New research from UC Davis and the University of Reading shows that everyday pairings—such as bananas blended with berries or coffee consumed with meals—can sharply reduce the body’s uptake of key nutrients.. The studies, highlighted in UC Davis press releases in October 2025 and May 2026, pinpoint enzymes and polyphenols as the culprits, suggesting that modest changes in timing or ingredient choice could restore much of the lost nutrition.

Banana‑Berry Smoothies Slash Flavanol Absorption by 84%

The most striking findinng comes from a controlled trial where participants drank a berry‑only blend versus a banana‑berry blend. According to the researchers, the banana‑containing drink cut flavanol absorption by 84 percent. The mechanism was traced to polyphenol oxidase (PPO) in bananas, which degrades flavanols during both blending and digestion.

Funding for the work came from Mars Inc., a company that markets a flavanol supplement, but independent labs have replicated the PPO effect in other high‑PPO foods such as apples, peaches, avocados, mangoes, eggplant, potatoes and mushrooms. swapping bananas for pineapple, oranges or mangoes preserved flavanol levels in the test smoothies.

Coffee’s Polyphenols Cut Non‑heme Iron Uptake by Up to 66%

Separate research highlighted that coffee’s polyphenols and tannins can lower non‑heme iron absorption in iron‑deficient women by 54 to 66 percent. The study recommends waiting at least one hour after drinking coffee before consuming iron‑rich foods to avoid the interaction.

Calcium compounds the problem: it competes directly with iron and can reduce iron uptake by as much as 50 percent when consumed together. Separating calcium‑rich foods or supplements from iron‑laden meals by a minimum of two hours is an effective, though little‑known, strategy.

Vitamin D, Caffeine and Fat: A Tricky Trio

Analysis of NHANES data linked higher caffeine intake to lower serum vitamin D levels, and cell studies suggest caffeine may suppress vitamin D‑receptor expression. Taking a vitamin D supplement alongside coffee—especially on an empty stomach—exacerbates the issue because vitamins A, D , E and K all require dietary fat for optimal absorption.

Conversely, vitamin C acts as a potent iron enhancer, doubling non‑heme iron uptake when paired with iron‑rich foods. Adding citrus, bell peppers or tomatoes to meals is a simple way to boost iron status.

Phytates, Oxalates and Simple Food Prep Fixes

Whole grains and spinach contain phytates and oxalates that bind iron and zinc, limiting their bioavailability. however, soaking, fermenting or cooking these foods markedly reduces phytate content, mitigating the inhibitory effect.

The broader message is not to abandon beloved foods like smoothies, coffee or spinach, but to adjust timing and pairings.. Skipping the banana in a berry smoothie , delaying coffee after breakfast, and pairing vitamin D with a fat‑containing meal can all improve nutrient utilization without overhauling grocery lists.

Who Still Needs to Verify the Findings?

While the UC Davis‑Reading studies are peer‑reviewed, the involvement of a supplement‑selling sponsor raises questions about potential bias. Independent confirmation of the exact magnitude of nutrient loss in real‑world eating patterns remains limited.. Moreover, the optimal timing windows for separating calcium and iron, or coffee and iron, vary among individuals and have not been standardized in clinical guidelines.