Fat? Thin? Molecular switch may turn obesity on or off

Identical twins may be alike in everything from their eye color to their favorite foods, but they can diverge in one important characteristic: their weight. A new study uncovers a molecular mechanism for obesity that might explain why one twin can be extremely overweight even while the other is thin.

Heredity influences whether we become obese, but the genes researchers have linked to the condition don’t explain many of the differences in weight among people. Identical twins with nonidentical weights are a prime example. So what accounts for the variation? Changes in the intestinal microbiome—the collection of bacteria living in the gut—are one possibility. Another is epigenetic changes, or alterations in gene activity. These changes occur when molecules latch on to DNA or the proteins it wraps around, turning sets of genes “on” or “off.”