Oral glucose tolerance testing – is carb loading still necessary?

23rd March 2025, A/Prof Chee L Khoo

carb loading or not

I thought I read somewhere many years ago that we don’t have to advocate the traditional 72-hour carbohydrate (carb) loading prior to an oral glucose tolerance test (OGTT). I have not heard that issue being brought up of late. Well, does it increase or decrease the glucose levels of the OGTT. Does the pre-loading up of the liver and muscle affect the glucose challenge? Does carb loading fill up the stores and the glucose challenge is an actual challenge to the insulin secretion or will carb pre-loading sensitise the insulin system prior and the OGTT readings become lower? Perhaps, in the non-pregnant patient, the minor effect may not matter much as we will be treating the metabolic dysfunction anyway. But we need to know in pregnant women as our GDM management depends on the accuracy, reproducibility and reliability of the results.

The plasma glucose levels following a glucose load is determined by beta cell function and the prevailing insulin resistance. The OGTT is used to obtain information about the resultant of the beta-cell secretory function and the peripheral action of insulin. However, both beta-cell secretory function and insulin sensitivity are not constant entities, particularly, in young women without pre-existing diabetes.

“Hunger diabetes” was first described in the late eighteenth century after the discovery of glycosuria in starving dogs fed a high-carbohydrate meal [1]. Conn discovered that malnourished individuals on low carb diets often met criteria for glucose intolerance or diabetes on OGTT. He recommended a standard 300g carbohydrate diet prior to OGTT. Wilkerson et al revised that to 150g of carb for normally nourished individuals (2).

Rosenberg et al observed an inverse linear relationship between carbohydrate intake (median 237g and 60-minute OGTT glucose. For every 50g reduction in carbohydrate intake, there was an 8.9 mg/dl increase in 60-minute OGTT glucose (P < 0.01) in an adjusted model. Lower carbohydrate intake was also associated with higher 30-minute (P < 0.01) and 120-minute OGTT glucose (P = 0.01) (3).

Hao et al also noted a relationship between the pre-pregnancy low carb (LC) diet and more detrimental OGTT values in patients with GDM (4). He conducted a small case control study of 20 women with GDM on low carb (<130g/day) prior to the diagnosis at 24-28 weeks gestation and compared their OGTT results with those on a normal carb diet and women who did not have GDM. They found that compared with women on normal carb diet, women on low carb diet before pregnancy had higher 1-hr and 2-hr OGTT readings.

Sandra Bonuccelli et al demonstrated that there was improved carb tolerance following repeated glucose administration (5). This was referred to as the Staub-Traugott effect.

While low carb diet prior to the OGTT can give rise to erroneous diagnosis of diabetes or GDM, pre-loading with high carb may improve the body response to later glucose challenge and give rise to lower glucose readings.

For a long time, the need for carb preloading before an OGTT is accepted into guidelines. World Health Organization (WHO) has long recommended preparation for OGTT with greater than 150 grams carbohydrate per day but the reference is buried somewhere in the Appendix back in 1985 (6).

Perhaps, the importance of carb loading may have been watered down over the last few years with the release of conflicting data. Harlass et al showed in a small crossover trial in pregnant women who failed a 50g 1-hour OGTT, carb preloading in subsequent OGTT did not affect the results of the fasting, 1-hour, 2-hour or 3-hour readings (7).

Similarly, Entriken et al compared the effect of a high carbohydrate preparatory diet on the performance of the 3-hour oral glucose tolerance test (GTT) in pregnancy (8). They found that compared to usual dietary intake, recommending a high carbohydrate diet had a negligible effect on the 1-hr, 2-hr and 3-hr glucose readings in the 100g oral GTT in pregnancy.

Buhling et al compared the effect of low carbohydrate (40%) with high carbohydrate diet (50%) one week prior to an OGTT in pregnant women in a cross over trial (9). There were no differences in the results of the OGTTs either.

The 2008 ACE/AACE guidelines for the diagnosis and management of prediabetes indicates the necessity of adequate carbohydrate intake prior to OGTT but does not define the amount of carbohydrate required [10].

The RACGP Management of T2D Handbook for General Practice does recommend “three days of unrestricted diet (containing at least 150 g/day carbohydrates)” (11). It cites Eyth et al’s paper in 2023 but the reference provided in the paper did not provide the evidence (12).

The Australian Diabetes in Pregnancy Society (ADIPS) specifically mentioned that “There is also no need for a 3-day high carbohydrate diet before the pregnancy OGTT” in its 2014 guidelines (13).

The Royal Australian College of Pathologist also recommend 3 days of carb loading prior to an OGTT (14).

It would seem that the data and recommendations are conflicting over carb pre-loading and it is important to take a common stance to avoid confusing both women and health care professionals. We await a position statement from the ADIPS and RCPA which should come out anytime soon.

References:

  1. Conn JW. Interpretation of the glucose tolerance test. The necessity of a standard preparatory diet. Am J Med Sci. 1940;199:555-564.
  2. Wilkerson HL, Butler FK, Francis JO. The effect of prior carbohydrate intake on the oral glucose tolerance test. Diabetes. 1960;9:386-391.
  3. Rosenberg EA, Seely EW, James K, Arenas J, Callahan MJ, Cayford M, Nelson S, Bernstein SN, Thadhani R, Powe CE. Relationship between carbohydrate intake and oral glucose tolerance test results among pregnant women. Diabetes Res Clin Pract. 2021 Jun;176:108869.
  4. Hao Y, Qu L, Guo Y, Ma L, Guo M, Zhu Y, Jin Y, Gu Q, Zhang Y, Sun W. Association of pre-pregnancy low-carbohydrate diet with maternal oral glucose tolerance test levels in gestational diabetes. BMC Pregnancy Childbirth. 2022 Sep 26;22(1):734.
  5. Bonuccelli S, Muscelli E, Gastaldelli A, Barsotti E, Astiarraga BD, Holst JJ, Mari A, Ferrannini E. Improved tolerance to sequential glucose loading (Staub-Traugott effect): size and mechanisms. Am J Physiol Endocrinol Metab. 2009 Aug;297(2):E532-7.
  6. WHO Study Group on Diabetes Mellitus & World Health Organization. Diabetes mellitus. Report of a WHO Study Group. World Health Organ Tech Rep Ser. 1985;727:1-113.
  7. Harless FE, McClure GB, Read JA, Brady K: Use of a standard preparatory diet for the oral glucose tolerance: Is it necessary? J Repro Med 36:147–150, 1993.
  8. Entrekin K, Work B, Owen J. Does a high carbohydrate preparatory diet affect the 3-hour oral glucose tolerance test in pregnancy? J Matern Fetal Med. 1998 Mar-Apr;7(2):68-71.
  9. Buhling KJ, Elsner E, Wolf C, Harder T, Engel B, Wascher C, Siebert G, Dudenhausen JW. No influence of high- and low-carbohydrate diet on the oral glucose tolerance test in pregnancy. Clin Biochem. 2004 Apr;37(4):323-7
  10. Garber AJ, Handelsman Y, Einhorn D, et al. Diagnosis and management of prediabetes in the continuum of hyperglycemia: when do the risks of diabetes begin? A consensus statement from the American College of Endocrinology and the American Association of Clinical Endocrinologists. Endocr Pract. 2008;14(7):933-946.
  11. https://www.racgp.org.au/clinical-resources/clinical-guidelines/key-racgp-guidelines/view-all-racgp-guidelines/management-of-type-2-diabetes/gestational-diabetes#ref-num-20. (Accessed 23/3/2025).
  12. Eyth E, Basit H, Swift CJ. Glucose Tolerance Test. [Updated 2023 Apr 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532915
  13. https://www.adips.org/downloads/2014ADIPSGDMGuidelinesV18.11.2014_000.pdf (Accessed 23/3/2025)
  14. https://www.rcpa.edu.au/Manuals/RCPA-Manual/Pathology-Tests/G/Glucose-tolerance-test (Accessed 23/3/2025)