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Adipose tissue (fat tissue).

The brain.

The brain has a border crossing for all cells called the blood-brain barrier that controls what enters it. Only lipid-soluble molecules such as oxygen, carbon dioxide, and steroid hormones can freely cross this border. All other molecules either need to be shuttled in or can’t enter at all. This is important to remember, because many companies claim that their supplements have effects on the brain, but in reality they don’t, since they can’t cross the blood-brain barrier. An example of this is GABA, a neurotransmitter that is involved in elevating mood. Often, those who suffer from anxiety have low levels of GABA and try to increase them with supplements (for supplements that do help alleviate anxiety, see ssss1, Supplements for Mountain Athletes).

Two systems control the digestive process: the endocrine system and the nervous system. When the endocrine system senses changes in the body, it sends a chemical message in the form of hormones to tell the body how to respond. For example, back to the digestive journey of the strawberry: once you ingest it, the small intestine detects the number of carbohydrates entering your system and helps your pancreas stimulate an insulin response. The nervous system works to regulate the digestive system by using nerve impulses and releasing neurotransmitters. Some of the control originates from nervous and hormonal connections between the digestive system and the central nervous system.