It's not surprising that the fitness industry has more than it's fair share of bullshit. It's just demand and supply: you have a lot of people who are uneducated about the ins and outs of getting in shape and want a shortcut to their perfect body. Then a bunch of companies who are willing to sell them products as these shortcuts, promising unrealistic results.
Like 'fat burner' pills, which are often marketed as if you can sit around pounding Big Macs while they do the work. And as long as you fit in that fortnightly gym visit, your abs will shine through in no time!
So when you see protein bar packaging talking about "impact carbs" (also known as "net carbs" or "active carbs") it's entirely fair that you might assume this is just another fitness marketing gimmick. But is it? Well, kind of - though it can also be useful.
Impact carbs are slightly misleading as they can be seen to suggest that they are the only carbs in the bar. In reality, they are the only carbs in the bar which will affect your blood sugar level. But there are still other carbs in the bar, and those other carbs still contain calories.
For your macros, this means you should still be counting the total grams of carbs listed on the product's nutritional info, not the impact carbs amount. But blood sugar levels do affect fat burning, so impact carbs is useful to be aware of.
Where Did 'Impact Carbs' Come From?
Impact carbs are calculated by taking the total carbs in a product, then subtracting fibre and sugar alcohols. Sugar alcohols are a confusingly named artificial sweetener that are neither sugar or alcohol. They do provide the sweetness of sugar though, but fortunately (unfortunately?) not the buzz of alcohol.
The concept of impact carbs may have been first popularised by the Atkins diet. Naturally it was then seized upon by food manufacturers during the low carb craze. It must help sales - a lot of people would surely just rather believe the product had lower total carbs and leave it there.
But for the three or four of you that do want the truth and have stumbled onto this blog, here it is:
Impact Carbs is a Useful Label
The idea of impact carbs is to separate fibre and sugar alcohols from simple carbs. This distinction is handy because consuming simple carbs pushes up your blood sugar levels, which means:
1) To reduce your blood sugar back to a normal level, your pancreas releases insulin.
2) Insulin then takes the sugar out of your blood and stores it in muscle and fat cells.
3) This can promote fat storage, or at least make the fat burning process slower.
So products that are low in impact carbs will not lead to these blood sugar spikes, and so won't in turn cause an insulin release that could slow your fat loss efforts. In this sense it's good to be able to easily see how high or low the impact carbs are.
Impact Carbs is Also a Little Misleading
At a glance, someone could easily think impact carbs is the total carbs. Or the amount of carbs which contain calories. Or that if impact carbs are low, the total carbs won't be much higher. All of which are not true.
It is true that fibre (which is not counted in impact carbs) contains little to no calories, as it cannot be digested. Sugar alcohols, on the other hand, have lower calories than sugar but are not calorie free.
So if you're lucky enough to have five protein bars in front of you like I do now, each with 2g impact carbs, this means you can't just munch 'em all down for a grand total of 10g carbs and a few hundred calories. Not to say that that should stop you (it probably should though).
So for calculating your macros, stick with the nutrition label and don't try and take any shortcuts based on impact carbs.
Now You Know
Low impact carbs means a product is low in simple carbs, which is definitely a useful distinction if you're looking to stay in shape. It's especially handy if you are cutting, as you are probably looking to minimise consumption of any simple carbs.
Just don't fall into the trap of thinking the other carbs in these products don't count.