Q: Is it true that the change in diet to milky coffee has
reduced calcium deficiency in Americans?
There seems to be no official data about how many Americans have changed their diets to milky coffee, so we don't know if drinking it has decreased calcium deficiency in them, but we can ask if it can reduce it.
In summary, there is very little evidence to say that drinking milky coffee can significantly decrease calcium deficiency in Americans in general. It may, however, prevent calcium deficiency in those individuals who would otherwise have very low calcium intake.
With other words, if you regularly drink 16 oz of cola (5 mg calcium) or orange juice (274 mg calcium) and switch to a milky coffee, like Grande Latte which has 450 mg calcium/16 oz, which is 45% Recommended Dietary Allowance (RDA), you will increase calcium intake, but this would decrease your risk of calcium deficiency only if your calcium intake without that would be very low - and it's not clear how low your otherwise calcium intake would need to be for milky coffee to help you.
What is calcium deficiency?
Calcium deficiency means low body calcium stores, which are mainly reflected as low bone mineral density (ncbi.nlm.nih.gov), also known as low bone mass or osteopenia, which may progress to osteoporosis. Even if long-term low calcium intake can result in low bone mineral density, it is only rarely associated with low blood calcium level (hypocalcemia), because the missing calcium is leached from the bones into the blood (Office of Dietary Supplements).
1) In several observational studies, there was no association between high milk intake and osteoporosis rate (Advances in Nutrition, 2019, Critical Reviews in Food Science and Nutrition, 2019).
2) From the maps that show worldwide milk and calcium intake and blood vitamin D levels and the incidence of hip fractures (mainly due to osteoporosis) you can see that milk/calcium intake is not associated with a lower incidence of hip fractures, especially in China and India with very low milk/Ca intake (<300 mg/day) and low incidence of hip fractures, and in the United States and Sweden with high milk/calcium intake (~1,000 mg/day) and high incidence of fractures.
According to Harvard School of Public Health:
...high calcium intake doesn’t
actually appear to lower a person’s risk for osteoporosis. For
example, in the large Harvard studies of male health professionals and
female nurses, individuals who drank one glass of milk (or less) per
week were at no greater risk of breaking a hip or forearm than were
those who drank two or more glasses per week....When researchers
combined the data from the Harvard studies with other large
prospective studies, they still found no association between calcium
intake and fracture risk...A 2014 study also showed that higher milk
consumption during teenage years was not associated with a lower risk
of hip fracture in older adults.
Additional evidence further supports the idea that American adults may
not need as much calcium as is currently recommended. For example, in
countries such as India, Japan, and Peru where average daily calcium
intake is as low as 300 milligrams per day (less than a third of the
U.S. recommendation for adults, ages 19 to 50), the incidence of bone
fractures is quite low.