Tate & Lyle shares tumbled 9% after the sweeteners and corn processing group warned over the US ethanol industry, where poor margins will drag the group's commodities division into the red.
The London-listed group revealed that a weaker Mexican peso and Brazilian real would deliver some dent to pre-tax profits, in sterling terms, which it now saw coming in "modestly" below the £193m that it had previously guided to.
The figure is also below the £196m that analysts have forecast.
However, a caution that its commodities business would suffer a "small loss", after having previously been fore4cast to record a "small profit", attracted more comment among investors, who sent Tate & Lyle shares to a five-month low of 529p at one point.
The drop wiped more than $350m from the group's stockmarket value.
"Commodities continue to have a material adverse impact on performance, especially due to further weakness in the US ethanol market," Tate & Lyle said.
At house broker Jefferies, Martin Deboo said he was "a little bemused" by a tone in Tate & Lyle's statement which was "notably more cautious than recent updates".
Credit Suisse said it was cutting its profits forecast for Tate & Lyle by £10m "entirely reflecting the commodity [division] outlook".
The bank added that it was "worth remembering the ethanol price is linked to oil, while the input cost is corn, which hasn't moved while oil has fallen".
Indeed, Tate & Lyle's comments underline the difficulties experienced by operators in the US ethanol market, where margins are being sliced by a reluctance among producers to slow output, allowing US inventories to hit a record 23.0m barrels, official data on Wednesday showed.
US ethanol producer Green Plains, announcing overnight a fall into a $3.59m after-tax loss for the October-to-December quarter, from a $42.2m profit a year before, highlighted a "challenging margin environment".
"Even with strong global and domestic ethanol blending growth, US production continues to outpace demand for the time being," said Todd Becker, the Green Plains chief executive.
The Andersons, the US-based silos to railcar group, noted a "very challenging energy environment" as it unveiled a 45% drop to $7.7m in profits at its ethanol division.
Last week, Archer Daniels Midland, citing "uncertain" ethanol industry margins, unveiled a strategic review of its corn dry mills, which turn the grain into ethanol rather than the higher-value products enabled of wet mills.
"We are concerned about the long term from the dry mill ethanol part of the industry," ADM chief executive Juan Luciano told investors.
"Production in the industry has kept this industry margins very, very low and we are really surprised by that."
Tate & Lyle chief executive Javed Ahmed added in comments to investors on Thursday that "I personally do not see ethanol situation getting better… in the foreseeable future.
"Because if you look at it, the underlying dynamics of the market, there's 15.5bn gallons out there" in capacity, above the 14.5bn gallons of annual ethanol use that the US renewable fuels mandate calls for.
"You look at the inventory situation, it's the highest inventory level of the past three years. We haven't seen too many things being mothballed up to this point.
"So I somehow don't see that situation getting any better."
In fact, there are a few signs of the industry's woes forcing output cuts, with Poet, the second-biggest US ethanol producer, halting corn receipts later this month at plants in Indiana and Iowa.
Small player Distiller Southwest Iowa Renewable Energy has also unveiled output cutbacks.
Tate & Lyle shares recovered a little ground to stand at 537.5p in lunchtime deals, a drop of 7.6% on the day, well ahead of the 2.5% drop suffered by the average stock in London's FTSE 100 index.
By Mike Verdin