One kilo of lard contains 9,020kcal of energy. A chunky 40kg of lard represents 360,800kcal.
In the simplistic world of calories-in versus calories-out, how many excess calories per day is this, spread over a reasonable weight gain period? Let's assume it was gained over 20 years, or 7,300 days, ignoring leap years.
That gives 49kcal/day of stored fat. If calories-in equals calories-out anyone carrying 40kg of excess weight could have stopped it by eating just one tiny spoonful less of butter per day. EASY. For goodness sake, these people must be such weak willed, greedy slobs that we should withdraw NHS medical care until they get their act together and stop pigging out on a spoonful of excess butter per day.
But just a minute, how many obese people WANT to be obese? If you believe the calories-in calories-out drivel, weight loss should be easy. Anyone can back off this much butter. Weight Watchers would go bankrupt.
But telling an overweight person to avoid eating fat (gasp, all those calories per gram) means that their diet must be based around carbohydrate. Which raises insulin. Which blocks fat loss. Just 49kcal of blocked fat loss each day over 20 years equals an obesity epidemic. It's not the calories from dry bread, spread with low fat goo, that does this but the insulinaemia that goes with it.
Most people do not read biochemistry texts for fun, they expect their nutritionist to have been paid to do that.
PS How DO people loose weight on low fat diets? They eat low everything, get hungry and dip their insulin low enough between meals to loose weight, especially in the early hours of the morning. This is hard, especially to sustain. Fat is your friend when you want to loose weight. No hunger in ketosis.