Why isn’t carb counting working?

Barry Rogers
Barry Rogers

One slice of Hovis medium white bread is 17.9g of carbs according to the nutritional label. Seems very specific so it must be pretty accurate, right?

People like precision. We like as much detail as we can get when it comes to numeric values. The value of pi has been calculated to 31 trillion digits. That’s a lot of precision. So when we see 18g, 17.9g and 17.91g next to each other, we put a lot more faith in the latter than the former. 

But when it comes to nutritional values on food packaging, should we have that much faith?


The 20% Tolerance Rule

Around the world, guidance is given by regulators for how accurate, or inaccurate, nutritional labels are allowed to be. 

For the sake of simplicity, we’re going to summarise this as the 20% Tolerance Rule. What this means is that the nutritional values for foods can be up to 20% higher or lower than the label value when tested and still be considered acceptable. If you are interested, full details are available for the UK/EU and the USA below with guidance for other countries found online. Warning: reading them may send you to sleep.

So what does this mean in practice?

For the 17.9g of carbs in our slice of bread from earlier, it would be acceptable to be anywhere from 14.3g to 21.5g. And that’s assuming we haven’t got one of those squashed loaves where sometimes you get a smaller slice or one with more holes than you would like.

So if you’re having two slices of bread for lunch, it can be useful to know that you could be eating anywhere between 28.6g and 43.0g of carbs and this wouldn’t be considered unusual. The same 20% rule applies to the protein, fats, vitamins and total calories.


Accuracy and insulin

Obviously, this makes carb counting and deciding how much insulin to dose incredibly difficult. For someone who takes 1 unit of insulin for 5 carbs, the difference between 28.6g and 43.0g could mean taking anywhere between 5 units and 9 units. 

There is no right answer. 

There’s also no wrong answer. It’s impossible to know exactly how many carbs or how much protein is in the food you are eating. It’s also impossible to know how this will be broken down by your body once eaten or how much your body will process as waste. Or whether the food you’re eating actually fits within the regulations at all.

Hundreds of millions of investment has been spent on some of the world’s smartest researchers and scientists to try and calculate exactly how much insulin to dose over the last 50 years. They haven’t solved it. 

All you can do is make a decision at the time with the knowledge you have available and see how it goes.

If it doesn’t go as you would like every time, it’s not your fault. Leave it behind you, it cannot be changed now. The next time you need to make a similar choice, Quin will help by showing you what has happened before. If there are any learnings, they can help inform your next diabetes decision.


Quin and carbs

Carb estimating can be a more useful term than carb counting. Knowing there’s no exact figure can better prepare you for the potential range of blood sugar outcomes over the next few hours whilst continuing to inform on how to dose. 

In Quin, you can store a carb range for personal portion sizes for each of your foods and drinks. This allows you to estimate the range of carbs once for your portion and then the next time you eat the same food or drink, the carb range is available as a reminder so you don’t need to estimate it again. Just come into Quin before you decide what to dose, select the food or drink and see how many carbs were in the portion. Even better, use the ‘What have I done in the past?’ button after selecting your food or drink and also see how many units of insulin you dosed alongside your starting blood sugar level and any other actions you took.

There is some comfort to having specific numbers and we can completely understand how this makes thinking about carbs and dosing easier. With Quin, you can store a precise number of carbs for each of your personal portion sizes by selecting the same values for the range. 

Hopefully, knowing the 20% Tolerance Rule will give you a little more understanding of why your blood sugar can vary so much after insulin dosing for food and drink. If your blood sugar doesn’t end up exactly as you expect, it’s not your fault, there’s too much ambiguity in food and our bodies to get it right every time. You can’t change the past but with Quin you can make more informed diabetes decisions in the future.







Barry Rogers
Barry Rogers