Karen K. Ryan and Randy J. Seeley. Food as a Hormone. Science 22 February 2013: 918-919.
In an opinion piece in the February 22 issue of Science, KK Ryan and RJ Seeley argue for an alternative approach to look at diet and food – not in terms of nutritional epidemiology (“this is what healthy people eat”), nor in terms of macronutrients (“fat makes you fat!” “sugar makes you fat!”), but to view food as a grab bag of exogenous (coming from the outside) hormones.
Wait, what!? Food as hormones?
Hormones are long-range messengers that certain cells spit out into the bloodstream to act on other cells. Hormones can either bind to receptors on the cell surface(like Human Growth Hormone), or activate receptors in the cell nucleus and activate gene expression (like testosterone). Chemically, hormones are not that special – they’re usually short amino acid chains (peptides), fatty acids (lipids), or modified types of aromatic amino acids called monoamines. What they’re called doesn’t really matter – what’s important is that some of these chemicals can be found in the stuff we eat.
A well-studied example is omega-3 fatty acid, which has been shown to boost heart health, lower risk of stroke, and (may) help with weight loss and depression (WebMD). From a standard biochemical point of view, where omega-3 is regarded as an energy source, these specific effects are difficult to explain. A study by DY Oh yields a clue: omega-3s can not only be used as an energy source, but can also bind to and activate a receptor called GPR120 which is expressed on fat and muscle cells. Since loss of GPR120 activation is associated with weight gain, inflammation and inadequate blood sugar control, it is conceivable that omega-3 may directly control your metabolism through its activity on GPR120.
Omega-3 isn’t the only example of food directly acting on your cells. Branched-chain amino acids, such as Leucine, are often consumed by fitness enthusiasts as supplements to build muscle. Why would Leucine be superior to a “straight” amino acid, such as glycine? Leucine can directly act on a central growth-regulating pathway called mTOR. In the periphery (body), mTOR activation can induce increased translation of components of skeletal muscle, leading to bigger muscle growth. In the central nervous system, mTOR activation signals satiety (feeling of fullness), and so can reduce food intake and body weight. Not bad for a simple amino acid found in cottage cheese!
There are many more similar examples, all demonstrating that the relationship between foodstuff and our bodies is not simply one of energy consumption. Following this line of thought, Ryan and Seeley propose that we should design diets “from the bottom up” – based on how they act on signalling systems (such as mTOR) that we know are linked to metabolic diseases.
It is an interesting proposal, though (traditional foodie as I am) I have the nagging feeling that this seems too much, too soon. How much do we REALLY know about what’s going on at the cellular level? For any food component, how much gets used as energy and how much is left to directly act as a hormone? Does the interaction between components alter signaling abilities? How does digestion and exercise fit into the gist of things?
While a lot of the above research has gone into fitness nutrition planning and fad diets, it’ll probably still be a while (if ever) before we enter the realm of “designer diets”. In the meantime, GIMME THAT STEAK!
More reading (paywall):
1) Omega-3: D. Y. Oh et al., Cell 142, 687 (2010).
2) Leucine reducing food intake: D. Cota et al., Science 312, 927 (2006).
3) Effect of Leucine on protein synthesis:
Orginal source: Merrick, W. C., & Hershey, J. W. B. (2000) The pathway and mechanism of initiation of protein synthesis. In: Sonnenberg N, Hershey JWB, Mathews MB, editors. Translational control of gene expression. Cold Spring Harbor Laboratory Press.
Ryan, K., & Seeley, R. (2013). Food as a Hormone Science, 339 (6122), 918-919 DOI: 10.1126/science.1234062