The calculations
Required background knowledge before getting started.
Insulin. Nearly all insulin used will have a concentration of 100U/ml, but there are more concentrated versions available. Low doses of insulin are typically used in a keto diet, so a higher concentration of 300u/ml is not required because low insulin volumes with high concentrations of insulin could lead to dosing errors. Ensure the patient has the correct insulin concentration. It is highly likely they will.
Insulin Carb Ratio. ICR. This ratio is based on the number of grams of carbohydrates that will be adequately metabolised by 1 unit of insulin. Typically people start with a ratio of 1:10. so, one unit of insulin is required for every 10g of carbohydrate. It is a variable figure and can change, even throughout the day. Many people with Type 1 find that applying ICR as a fixed number is not practical. For example, many find that the ICR is lower throughout the morning and much higher in the afternoon. It is not a problem but something to acknowledge and work with. This is refined through trial and error. Start with the ratio the patient is using and refine the ratio as needed.
Insulin Protein Ratio IPR. The starting ratio is usually twice that of the ICR. It is explained at the end of this module. For example, if the ICR is 10:1, IPR is 20:1
ISI Insulin Sensitivity Index. ISI is used widely in the management of Type 1 diabetes, but informally in most cases. Insulin Sensitivity is the amount of insulin required to lower blood glucose. It reflects the ability of cells to remove glucose from the bloodstream under the influence of insulin. The more insulin sensitive, the less insulin is required for this action.
People have different sensitivities at different times of day, with certain foods and over time. Chronic use of higher volumes of insulin can lead to progression to a Type 2 diabetes picture on top of Type 1 diabetes. This is sometimes called double diabetes. In this situation, the ISI drops so that more insulin is needed to get a reduction of blood glucose. It is a dynamic situation, but in each individual, it is relatively easy to adjust to both short-timescale and longer-timescale changes in ISI. Be aware that the ISI changes. It can be noticeable in menstrual-aged females every month. But they learn to anticipate and work with the change. People who use pumps are able to adjust insulin delivery rates to change hourly if needed. For users of pens, splitting long-acting insulin into 12-hour intervals will allow more flexibility in the adjustment to changes in insulin sensitivity.
It has been widely stated that in an insulin-sensitive adult, 1 unit of insulin will lower blood glucose by 2.5 mmol/l approximately. It can vary a lot. Some children have an ISI of 5. People who have progressed to double diabetes might find it as low as 1 or even lower. ISI is used in dose correction calculations, so it is important for each person to know roughly their ISI bearing in mind its variability. Work on an ISI of 2.5 initially. Continuous glucose meters help to make life easy, and the gradient of glucose reduction after an insulin injection will enable that calculation to be easier to make. The insulin carb ratio will also give a crude idea of ISI, but this will be against a background of mixed carbohydrates in most cases, so is a rough estimate. Injecting a small amount of insulin after a fast will give a clear idea of ISI, but the possibility of a hypoglycaemic event must be uppermost in the mind. The problem with ISI is its variability, as explained above. It can only be an approximation, but it will be needed to calculate correction doses.
https://www.dtu.ox.ac.uk/homacalculator/
A model was developed to assess Insulin sensitivity by measuring something called the Glucose Disposal Rate. It is not routinely adopted, but it uses three pieces of easily obtainable data and seems to correlate with other assessment methods.
The eGDR (mg/kg/min)= 21.158 − (0.09 * Waist Circumference in cm) − (3.407 * Hypertension (1=yes,0=no)) − (0.551 * HbA1c (%))
A higher number ( so the more efficient a person is at glucose removal from the blood) is more protective against stroke in Type 2. https://cardiab.biomedcentral.com/articles/10.1186/s12933-021-01394-4#Sec2
Insulin Resistance
Insulin resistance is the reciprocal of insulin sensitivity. There is a model of insulin resistance called the Homeostatic Model Assessment (HOMA-IR), which is mostly used in Type 2 diabetes. It assumes that insulin levels are maintained by a compensatory mechanism when there is reduced beta-cell function. Fasting insulin levels are elevated in direct proportion to diminished insulin sensitivity. HOMA requires measurements of insulin and glucose and, for that reason, is not practical with the current lack of availability of insulin tests. This situation might change, and it will be helpful to measure insulin resistance in the future. There are commercial labs that provide insulin measurement.
But for practical purposes, while it is important to be aware of these calculations, a simple estimate of the amount of insulin required to maintain normal glucose is sufficient. In practice, the basal requirements will highlight the level of insulin resistance. The higher the volume of insulin, the more likely it is that insulin resistance exists.
Calibrating the Insulin Basal Dose
Adapting insulin management to a ketogenic diet consists of the same three steps
. Basal calibration,
Carbohydrate counting,
Insulin bolus injection.
Patients will consciously or unconsciously apply all of them during their daily management of Type 1 diabetes. It is common for people to forget their training and then get into a routine that works for them to stay out of trouble. The only difference for a keto diet is that they will also need to use bolus doses for protein. This is explained below. If you find that the patient’s HbA1c is higher than ideal (and this is the case in 95% of Type 1 on conventional management), then you should initially go through a revision of the basics. This equally applies to a keto diet. The next module has a practical session on insulin estimation and carbohydrate counting.