How do you tell someone who does that that sugar, in such large amounts, is not good for you? How? I mean, these are overall smart girls and they should absolutely know better.
Math? I assume "heaping spoonful" is a tablespoon since you used a tablespoon earlier. So that is five tablespoons which is fifteen teaspoons which is sixty-three grams of sugar. A young lady should consume at most seventy-seven grams a day. This is not including the sugar in the sweetened cream. I'd have literally done the math then and there then explained she just consumed a almost full day's worth of sugar in a cup. I presume she didn't realize that she was consuming a diabetic amount of sugar; if she knew she was I'd think she would have choose not to.
Good math, but if you really want someone to cut down their sugar and illustrate just HOW BAD it is, you need to be going with the WHO recommendations which is less than 10% of calories from added sugar, ideally less than 5%. 10% for the average woman would be about 25 grams or 6 tsp. THE AMOUNT RECOMMENDED FOR CHILDREN BEFORE THE AGE OF 4 IS 0.
OP - I feel your pain, I use to work child development in low-income daycares and the shit that parents would send their children to school with and feed them for meals use to make me feel pretty hopeless about our future.
I have found you can't lecture, but if you can show them out of love, and make it concrete you might have a chance. Most people, yes even smart ones, aren't always aware of how much sugar is in stuff, or they see grams and such and can't equate it to real life. Also, you are battling what is being more and more recognized as an actual addiction. There is a host of chemical, biological, and psychological stuff wrapped up in sugar addiction. If it were a matter of data and logic and cold hard facts, none of us would be consuming it, at all.
If you can come at it from a "learning" mindset, instead of a lecturing "this is bad for you and I know better" way you will have much more luck. Also, this is probably something your wife should be doing, she'd probably have better luck. I have had many versions of the following that I have pulled out ith family, with friends, in the classroom.
Opening: Hey cuz, I got a little worried about mom (whoever they love most) because doc said they are getting diabetes and I wanted to understand it - so I started looking into what causes it. I had NO IDEA that sugar played such a big part of it! So I started looking at labels and I got super confused. Did you know there are over 50 different names for sugar? Talk about making it hard! I also had a hard time wrapping my brain around grams. Can you help me figure it out for some of our favorite foods?
I looked online and apparently, there is 4 grams of sugar in a teaspoon, so let's use that, way easier! I'm a visual person, will you help me?
Hmm let's look at the coke. Okay so this says there are 39 grams of sugar in a can, that is, let's see...39/4 ...wow! almost 10 tsp of sugar, I wonder what that looks like? *Grab sugar bowl and measures out 10 tsp into a bowl*
But we don't have coke too often, I wonder how much sugar is in: fruit flavored yogurt, juice, cereal? <---Take a look and repeat process of measuring out sugar.
This is crazy! We are only supposed to be having 6 tsp in an entire day! I can't believe they put sugar in EVERYTHING! You know, I think I want to try and cut down my sugar. Maybe do one of those challenges I keep seeing on Facebook. Any chance you want to join me? We could do it together!
Reasons why this works:
It is visual - a good online resource for this is Sugar stacks it shows you pictures of various foods and how many sugar cubes are in it:http://www.sugarstacks.com/beverages.htm
You are not lecturing, implying anything about their knowledge or parenting skills. They will be more open to the idea if they are not feeling personally attacked.
You are asking for their help. People are much more willing and open to things if they are helping you with something.
Coming at it from a "Beginner's mind" and not being fake about it, even if you know all of this, helps people become more open to learning. It's not knowledge you are lording over them, it is something you are empowering them to learn with you.
You are creating an us vs. them (food industry) situation instead of a "me vs. you" situation. Yes, you need a certain amount of personal accountability and responsibility, but when you are trying to change world views, giving them a personal out (for now) can help them get on board as it is not like you are personally criticism them and making them feel like shitty people.
It turns it into a shared experience that allows you to help and slowly dribble out advice and resources longer term.
Going after their parenting and trying to change how the feed their kids does ZERO good if the parents aren't fully on board and doing it for themselves as well. It won't stick.
Like I said, this whole experiment would be way better coming from your wife, since it is her family and she knows them and she may be in a better headspace than you to engage them.