The gym teacher was kind of right, actually. Eating fat, even healthy animal fat will, at least temporarily, make you fatter as the fat requires very little energy to digest, transport and store (0-3%). It'd be very difficult to become obese like this, but I absolutely guarantee that you could get flabby eating a lot of LCTs, especially when added to a mixed diet. I wouldn't say it's wrong to do so, unless you're trying to lean out. What is wrong is telling 8-year old kids to eat less, especially since the good sources of fat are also highly nutrient-rich. No fat kid you have ever seen got that way from eating too much fat attached to meat.

I never talk about calories because they're rarely equivalent unless they're coming from the same macronutrient class. The thermic effect of carbohydrates is 5-10% and protein is 20-30%. Evolution favors the most efficient process, and clearly our preferred long-term energy storage method is packing triglycerides into adipocytes. If you hand your body a fatty horn of plenty, it will rejoice and store it away in your fat cells. It would be insane if we evolved to do anything else.

You don't really have to go into a lot of scientific detail at that age unless you really want to. The information she was given was pretty simplistic, so all you really need to do is explain that your family has different beliefs and explain why. At the most basic level you can talk about how the way food is produced/grown makes a big difference in how many nutrients it has and we want to fill our tummies with foods that have lots of nutrients to keep us strong and healthy instead of processed food that fills us up without giving us the nutrients we need to live and grow. One of the ways we know which foods are best for us is by looking at the foods our ancestors ate many, many years ago and making sure we are also eating plenty of those foods. Calories aren't bad or good. They are simply a measure of energy. It's not as important to think about calories as it is to choose healthy whole foods like fruits, veggies, meats, and fish.

You can also explain how for a long time people thought that eating fat would make you fat, but we now know that isn't true even though some people still believe it. There are some fats that are unhealthy to eat (manufactured transfats, industrial seed oils, etc.), so your family doesn't eat them.

If you want to get into more detail, there are some great teaching resources designed for kids and young adults. While not specifically paleo, the Omnivores' Dilemma has a kids' edition which explains the problems with factory farming and emphasizes the importance of eating whole foods. Food Renegade has two nutrition programs for kids. One is for ages 6-11 and the other is for teens and adults ages 12 and up. Again, it's not specifically paleo. It follows the WAPF philosophy, but it is still going to be pretty close in line with paleo beliefs. You can always skip the dairy (if you avoid dairy) and grain/legume chapters.

One more thing: your child is likely going to wonder why the teacher gave her bad information. I would definitely make sure you take the time to explain that the teacher isn't "lying"; s/he believes the information will help the students. Explain that different people, and even the government, have different beliefs about nutrition and because there are so many different theories it can be very confusing for people. But, you have spent a lot of time reading about nutrition and you believe this is the best way for your family to eat to stay strong and healthy.

I might have said that the gym teacher was actually pretty close to being right ... we're eating too many of the "wrong" calories (in the form of hyper-palatable, processed foods made from wheat, refined sugar, and vegetable oils). They are "wrong" for us because they are newer foods that our bodies haven't had time to adapt to eat. They are also "wrong" because it is really easy to overeat regularly (i.e., eat a day's worth of calories in a single meal, especially when liquid calories from soda or juice are factored in) which is something our bodies cannot handle well (it's like continuing to pour gas into a tank that's already full) ... we're designed for dealing with famine, not perpetual feasting.

I agree with Kewpie that it's worth pointing out that people aren't lying when they say one thing or another, but that nutrition is complicated and we're finding out new things every day. But for us, the principle is whether or not it makes sense from an ancestral perspective.

My 17-year-old grandson has shared some conflicting statements he's heard at school. I've found I don't need to debate them; I don't even think he wants me to. I just try to be very clear about what I'm doing and why and he seems to give me a lot of credibility over what others, even teachers, might say.

A larger concern for me is that he's worried about staying slim and is threatening to take the don't-eat-anything mode. Since he's a cross-country athlete I've stressed the need for his joints and muscles to have fuel. I pointed out that he's abusing his joints and needs to eat gristle and tendons and bone broth, etc. Since he's burning so much energy and is extremely slender now I haven't tried to bad-mouth carbs in general, but I do bad-mouth his beloved junk cereals and I hope to make him think over time. Happily for me, he likes meat and fruit. Veggies not so much though he occasionally surprises me by taking some.

My bottom-line recommendation: do a lot of communicating but don't trigger tugs-of-war or take extreme positions that will endanger your ability to be heard over time.

If you over-eat excess calories you will gain weight.