Nutrition and Energy Metabolism

The energy used by your body to function at rest. It includes basic tasks like breathing, heart beating, and organ function.

The energy needed to perform everyday activities like walking, running, or lifting weights.

The energy your body uses to digest and absorb food.

The total amount of energy your body uses in a day, including BMR, physical activity, and DIT.

Nutrients that the body cannot produce on its own and must obtain from the diet.

The average daily intake of a nutrient needed to meet the requirements of nearly all healthy individuals.

A recommended intake level used when there isn't enough scientific data to establish a precise RDA.

Organic compounds necessary for various biochemical processes in the body, obtained from the diet.

Most water-soluble vitamins are absorbed from two sources: the diet and synthesis by microbes in the large intestine.

Biotin, folic acid, pantothenic acid, riboflavin, and thiamin are all absorbed from both dietary sources and microbial synthesis.

Niacin can be produced in the body from tryptophan, but it is also absorbed from dietary sources.

Water-soluble vitamins from the diet are mainly absorbed in the small intestine, while those produced by microbes are absorbed in the large intestine.

Thiamin (B1) is absorbed in the proximal small intestine using transport proteins THTR-1 and THTR-2.

Riboflavin (B2) is absorbed in both the small and large intestines. The active transport does not need sodium or specific pH levels.

Niacin (B3) can be absorbed through the stomach wall.

Pantothenic acid (B5) is absorbed in the small intestine using a sodium-dependent carrier, but in high concentrations, it can also be absorbed directly.

Vitamin B12, after being released from food, binds to Intrinsic Factor, a protein produced in the stomach. This binding protects B12 from digestion in the small intestine, ensuring it reaches the terminal ileum for absorption.

Fat-soluble vitamins like A, D, E, and K are absorbed differently than water-soluble vitamins. They are first incorporated into micelles, small spheres that help them travel through the small intestine. Then, they get packaged into chylomicrons, which are transported through the lymphatic system before entering the bloodstream.

Vitamin A, a fat-soluble vitamin, is found in foods like carrots, sweet potatoes, and spinach. It can be absorbed directly from these sources or generated from carotenoids, which are plant pigments. Vitamin A, in the form of retinol, is transported as part of micelles and also stored in liver cells.

Vitamin D, another fat-soluble vitamin, is absorbed by passive diffusion in the duodenum and ileum. This means it moves directly through the intestinal lining without needing special proteins or active transport.

Vitamin E, a fat-soluble vitamin, is absorbed by passive diffusion in the distal small intestine and ileum. It follows the same principle as Vitamin D, moving directly through the lining of the intestines without any special help.

Vitamin K, the last of the fat-soluble vitamins, is absorbed by passive diffusion in the small intestine. It also travels through the intestinal lining without active transport, similar to Vitamin D and E.

Sodium absorption involves different transport mechanisms depending on location. In the small intestine, it's mainly coupled with other substances. In the ileum and colon, sodium/proton transporters and aldosterone-responsive channels play a role.

Chloride absorption is closely linked to sodium absorption to maintain electrical neutrality. While chloride is usually absorbed through channels, it can also be secreted through cAMP/ATP-gated channels.

Potassium absorption in the small intestine primarily occurs through passive paracellular diffusion, driven by the concentration gradient. This means potassium moves from a higher concentration area to a lower concentration area through gaps between cells.

Magnesium absorption is primarily passive (90%) through paracellular pathways, but some (10%) also happens via active transcellular facilitated transport in the distal small bowel. This means magnesium moves passively through gaps between cells and actively through cells with the help of transporter proteins.

Calcium absorption occurs in the duodenum through an active transcellular process, requiring energy and transporter proteins. In other parts of the gut, passive absorption through gaps between cells also occurs if calcium levels are high or normal.

Water absorption happens almost entirely (100%) in the small intestine, mainly in the proximal (beginning) part. It's driven by osmosis, following the concentration gradient of electrolytes (salts) that are being actively or passively absorbed.

Intracellular fluid (ICF) makes up 40% of total body water, while extracellular fluid (ECF) comprises 20%. ECF encompasses interstitial fluid (75% of ECF) and plasma (25% of ECF).

Insensible water loss occurs through skin and lungs, and varies depending on environmental temperature and exercise intensity. Sweat loss greatly increases during hot weather and heavy exercise.

Maintaining water balance requires matching water intake with daily water loss. This loss includes insensible loss (through skin and lungs), sweat, feces, and urine.

The body's ability to absorb nutrients varies depending on the specific mineral or substance. Some, like potassium and magnesium, are absorbed passively, while others like calcium require active processes. Water is absorbed nearly entirely in the small intestine, mainly in the proximal portion.