My favourite summary of offal is from Pay Now Live Later.

For what it's worth, I think you're already eating the most nutrient dense part - the liver.

Eating any of those things sounds offal to me. ;)

I'll second the fact that liver is the best. Kidneys are also an excellent staple, lots of a variety of nutrients; rather like a super-muscle meat, all the same nutrients but lots more of them. Also pretty unobtrusive compared to tripe or some other offals.

I've never seen any nutritional statistics for (thymus) sweetbreads, but would have thought they should contain lots of nutrients. (Have wondered if they'd be a source of iodine). I can't speak for (pancreas) sweetbreads.

On the liver and vitamin A thing. The risks are substantially over-rated. I think the most important thing is your vitamin A to vitamin D balance: more vitamin A requires more vitamin D. Also this is why I go for chicken livers rather than any others, they're about as nutritious but contain relatively less vitamin A. I feel safe having 400g chicken liver around twice a week, but would have some concerns about having lots of other livers that frequently. It's also possible that getting adequate vitamin K2 would be important for vitamin A safety, since it's integral to regulating vitamin A and D. (Lucky then, that liver is a good source of vitamin K2!)

Liver is fantastic for you. Avoid eating the livers out of polar bears and seals and you shouldn't have to worry about too much vitamin A, although some people can be unlucky with normal farm-animal liver, as Peter @ hyperlipid points out.

"Sweetbreads" are thymus and pancreas glands, and can be delicious. Brains are really tasty but I honestly would only go for this if you trust your butcher or if you knew the animal it came from. (Some people are quite worried about prions and will never touch the stuff.)

I think if you're getting liver and marrow you're probably not missing much of anything, nutrition-wise.

It's illegal in most of the US to sell the brains of cows, etc., because they are condemned by the USDA inspectors. On the other hand, you can buy fish heads from the supermarket for very cheap, and there's a lot of things you can do with those. You can steam them and eat the eyes and brain intact or boil the heads in a soup.

I would say buy pigs or calf's feet - OK not exactly offal, but here in France they are in the 'abbats' section of the supermarket (think abbatoir) so I say they count as offal.

They can be boiled up to extract their gelatin (and the amino acids proline and glycine) which is very nutritious and has many health benefits. They are not expensive and seem to be overlooked by many as unworthy of consideration, but they would have been utilized by hunter/gatherers, who indeed would not have wasted any part of the animal.

Remember to add vinegar to your pig's/cow's feet broth; this extracts the maximum amount of gelatin from the hooves and makes for a very rich and jelly-like stock, which can be kept in the fridge or frozen and then used in gravies, sauces and soups....

In traditional cultures, liver would have been given to pregnant, nursing and sick individuals (sometimes raw), so I would say don't go mad on the liver, once a fortnight is probably ample - calf's liver tastes the best, but is the most expensive.

You mentioned lamb heart... Have you tried beef or veal heart? I've recently discovered veal heart, and they are surprisingly delicious if cooked the right way (either briefly like a rare steak, or slow-cooked in a stew for several hours - I much prefer the former). Not sure about any particular nutrition benefits of heart versus other meat or offal, but I would venture that since it's a muscle it would probably have a similar composition to (lean) muscle meat such as steak. With the bonus of being considerably cheaper, however. Worth trying.

It may not be just about nutrition in the normal sense of the word. I found my way here because there's research that says Calf Thymus Extract "accelerated regeneration" of the thymus gland in mice treated with it link. Your thymus gland is where T-cells mature. It usually atrophies with age. I don't know if this means that eating thymus sweetbreads directly can do the same thing for humans - and therefore possibly improve immune function in old age - that's what I'm trying to find out.

Rob