A Random Blood Sugar Gives You a Number. A Timed Blood Sugar Gives You an Answer.

I recently visited a well-known hospital in Hyderabad to discuss trialling Ruby Roti in inpatients with diabetes. We know it works wonders but we wanted the to challenge the hospital to test and see.

The plan was simple: serve a Ruby Roti meal and measure its effect on blood glucose. However, the proposed assessment relied on random blood sugar readings, and some of the dietitians were unfamiliar with continuous glucose monitoring (CGM).

This highlighted an important gap. If dietitians and nutritionists advise people about carbohydrates, glycaemic index, glycaemic load or “diabetic-friendly” foods, they must also understand how to measure the glucose response correctly.

A random glucose reading cannot assess a meal

A random blood glucose measurement may be taken at any time—after food, medication, insulin, exercise or during illness. Without this context, it is difficult to interpret.

Most importantly, it cannot reliably show whether a particular food caused a glucose spike.

To assess a meal, take two timed readings:

1. Immediately before eating

2. At a fixed time after the first bite—usually 90 minutes or two hours

Then calculate:

Glucose rise = post-meal glucose − pre-meal glucose

For example:

• Before the meal: 120 mg/dL

• Two hours after the meal: 155 mg/dL

• Glucose rise: 35 mg/dL

The starting value matters. A post-meal glucose of 155 mg/dL means something very different if the pre-meal value was 80 rather than 150 mg/dL

Make every glucometer strip useful

If you want to understand how food affects your glucose, avoid repeatedly testing at unplanned times. Instead:

• Check fasting glucose on waking

• Check immediately before a meal

• Check 90 minutes or two hours after the first bite

• Record the food, portions, medication, insulin and activity

• Use the same method when comparing meals

Random testing remains important when someone feels unwell or may have low or very high glucose. People using insulin may also need additional safety checks.

The message is therefore:

Do not use an isolated random reading to judge the effect of a meal.

Which test answers which question?

No single test is best for every purpose. HbA1c shows the long-term picture, while paired pre- and post-meal readings—or CGM—are better for assessing food.

Diagnostic ranges

Abnormal diagnostic results usually require confirmation unless there are classic symptoms with unequivocal hyperglycaemia. These thresholds are consistent with NIDDK diagnostic criteria and the ICMR Standard Treatment Workflow.

For many adults already diagnosed with diabetes, commonly used treatment targets are:

• Before meals: 80–130 mg/dL

• One to two hours after starting a meal: below 180 mg/dL

• HbA1c: commonly below 7%

These are management targets, not diagnostic cut-offs, and must be individualised.

How hospitals should decide on a ‘diabetic meal’

A random glucose reading is not enough to compare a new food with a patient’s usual meal.

A practical hospital protocol should record:

1. Glucose immediately before the first bite

2. Glucose at the same fixed time after eating

3. The complete meal and portion sizes

4. How much the patient actually ate

5. Insulin and medication doses and timing

6. Relevant factors such as infection, surgery, steroids, intravenous glucose or activity

7. The calculated glucose rise

Where possible, compare the new meal with the patient’s usual meal at the same time of day, using similar portions and comparable medication.

A lower post-meal glucose after a larger insulin dose cannot automatically be attributed to the food. Likewise, one meal in one patient is a demonstration—not proof.

Where do CGM and C-peptide fit?

A CGM sensor measures glucose throughout the day and night. It can show:

• The size and timing of meal-related rises

• How long glucose remains elevated

• Glucose direction and variability

• Overnight patterns and low glucose episodes

• Time spent within the target range

This provides far more information than isolated finger-prick readings, although hospital treatment decisions may still require bedside confirmation.

C-peptide estimates how much insulin the pancreas is producing. It can help distinguish diabetes types when the diagnosis is uncertain, but it is not a routine test for assessing meals or daily glucose control.

The practical takeaway

To understand how a meal affects your glucose:

1. Check immediately before eating.

2. Check again at the same fixed time after the first bite.

3. Calculate the rise.

4. Record the food, portions, medication and activity.

5. Repeat before drawing conclusions.

Do not change prescribed medication or insulin based on a single food test.

For the general public:

If strips are limited, make each one answer a useful question: check fasting, or check before and after a meal.

For hospitals, dietitians and nutritionists:

If we recommend food for diabetes, we should know how to measure what that food actually does.