Molecule of the Day has a post up about isotopically-enriched food that caught my eye for a couple of reasons. Firstly, the idea is wildly outrageous, and, secondly, this is something that actually gets joked about quite a bit in an NMR (nuclear magnetic resonance) lab.
Any given element can come in various isotopes, which differ in the composition of their nuclei. The nuclei of different isotopes of the same element have the same number of protons, but they vary in their number of neutrons. Because the number of neutrons in a nucleus does not significantly affect the chemical properties of an atom (except in a few specific cases), the various isotopes of an element all have virtually the same chemical properties. Only certain nuclei are visible in NMR experiments, and three of the most commonly used are 1H, 13C, and 15N. With the exception of 1H, these are not the most abundant isotopes of these elements (12C and 14N are, but neither is NMR-active).
To perform NMR experiments on proteins, then, we often have to produce proteins that are isotopically labeled with 15N and/or 13C. Isotopically-labeled molecules are not cheap, but simpler molecules are cheaper than more complex molecules. Therefore, it's advantageous to produce our proteins in simpler organisms, such as bacteria, which can synthesize all of their own amino acids (the building blocks of proteins) from simple molecules. More complex organisms--such as humans--can't do this. One gram of 15N-labeled ammonium chloride costs about $15, and 1 g of 13C-labeled glucose costs about $60. On the other hand, 1 g of 15N- and 13C-labeled amino acids costs about $2,000, so there is quite an financial incentive for producing isotopically-labeled proteins from simple precursors in simple organisms. (None of these isotopes are radioactive, which is a phenomenon that only occurs when the nucleus of an isotope is unstable.)
Since the various isotopes of nitrogen have virtually the same chemical properties as each other (and the same applies to the various carbon isotopes) there wouldn't be much of a reason for a human to eat food labeled with 15N or 13C (although your shit would literally be worth much more than its weight in gold). No, the isotope that Molecule of the Day wrote about ingesting is deuterium (D or 2H), which is an isotope of hydrogen that features one proton and one neutron in its nucleus (as opposed to one proton and no neutrons in the much more abundant 1H nucleus). In this case, however, the presence of deuterium versus hydrogen actually does influence the chemistry of a molecule. Specifically, it affects the rates of chemical reactions through the kinetic isotope effect.
If an atom participates in a chemical bond that is broken in the rate-determining step of a chemical reaction, then the rate of the chemical reaction is influenced by the atomic mass of that atom. The heavier it is, the slower the reaction proceeds. This effect is minimal for most elements, but it can be very significant for hydrogen. Since deuterium has double the atomic mass of hydrogen, replacing a hydrogen atom with a deuterium atom can slow a reaction by a factor of ten or so.
It was reported last year that scientist Mikhail Shchepinov was able to increase the lifespan of worms 10% by feeding them heavy isotopes. As far as I can tell, this work has still not been published, so I have no way of evaluating it--although I can say that the press release is incredibly exaggerated, and its claim that this 10% increase in the lives of worms will translate to a ten-year increase in human lifespan is just plain irresponsible. Shchepinov has laid out his hypothesis elsewhere, however, and the idea is that enriching our diet in heavy isotopes would limit the oxidative damage that contributes to aging, because the kinetic isotope effect would slow down the generation of reactive oxygen species. This is a reasonable hypothesis, but I would have to see some hard experimental evidence before I could fully believe it.
The most relevant isotope for this would be deuterium, but there's a problem: D2O (deuterium oxide, water with its two hydrogen atoms replaced by deuterium) is toxic to humans--although it's not fatal until about 50% of your body water is replaced by deuterium oxide. Also, it's expensive, costing up to about $1,000 per liter. Due to its toxicity, then, deuterium could only safely be introduced into our diet in very limited quantities, and probably just through selectively-labeled molecules (this means molecules that only have deuterium present at specific key positions). This wouldn't get around the cost. Quite the opposite, in fact: this would require ingesting these isotopes in more complex molecules, and the cost of isotopically-labeled molecules increases quite a bit with molecular complexity.
Therefore, I have a lot of difficulty imagining humans regularly ingesting isotopically-enriched foods, at least in the foreseeable future... although it would increase your intrinsic value quite substantially.
In addition to the cost and the relative toxicity, heavy water has other interesting effects. For example, it causes your circadian clock to run slower. This might have longer-term negative ramifications, since the circadian cycle influences many physiological processes, and messing with the central timekeeper could muck up lots of those.