New evidence from continuous glucose monitoring data indicates that fat, protein, and glycemic index, may also have a major effect on postprandial glucose levels.

Researchers conducted a systematic review of biomedical databases MEDLINES, Embase, CINAHL, and the Cochrane Central Register of Controlled trials, to identify the effects of protein, fat, and the glycemic index (GI) on acute postprandial glucose control in type 1 diabetes and prandial insulin dosing strategies for these dietary factors.

Researchers found seven studies that examined the effect of dietary fat on postprandial glucose in 103 patients with type 1 diabetes. All of the studies added between 6.6 - 52 g to the patient’s meal, and reported a change in postprandial glucose. One study found no increase in glucose concentration, but it may be because postprandial levels were only monitored for three hours. Two studies reported that the additional dietary fat reduced the area under the curve in the first two to three hours, possibly due to delayed gastric emptying. Evidence suggests that meals containing carbs and high levels of dietary fat cause sustained late postprandial hyperglycemia, possibly due to the fact that free fatty acids directly induce insulin resistance.

Next, researchers examined the effect of dietary protein on postprandial glucose levels in 125 patients with type 1 diabetes in seven of these studies. All of the studies kept carbohydrate content consistent, with six adding protein to the test meal, thereby increasing the energy content. The other study kept the carbohydrate level consistent and varied the levels of the both the protein and fat to keep the energy content constant. All studies showed that postprandial glucose levels were modified by the addition of protein, with all except one showing significant differences. All studies agreed that protein leads to increased glucose concentrations in the late postprandial period. When protein was the only macronutrient consumed, glucose levels rose after 100 min for protein loads of ≥ 75 g. When carbohydrates were also consumed with the protein, glucose levels rose after three to four hours.

Lastly, researchers examined seven studies to examine the effects of GI on postprandial glucose in 98 subjects with type 1 diabetes. The studies compared high GI foods or meals versus lower GI foods or meals. All of the studies showed significant differences in blood glucose levels, with lower GI foods and meals eliciting lower glycemic responses. Three of the seven studies suggest that the risk of mild hypoglycemia is greater with low GI foods. Low GI foods are more likely to cause early hypoglycemia, with each unit increase in GI delaying hypoglycemia by one minute.

The researchers concluded that GI, protein, and fat can drastically affect glucose concentrations in individuals with type 1 diabetes. The effect on three hour postprandial glucose concentrations with the addition of 35 g of fat and 40 g of protein to a meal is equivalent to that resulting from the consumption of 20 g of carbohydrates without insulin. The addition of 50 g of fat to a meal can increase insulin requirements for by greater than two fold. This shows us that in order to optimize postprandial glucose control, prandial insulin doses need to be adjusted based on the complete meal composition rather than just the carbohydrate content. The research also showed that prandial insulin taken fifteen to twenty minutes before meals was more effective than doses taken immediately before or after a meal.

Practice Pearls:

  • 1. Prandial insulin doses taken 15-20 minutes before a meal may be more effective in modulating postprandial glucose levels in type 1 diabetes patients.
  • 2. Clinicians should remain aware of the inverse relationship between diabetes and fitness, and regularly monitor pre-diabetic patients.
  • 3. Type 1 diabetes patients should be educated on how their complete meal, carbohydrates, protein, fat, and GI, can impact their blood glucose levels.

Bell KJ. Impact of Fat, Protein, and Glycemic Index on Postprandial Glucose Control in Type 1 Diabetes: Implications for Intensive Diabetes Management in the Continuous Glucose Monitoring Era. Diabetes Care. June 2015. 38(6)1008-15.


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