The following is vitamins/minerals and where they are absorbed

DUODENUM:
Riboflavin (B2)
Niacin
Calcium
Iron(iron absorbs better when taken with Vit C)
Protein
Carbs
Starches
Magnesium
Thiamine (B1)

JEJUNUM:
Sodium
Electrolytes
Glycerol
Iron
Calcium
Fatty acids
Cholesterol
Carbs
Peptides
Starches
Amino Acids
Water: 85% is absorbed in jejunum
Folic Acid
Thiamine (B1)
Bile Salts (small amount)
Nicotinic Acid

ILEUM:
L-Absorbic Acid
Vit. C: proximal ileum (iron absorbs better when taken with Vit C)
B12- terminal ileum
Fats
Bile Salts: Mostly in ileum (terminal)
Fat Soluable Vitamins (A, D, E, K): Ileum

Residual Water: Colon

And, here is a list of tests and what they are for (a special list
for people with malabsorption issues):

A: Fat Absorption:
1) Fecal fat microscopy - normal results are less than 100 globules
2) Quantitive Fecal Fat Analysis (one of the best tests for fat
absorption) - normal is 3-5 gm fat/day excreted. More than 6 gm/day
is significant
3) Serum Carotene - carotene level falls in fat malabsorption
4) Serum cholesterol - cholesterol is decreased in malabsorption
5) Fat Breath Tests

B: Carbohydrate absorption:
1) D-xylose absorption test: monosaccaride absorbed in jejunum.
Decreased level seen in jejunal mucosal disease and bacterial
overgrowth syndrome (breakdown of xylose)
2) Glucose tolerance test: results in a flat curve on test
3) Lactose tolerance test: shows an intolerance in infants due to
lactase deficiency or in adults with lactase deficiency
4) hydrogen breath test: disaccharidase deficiency and bacertial
overgrowth. Increased H2 (hydrogen) in breath if dissacharidase
level is low due too increased sugar in gut

C: Protein absorption:
1) fecal microscopy - shows animal skeletal muscle fibre if positive
2) fecal nitrogen - Normal is 2 to 2.5 gm/day.
3) protein losing enteropathy - increased fecal clearance of alpha-1-
antitrypsin. There is increased protein leakage in the intestine
when positive.
4) serum albumin - albumin levels are decreased

D: Vitamin Absorption:
1) B12 deficiency - Schilling Test
2) Serum folate - indicator of jejunal dysfunction (not enough folate)
3) Prothrombin time - Vit K decreased

E: Bile Salt Reabsorption
1) Bile Acid Breath Test: glycine is used. Bile acid glycocholate is
formed. In ileal dysfunction, jejunal bacterial overgrowth, short
bowel syndrome, there is labelled bile acid breakdown in colon

F: Mineral Absorption:
Serum calcium, phosphorus, magnesium, iron and TIBC

Pancreatic Function Tests:
Secretin test: duodenal contents are assayed for enzymes
Lundh test: Duodenal juice is analyzed for tryspin activity

Other Tests:
Duodenal aspirate: microscopy - girdia, culture- bacterial overgrowth