al.920

https://doi.org/10.1079/BJN20051571 Published online by Cambridge University Press

Brand-Miller J, Foster-Powell K & Colagiuri S (2003) The New Glucose

Revolution. New York: Marlowe and Company.

Englyst HN & Cummings JH (1987) Digestion of polysaccharides of

potato in the small intestine of man. Am J Clin Nutr 45, 423–431.

Food and Agriculture Organization/World Health Organization (1998)

Carbohydrates in Human Nutrition. Report of a Joint FAO/WHO

Expert Consultation. Rome: FAO.

Fernandes G, Velangi A & Wolever TMS (2005) Glycemic index of potatoes

commonly consumed in North America. J Am Diet Assoc 105, 557–562.

Foster-Powell K, Holt SHA & Brand-Miller JC (2002) International table

of glycemic index and glycaemic load values: 2002. Am J Clin Nutr 76,

5–56.

Frost G, Leeds AA, Dore CJ, Madeiros S, Brading S & Dornhorst A

(1999) Glycaemic index as a determinant of serum HDL-cholesterol

concentration. Lancet 353, 1045 – 1048.

Frost G, Leeds A, Trew G, Margara R & Dornhorst A (1998) Insulin sen-

sitivity in women at risk of coronary heart disease and the effect of a

low glycemic diet. Metabolism 47, 1245–1251.

Frost G, Wilding J & Beecham J (1994) Dietary advice based on the gly-

caemic index improves dietary proﬁle and metabolic control in Type 2

diabetic patients. Diabet Med 11, 397–401.

Gilbertson HR, Brand-Miller JC, Thorburn AW, Evans S, Chondros P &

Werther GA (2001) The effect of ﬂexible low glycemic index dietary

advice versus measured carbohydrate exchange diets on glycemic con-

trol in children with type 1 diabetes. Diabetes Care 24, 1137–1143.

Holt SHA, Miller JCB, Petocz P & Farmakalidis E (1995) A satiety index

of common foods. Eur J Clin Nutr 49, 675–690.

Jenkins DJA, Wolever TMS, Collier GR, Ocana A, Rao AV, Buckley G,

Lam Y, Mayer A & Thompson LU (1987b) Metabolic effects of a low

glycemic index diet. Am J Clin Nutr 46, 968–975.

Jenkins DJA, Wolever TMS, Kalmusky J, et al. (1987a) Low-glycemic

index diet in hyperlipidemia: use of traditional starchy foods. Am J

Clin Nutr 46, 66–71.

Jenkins DJA, Wolever TMS, Taylor RH, Barker H, Fielden H, Baldwin

JM, Bowling AC, Newman HC, Jenkins AL & Goff DV (1981)

Glycemic index of foods: a physiological basis for carbohydrate

exchange. Am J Clin Nutr 34, 362–366.

Meyer KA, Kushi LH, Jacobs DR, Slavin J, Sellers TA & Folsom AR

(2000) Carbohydrates, dietary ﬁber, and incident type 2 diabetes in

older women. Am J Clin Nutr 71, 921–930.

National Statistics (2004) Family Food. A Report on the 2002–03 Expen-

diture and Food Survey. London: TSO.

NIAB (2004) Pocket Guide to Varieties of Potato. Cambridge: NIAB.

Salmeron J, Ascherio A, Rimm EB, Colditz GA, Spiegelman D, Jenkins

DJ, Stampfer MJ, Wing AL & Willett WC (1997a) Dietary ﬁber, gly-

cemic load, and risk of NIDDM in men. Diabetes Care 20, 545–550.

Salmeron J, Manson JE, Stampfer MJ, Colditz GA, Wing AL & Willett

WC (1997b) Dietary ﬁber, glycemic load, and risk of non-insulin-

dependent diabetes mellitus in women. JAMA 277, 472– 477.

Soh NL & Brand-Miller J (1999) The glycaemic index of potatoes: the

effect of variety, cooking method and maturity. Eur J Clin Nutr 53,

249– 254.

Storey RMJ & Davies HV (1992) Tuber quality. In The Potato Crop. The

Scientiﬁc Basis for Improvement, pp. 507 – 569 [PM Harris, editor].

London: Chapman and Hall.

Thomas DE, Brotherhood JR & Brand JC (1991) Carbohydrate feeding

before exercise: effect of glycemic index. Int J Sports Med 12,

180– 186.

Van Amelsvoort JM & Westrate JA (1992) Amylose–amylopectin ratio in

a meal affects postprandial variables in males volunteers. Am J Clin

Nutr 55, 712–718.

Warren JM, Henry CJK & Simonite P (2003) Low glycemic index break-

fasts and reduced food intake in preadolescent children. Pediatrics 112,

e414– e419.

Wolever TMS (1990) The glycemic index. World Rev Nutr Diet 62,

120– 185.

Wolever TMS (2003) Carbohydrate and the regulation of blood glucose

and metabolism. Nutr Rev 61, S40– S48.

Wolever TMS, Jenkins DJA, Jenkins AL & Josse RG (1991) The glyce-

mic index: methodology and clinical implications. Am J Clin Nutr

54, 846 – 854.

Wolever TMS, Jenkins DJA, Vuksan V, Jenkins AL, Buckley GC, Wong

GS & Josse RG (1992) Beneﬁcial effect of a low glycemic index diet in

Type 2 diabetes. Diabet Med 9, 451 – 458.

Glycaemic index and potatoes 921

https://doi.org/10.1079/BJN20051571 Published online by Cambridge University Press