Gestational Diabetes & the oral glucose tolerance test (OGTT)

gestational diabetes health Jun 19, 2022
pregnant woman getting blood test

I’m not sure what people are more scared of, the oral glucose tolerance test (OGTT) or actually having gestational diabetes!


Yep, the OGTT is probably one of the most feared tests in pregnancy. I get it, no one wants to drink 75g of pure glucose, especially if you have been super diligent with your nutrition in pregnancy. No other testing is as standardised, e.g. the jellybean test and home glucose monitoring can leave lots of room for error. The reality is even if you have been consuming the cleanest, well balanced diet during pregnancy, you can still develop gestational diabetes (GDM). It turns out that whether you get GDM, might actually be completely out of your control in some regards. GDM is both a metabolic and endocrine disorder. Hormones are increased during pregnancy and due to multiple mechanisms, the action and even secretion of insulin can be decreased. As a pregnancy progresses insulin sensitivity decreases (7) which is an adaptive response of the mother to ensure sufficient nutrients to support the growing foetus and meet placental demands. When pancreatic beta cell function (where insulin is produced and secreted) is insufficient to overcome the increased demands of pregnancy and reduce insulin sensitivity, hyperglycaemia occurs (8). Pregnancy is a stress test!


Approximately 1/7 pregnancies in Australia develop GDM (6). Risk factors for developing GDM include; a family history of DM2, hypothyroidism (1), having micronutrients deficiencies and being overweight or obese (2).


Hyperglycaemia in pregnancy and GDM can lead to both maternal, foetal and birth complications (3). For the mother, increased blood glucose levels can predispose to preeclampsia, maternal hypertension, caesarean section delivery, induction of labour and birth trauma. Complications for foetus include macrosomia (BIG BABY), small for gestational age, hypoglycaemia and hyperbilirubinemia after birth, neonatal hyperinsulinemia, shoulder dystocia, and respiratory distress (4) as well as a predisposition to metabolic diseases later in life (5).


Therefore, I recommend every pregnant woman be tested between 24-28 weeks of gestation.


There is a lot of conflicting information on gestational diabetes and diet, especially when it comes to carbohydrate recommendations. Much of what is published is either based on type 2 diabetes (very different to GDM) or the authors opinion. There is a serious lack of high-quality evidence and consensus on effective GDM management. There is however an abundance of retrospective evidence that points towards a moderate carbohydrate diet (in line with the recommend min 175g per day) and favours a high intake of plant foods including vegetables, fruits, wholegrains, nuts, seeds and legumes. Interestingly, high intakes of animal proteins and fats have been associated with higher risk of GDM (5). So, there goes the evidence for paleo, keto, and any other high protein low carb diet. The good news is, you can still eat carbs!


Carbohydrates are an important macronutrient during pregnancy. Outside of pregnancy, there is actually no carbohydrate requirement or recommended daily intake. Carbs are considered essential during pregnancy to meet the glucose energy requirements of the foetus and placenta. The mother can use ketones as energy and ketone body synthesis helps to meet her energy demands while preserving glucose for the foetus, while fasting.


Mothers with glucose intolerance have a predisposition to developing ketonemia.  Ketonemia in pregnancy has been associated with growth abnormalities, reduced volume and distortion of organs including the brain, heart, thymus and spinal cord in the foetus (9). Therefore, I do not recommend low carbohydrate (<26%) or ketogenic diets in pregnancy. The management of blood glucose levels as well as preventing ketonemia is essential to ensure the health of your baby. Carbohydrate restriction in pregnancy could potentially put the developing foetus at risk of growth abnormalities (2).


Let’s start with prevention and what you can do before falling pregnant and in early pregnancy.


Moderate carbohydrate diet

  • As mentioned about, 175g is the minimum requirement of carbohydrates in pregnancy. This is not a lot! By the time you have 5 serves vegetables, 2 pieces of fruit, some oats, 2 serves of legumes a slice of wholegrain bread and a cup of milk, you are there!! You don’t have to follow the grain heavy food pyramid to get your carbs.
  • One very recently published study (7) showed a benefit with eating 50% of your daily intake of carbs in the morning vs in the evening meal. Hello porridge!
  • A diet the contains moderate carbohydrates, less red meat and animal fats has been shown in multiple studies (10–12) to reduce the risk of developing GDM.


Ensure optimal protein intake and focus on plant proteins

  • Plant protein over animal protein have been shown to reduce the risk of GDM (11,12).
  • Good news is that studies have also shown swapping a proportion of animal protein for plant based protein daily also improves fertility (13).
  • Plant protein AKA legumes + beans, are rich in protein, fibre and you guessed it, complex carbs. A one stop shop really!


Micronutrient adequacy – vitamin D, E, zinc and magnesium (plus more)

  • Vitamin D is involved in the secretion of insulin from the beta cells of the pancreas, which helps to lower blood glucose levels (11,12).
  • Diets high in vitamin C and E, both found in higher concentrations in plant foods, are also suggested to play a role in glycaemic control and reduce risk of GDM (11).
  • Zinc is an important mineral involved in the synthesis, storage, and release of insulin (14,15). Zinc deficiency may predispose glucose intolerance and insulin resistance. Nuts, seeds and seafood are all excellent sources of zinc.
  • Magnesium, helps to reduce insulin resistance but has no actual effect on blood glucose (16).



  • Regular exercise help to improve insulin sensitivity, which is basically how responsive your cells are to insulin. The more responsive they are, the less insulin your body needs to produce and the less likely you will end up with unchecked hyperglycaemia (high blood glucose) or diabetes.


If you’ve been diagnosed with GDM, it is really important you show improvements in glycaemic control in the first few weeks, otherwise you might need pharmaceutical intervention. Do yourself a favour, get a home glucose monitor and learn how to measure your own blood sugar. You will very quickly learn what spikes your blood sugar.


Here are a few nutritional tips for improving glycaemic control with managing GDM (I do get a bit stricter here):


Ditch the sugars + starch

  • Sugar is the biggest villain. Ditch any added sugars like table sugar, honey, and maple syrup, etc.
  • Removing starch is also helpful because starch is just very long chain of glucose and actually raises your blood glucose more than glucose itself! Basically, anything that has been made into a flour (bread, pasta, cake, crackers) and sadly cooked potatoes. Some fruits and vegetables are higher in starch (e.g. bananas), though these foods are generally higher in fibre, which slows glucose release (low GI) but again, use your home glucose monitor to see how YOU tolerate these foods.


Focus on low glycaemic index + complex carbs

  • Low GI foods as mentioned above, are slow release carbs. They usually contain fibre.
  • When I say complex, I mean complex. Not wholegrain bread but wholegrain sprouted breads are best.


Include plenty of low starch vegetables

  • Asparagus, broccoli, cucumber, cauliflower, cabbage, zucchini, spinach, onions, mushrooms, celery etc
  • Not only are the all a rich source of essential nutrients, including folate, they are also packed with fibre and have minimal effect on blood glucose levels.


Pair it with protein

Adding protein to a meal helps to lower the GI of the overall meal.

  • Oily fish, smaller species fish, like sardines would be my top pick as it is high in omega 3 fatty acids which also help with insulin sensitivity.
  • Eggs are packed full of protein, omega 3 and vitamin D.
  • Nuts and seeds are also a great source of healthy fats, protein, minerals and vitamin E. All nuts are great, with the exception of cashews which are higher in starch. Just don’t go eating a whole bag of cashews!!!


Meal timing and frequency

  • Some women will do better with 5-6 smaller meals, while others will be ok with 3 main meals and a snack. Do what work for you and make sure you are checking you blood glucose regularly.
  • Carbohydrates are generally better tolerated in the mornings and 50% of total daily carb allowance when eaten at breakfast, actually reduce fasting blood glucose in women with GDM, according to a recent study (7).


Ensure optimal micronutrient intake

  • Vitamin D, C and E plus zinc and magnesium all play a role in blood glucose management. Do yourself a favour, get you vitamin D and zinc levels checked at the very least.
  • Vitamin C supplementation, when use concurrently with insulin therapy, reduces the amount of insulin required. This can be beneficial but also potentially dangerous if not closely monitored.
  • Working with a nutritionist will help to identify deficiencies in the diet and they can advise on supplementation dosages that are appropriate for pregnancy and concurrent use of medications.



Myo-inositol is a simple carbohydrate that is given in supplemental form. It controls the action of insulin and assists with glucose entry into cells, reducing blood glucose levels. When supplementing with anything other than a prenatal and fish oil in pregnancy it is important to seek professional advice.

Your pre-pregnancy diet and micronutrient status can all help to reduce your risk of GDM. During pregnancy, it is important to ensure you meet the minimum 175g of carbohydrates daily but also focus on complex and less starchy vegetables. You baby and placenta need carbs! Be sure to balance meals with protein and healthy fats to ensure better glucose tolerance. At the very least, take a prenatal multivitamin that contains a complex of vitamin D, C and E, zinc and magnesium for blood glucose management and insulin sensitivity. If micronutrients testing is available to you, I do highly recommend testing in preconception or first trimester as well as in the late second or early third trimester, to ensure nutritional adequacy. And please don’t be freaked out by the OGTT, it’s really not that bad! It is important for your health and the health of your baby that GDM doesn’t go unrecognised in pregnancy.


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