Insulin to carb ratio and the 500/100 rule

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pawprint91

pawprint91

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I follow a few diabetic accounts on social media, and I saw one today posting about the ‘500’ rule to find your insulin to carb ratio (so you add up the total of the previous days insulin and then divide 500 by that number - and the total is the number of carbs you would need for one unit of insulin - I believe the 100 is to find your correction dose, adding it up and dividing that number by 100 to see how many mmol your BG will drop by if you inject one unit of insulin).

To be honest, both of these make little sense to me - I was wondering if anyone could shed any light for me, and check I am not misunderstanding etc?

According to that rule my insulin to carb ratio for food is 1:19 - I know that if I applied this my BG would be sky high!

I still go on the 1:10 rule that I was given at diagnosis, but I have seen others on here say they use different ratios for different meals etc - I didn’t even know that was a thing! I always just go on the basis if my BG is this number before I eat and I’m not doing anything that affects it after I need say 1 unit more/less - but I also know I’m more sensitive to insulin at different times of the day (or even the week!) and just adjust my bolus/basal rates accordingly.

Like any of us, I am not perfect but I have no issues with my TIR - is the ratio something I should be asking the DSN nurse about at my next appt? And should I be using the rule above, even though I really don’t think it would work for me?
 
I think it can be helpful to have starting points. However the 500 rule might sometimes be the 350 rule so you can’t ever really place too much confidence in them.

If yoyr doses are working ( or nearly working) changing by some average notional common starting point seems unlikely to be more effective than some tweaks to your current settings.
 
This from a conference presentation some years ago shows some of the from-to possibilities

1682109123706.jpeg
 
everydayupsanddowns said:
This from a conference presentation some years ago shows some of the from-to possibilities

View attachment 25486
Click to expand...
Thanks for this - out of curiosity, I worked mine out using the 350 one and that seemed more of a sensible number for me, so could be something to bear in mind. But as you say, tweaks to current settings could work better!
 
everydayupsanddowns said:
This from a conference presentation some years ago shows some of the from-to possibilities

View attachment 25486
Click to expand...

If I had that much basal I'd be dead by now! Though it does explain how they worked out the dose they put me on when I was first diagnosed, I did wonder about that. It was about three times as much as I need, and they reduced it pretty quickly.

The correction dose I was told to use, presumably also based on the figures given there, is roughly accurate for most of the day but would be no use at all first thing in the morning, when I need about twice as much insulin to have the same effect.

I've never heard the 500/350 thing and have just done the maths and it doesn't work for me at all either (especially as I need so much more insulin for carbs in the mornings than I do the rest of the day). I work out my insulin ratios for different times of day by raising/lowering my insulin depending on what the meal I eat at that time does to my bgl. They change quite frequently, depending on the weather and the 40 other factors ... .

I should stick with what works for you, @pawprint91
 
pawprint91 said:
I follow a few diabetic accounts on social media, and I saw one today posting about the ‘500’ rule to find your insulin to carb ratio (so you add up the total of the previous days insulin and then divide 500 by that number - and the total is the number of carbs you would need for one unit of insulin - I believe the 100 is to find your correction dose, adding it up and dividing that number by 100 to see how many mmol your BG will drop by if you inject one unit of insulin).

To be honest, both of these make little sense to me - I was wondering if anyone could shed any light for me, and check I am not misunderstanding etc?
Click to expand...
I too was puzzled by the '500 rule' or '350 rule' for insulin-to-carbohydrate ratio (ICR), and when applied to me it also produced crazy numbers!

So I went and looked for an explanation-- and looked, and looked-- and didn't find any ...

But, if you think about it logically-- these rules can ONLY work if you assume both of the following:

1) Your usual daily carbohydrate intake is roughly 500g (or 350g), AND
2) Your total daily dose of insulin is usually keeping your BG in a decent range.

If either of those things is not true, dividing 500 (or 350) by your TDD will give you stupid numbers! ... In particular, these 'rules' can only have been based on assumptions about daily carb consumption. I don't deliberately follow any particular diet, but my usual daily carb consumption is a lot less than 500 or even 350.

What I found far, far more sensible was, I think, in BERTIE (https://www.bertieonline.org.uk/ ): Start with an ICR of 1:10 and adjust until you find what works for you. Happily, I found BERTIE-- via this forum!-- before I came across 'the 500 rule' or '350 rule'.

And of course it helps to keep good records, for example in your Libre logbook, of how many grams of carbs you eat and how many units of insulin you inject for each meal-- and then after a while go back and, for each meal, look at the data for days when 'it worked'.

Oh and, Juliet-- snap! "If I had that much basal I'd be dead by now! Though it does explain how they worked out the dose they put me on when I was first diagnosed".

The basal dose they started me on turned out to be about 6 times more than I needed; fortunately they reduced it very rapidly and I didn't die of nocturnal hypo in the meantime. ; )
 
Spathiphyllum said:
But, if you think about it logically-- these rules can ONLY work if you assume both of the following:

1) Your usual daily carbohydrate intake is roughly 500g (or 350g), AND
2) Your total daily dose of insulin is usually keeping your BG in a decent range.

If either of those things is not true, dividing 500 (or 350) by your TDD will give you stupid numbers! ... In particular, these 'rules' can only have been based on assumptions about daily carb consumption. I don't deliberately follow any particular diet, but my usual daily carb consumption is a lot less than 500 or even 350.
Click to expand...
TDD is the combination of both basal and bolus. So the amount of carbs is not necessarily 500 or 359g.
I am not saying the calculation works for me when you include basal but it is closer.
 
helli said:
TDD is the combination of both basal and bolus. So the amount of carbs is not necessarily 500 or 359g.
I am not saying the calculation works for me when you include basal but it is closer.
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Thanks, Helli-- I did know that but forgot when writing the above!

So-- 'the 500 rule' and '350 rule' must be based on three assumptions:

1) Assumption about daily carb consumption,
2) Assumption that your TDD is keeping your BG in a decent range, and
3) Assumption that your basal will be about half your TDD (which I gather is the usual assumption).

And if any of those assumptions is wrong for you personally, these rules will yield crazy numbers.

The basic principle stands, though-- ignore these rules. The suggestion I got from BERTIE and Pawprint was given at diagnosis-- start with 1:10 and tinker depending on results-- is much more sensible!
 
The other assumption I would add is
  • This is guidance, not absolute. It is a useful starting point but will need adjustment to manage things like different insulin resistance at different times of the day, impact (and amount of) exercise, stress, etc
When a DSN first suggests doses, this may be where they get their numbers from. You have to start somewhere.
 
helli said:
The other assumption I would add is
  • This is guidance, not absolute. It is a useful starting point but will need adjustment to manage things like different insulin resistance at different times of the day, impact (and amount of) exercise, stress, etc
When a DSN first suggests doses, this may be where they get their numbers from. You have to start somewhere.
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Yes, Pawprint, you were lucky to be given any starting point at diagnosis! I had to look it up myself.

I still think, though, that advising 1:10 as the starting point for ICR, as BERTIE does, is much better advice-- because it is less likely than the 500 or 350 rules to yield massively wrong results.

As for ISF, or 'correction factor'-- the '100 rule' also yielded results which looked to me to be crazy, so I never even tried relying on it. I just experimented-- with extreme caution! aided by the fact that I have a half-unit pen-- recorded as ever in my Libre logbook-- until I worked out what correction factor usually works for me.
 
@Spathiphyllum my starting was lower than 1;10.
I think it was 1:15 because I was in the honeymoon phase so still producing some of my own insulin. ,I would definitely be hyping a lot if I was on 1:10. It is still less than 1:20.
Likewise, my basal was lower than the guidelines suggest.
Actually, I think I started on fixed dose andy basal was bought up very slowly from a very low base. This was for at least a year. Then I was given my I:C ratio. At the time my basal was less than 10. Not that it has risen much since: it is now 15.
Thankfully, the consultant who provide the dose was able to incorporate his experience into my dose. And have me enough knowledge and confidence to adjust it.
 
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I think the 1:10 rule is simpler and a better start point. As time goes by you may find like me that all the rules go out of the window and guess-work becomes more common.
 
Spathiphyllum said:
1) Your usual daily carbohydrate intake is roughly 500g (or 350g),
Click to expand...

I think it’s more the average RDA of 250ish. At least that seemed to be what it needed when I ran my numbers through it.
 
everydayupsanddowns said:
I think it’s more the average RDA of 250ish. At least that seemed to be what it needed when I ran my numbers through it.
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Yes, that would make sense of the three assumptions listed previously: *if* you're consuming around 250g carbs per day, and *if* your basal is about half your TDD, and *if* your TDD is yielding good results in terms of glycaemic control-- then 'the 500 rule' would work ... except even then it's not much use as a starting point for Robin's reason!
 
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