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Thursday, 18 May 2017

Fruit and Diabetes - some evidence

It's a commonly discussed paradox of sorts - how can fruit have a negative association with diabetes in epidemiology when it's full of sugar?

Two recent papers from China go some way towards clearing this up in my opinion. One is a prospective study of Type 2 Diabetes risk, in which a difference is seen between different classes of fruit; apples are good, tropical fruits - pineapples, mangos, and bananas are not, but the effect is staggered by gender.[1]

Results: In 494,741 person-years of follow-up, 5207 participants developed T2DM. After adjustment for lifestyle and dietary risk factors, high total fruit consumption was not consistently associated with lower T2DM risk [men: HR of 1.33 (95% CI: 1.04, 1.71) for 3 or more servings/d compared with less than 1 serving/wk (P-trend = 0.17); women: HR of 0.88 (95% CI: 0.71, 1.11) (P-trend = 0.008); P-interaction = 0.003]. The direct association in men was observed for higher–glycemic index (GI) fruit [HR: 1.51 (95% CI: 1.22, 1.86) for 1 or more serving/d compared with rarely consumed; P-trend = 0.001] but not for lower or moderate GI fruit. In women, the consumption of temperate fruit, but not of subtropical or tropical fruit, was associated with lower T2DM risk [HR: 0.79 (95% CI: 0.67, 0.92) for 1 or more serving/d compared with rarely; P-trend = 0.006].

Conclusions: The consumption of temperate fruit, such as apples, was associated with a lower risk of T2DM in women, whereas the consumption of higher-GI fruit, such as bananas, was associated with higher risk in men. The impact of fruit consumption on the risk of diabetes may differ by the type of fruit, which may reflect differences in the glycemic impact or phytochemical content.

A second Chinese paper looked at fruit consumption in the second trimester and risk of gestational diabetes.[2] (This was posted by gestational diabetes expert Lily Nichols @LilyNicholsRDN on her blog)

As epidemiology goes, this paper has signs of class - look at table 1, where they have actually gone to the trouble to check that their respondents are representative of the whole population canvassed by giving the baseline characteristics of the people who didn't want too be in the study, who are well-matched with the people they included. This is textbook stuff, but I can't remember the last time I saw it done. Fruit intake was fairly high - 740g a day in the upper quartile.

An increase in total fruit consumption during the second trimester was associated with an elevated likelihood of GDM (highest vs. lowest quartile: crude OR, 3.20; 95% CI, 1.83 to 5.60). After adjustment for age, education, occupation, income level, pre-pregnancy BMI, gestational weight gain, family history of diabetes, smoking status and alcohol use in Model 1, a significantly higher likelihood of GDM was still observed in the third and fourth quartiles for total fruit consumption (OR 2.81; 95% CI 1.47 to 5.36; OR 3.47; 95% CI 1.78 to 6.36, respectively). After adjustment for potential confounding factors in Model 1 plus the consumption of grain, vegetables, meat and fish, the ORs for the lowest to the highest quartiles of fruit consumption were 1.00 (reference), 1.08 (95% CI 0.50 to 2.34), 3.03 (95% CI 1.54 to 5.94) and 4.82 (95% CI 2.38 to 9.76), respectively.

These are some huge ORs - what about type of fruit?

Comparison of fruit subtypes revealed that a greater consumption of pome fruit was associated with a lower likelihood of GDM (crude OR 0.59; 95% CI 0.37 to 0.96). The OR of GDM in the highest tertile of pome consumption was almost half that in the lowest tertile. However, the association attenuated to null after adjusting for potential confounding factors in Models 1, 2 and 3. Compared with the lowest tertile, the second tertile for consumption of gourd fruit was inversely associated with the likelihood of GDM, but this inverse association was neither observed in the highest tertile nor in the overall trend (P trend = 0.346). The adjusted ORs in Model 3 across the lowest to highest tertiles of fruit consumption were 1.00 (referent), 0.27 (95% CI 0.11 to 0.66) and 0.94 (95% CI 0.45 to 1.95), respectively. In contrast, compared with the corresponding lowest tertiles, the highest tertiles for consumption of citrus and tropical fruit were each related to a higher likelihood of GDM (adjusted OR in Model 3, 2.26; 95% CI 1.29 to 3.99; adjusted OR in Model 3, 3.73; 95% CI 1.74 to 8.01, respectively). Berry consumption was initially positively associated with GDM, but this association was attenuated to null in Model 3 (highest vs. lowest tertile in Model 3: OR, 1.69; 95% CI 0.80 to 3.56).

Ignore the berry association, it's obvious from the CIs that people didn't eat enough berries to give much of a result. But pomes are apples and pears, and again they look good. Why?

They also assessed the results by GI:

The increased consumption of fruit with moderate to high GI values was significantly associated with a higher likelihood of GDM. Compared with the lowest quartile, the highest quartile for consumption of fruits with moderate to high GI was associated with a higher likelihood of GDM (crude OR 3.04; 95% CI 1.80 to 5.06; adjusted OR in Model 3, 2.94; 95% CI 1.47 to 5.88).

High GI fruits were pineapple, mango, citrus. The authors hypothesised about effects of polyphenols, but this didn't really go anywhere.
Here's what I think; apples and pears are the only fruits you can't juice with your bare hands. When you eat an orange, you're swallowing juice and pulp separately. When you eat an apple, you're still swallowing them together, mostly. And this, I think, is what makes the difference. It takes longer for the sugar to appear in your blood, so people with an already impaired phase 1 insulin response are less affected by it, and the slower digestion produces a more satiating and less insulinogenic gut hormone response.
Of course it's possible that people with a sweet tooth ate the sweeter fruit and that a sweet tooth indicates some sort of internal starvation predictive of diabetes, but even so, eating the sweeter, juiceable fruit is not going to help.

The amount of fruit associated with a lower risk of diabetes in meta-analysis, as with pome fruit here ("one or more serving/day") is relatively low and would fit in many low carb diets (the same is true of wholegrains and legumes - the studies that say that these foods are associated with protection don't say that very high intakes are needed at all). Not that this effect, whatever it is, would be important or needed in a low carb diet, but it is available unless your preferred carb intake is under 50g. If people do include sweet or starchy carbs in their diet, the types of carbs are important.
Very important.

Also see Gannon and Nuttall's study comparing a 40% carb diet high in intrinsic sugars (fruit, milk, root veges) with a 60% carb diet high in starch.[3]

[1] Alperet DJ, Butler LM, Koh W-P et al. Influence of temperate, subtropical, and tropical fruit consumption on risk of type 2 diabetes in an Asian population. Am J Clin Nutr. 2017: ajcn147090;ajcn.116.147090v1

[2] Huang W-Q, Lu Y, Xu M, Huang J, Su Y-X, Zhang C-X. Excessive fruit consumption during the second trimester is associated with increased likelihood of gestational diabetes mellitus: a prospective study. Scientific Reports. 2017;7:43620. doi:10.1038/srep43620.

[3] Gannon MC, Nuttall FQ, Westphal SA, Fang S, Ercan-Fang N. Acute metabolic response to high-carbohydrate, high-starch meals compared with moderate-carbohydrate, low-starch meals in subjects with type 2 diabetes. Diabetes Care. 1998 Oct;21(10):1619-26.


Observer Status said...

What if the old saying "An apple a day keeps the doctor away" is in fact a warning and not an exhortation to eat more fruit? In other words, eat no more sugars (especially fructose) than contained in one apple per day. Slightly modified, this saying appears to have arisen around the of Banting and reduced carbohydrate weight loss diet.

George Henderson said...

That's a good way of putting it, although fruit will always be better than lots of other things people could eat, depending what their choices are.
Did you know the saying "an apple a day keeps the doctor away" was an advertising slogan invented by the apple industry when it was under threat from Carrie Nation and her Temperance brigade? Puritans were chopping down apple trees because apples were used to make applejack, a cheap freeze-distilled liquor.

"Yet these bitter apples were perfect for something – the making of alcoholic, or "hard" cider, a potent libation that became the drink of choice for everyone from paupers to presidents. But by the late 19th century, drunkenness was on the rise, and an outcry, led by the hatchet-wielding prohibitionist Carrie Nation, arose against the evil apple.

It wasn't until after 1900 that the apple became the fruit we know today. Growers cloned the sweetest apples by grafting them, and ingeniously marketed them as the ultimate health food, guaranteed to "keep the doctor away." Soon, only a handful of varieties of apples were being produced, in vast operations that scientists call "monocultures," which grow just one genetic variety.

Stripped of the genetic diversity plants rely on to survive disease, the relatively few popular apple varieties grown in monoculture have become increasingly vulnerable to insects, bacteria and viruses. Today, apple growers are some of the biggest consumers of pesticides."

So yeah, maybe one a day is a wise limit!