Heating destroys some of the benefits of turmeric, but not all.
Heating foods generally does destroy nutrients. This is true for vitamins such as vitamin C, but some phenolic compounds are very resistant – this is true for lycopene, the pigment nutrient found in tomatoes.
Curcumin, the main active ingredient in turmeric is also a phenolic, and its benefits and stability is heavily researched. Research has shown that roasting curcumin at 180°C for 70 mins does cause it to breakdown. However, the product of this degeneration still exerts significant benefits (at least to cultured cells). These benefits include antioxidant activity and immune system stimulation, which are some of the key benefits of taking turmeric.
There isn’t any comparative study of un-roasted vs roasted curcumin to see if potency is lost (or even gained), and this study was done in cultured cells rather than humans. This does mean that some clarification is needed to make completely conclusive statements. However, the current reserach does show that the benefits of curcumin are not completely lost when roasted in extreme conditions. In real world terms, it is very unlikely that turmeric would ever get heated to these temperatures for this long, so normal cooking with turmeric is unlikely to cause degradation to this extent anyway. The fact that it shows some activity after being exposed to this is very positive!
The other nutrients in turmeric, of which there are many (such as turmerone) are much less tested. They are all phenolic compounds, so they may well be heat stable to some extent, and like curcumin they may also degrade into active compounds too – we just don’t know.
Although these other nutrients are an important part of the benefits of taking turmeric, curcumin is the primary active ingredient, and so arguably the most important. Despite the fact that curcumin does get degraded by heat, it doesn’t destroy all the health benefits that it exerts. The impact of this degradation on the potency of the health benefits is still unclear, but if research on other nutrients is anything to go by, the the assumption would be that the potency is reduced.