Basel, Switzerland
September 30, 2005
By Katharina Schoebi,
Checkbiotech
The body needs calcium to reduce
osteoporosis, a disease characterised by reduced bone density
and the primary cause of bone fragility. The recommended amount
of daily calcium intake depends on the person’s age. For
example, nursing children need between 220 to 400 milligram of
calcium per day. The older a child becomes, the more calcium it
needs. A teenager should consumer around 1.2 gram calcium per
day, whereas adults should eat 1.5 gram of calcium per day.
Dairy products are a good source of calcium. Cheese is
especially rich in calcium. A 100 gram piece of Parmesan
contains 1,180 milligram of calcium. Thus, the daily calcium
need would already be met by eating a piece of cheese and
drinking a glass of milk.
However, some individuals suffer from lactose intolerance and
thus limit their consumption of dairy products or even avoid it
totally, while some ethnic groups do not use dairy products in
their diet, or restrict them considerably. These individuals
have to meet their daily calcium needs by eating vegetables and
fruit. Unfortunately, widely consumed vegetables, such as
potatoes, grains and rice, do not contain enough calcium. For
example, an average tuber of boiled potato contains only 20
milligram of calcium. Given that an average person would eat one
boiled potato per day, one would consume 7.3 gram calcium in a
year – the amount recommended for only one week!
“If we double or triple the amount of calcium in a tuber, we
could provide several weeks worth of the dietary reference
intakes of calcium,” said Dr. Kendal Hirschi from the Baylor
College of Medicine in Houston, Texas, in his publication in the
Journal of Agricultural and Food Chemistry.
The reason for Dr. Hirschi’s excitement was that his publication
dealt with transferring a gene that encodes the calcium
transporter CAX1 to a potato variety. CAX1 is a calcium
transporter in the vacuole membrane of plants. The researcher’s
hypothesis was that increased activity of CAX1 in potatoes
should result in an increase in total calcium level. This
hypothesis was based on his past research that CAX1 increased
calcium levels in tobacco (Plant Cell, 1999).
Analyzing their data, the researchers found out that the
transgenic potato tubers contained 1.5- to 3-fold more calcium
than natural tubers, and in transgenic leaves, the calcium
levels increased up to 1.7-fold. Since even the third generation
of potato plants showed an increase in calcium content, the
researchers believe that this trait is stable through successive
generations.
Overall, comparing the mineral content and consistency of common
potato varieties and calcium-enhanced potatoes, Dr. Hirschi’s
group was not able to find any differences.
The researchers underline, however, that no single food source
will rectify calcium intake deficiencies alone and the
genetically enhanced potatoes follows suite. Thus, transgenic
potatoes will not cure osteoporosis, but they rather will
provide additional calcium and will be a model for the
modification of the calcium content in many more food crops, Dr.
Hirschi and his colleagues argue.
To assess the so called bioavailability of calcium (the amount
of calcium that can be digested, absorbed and used by the body)
further studies are needed. Right now, the researchers are
undertaking feeding studies in mice to see if the transgenic
potatoes are actually more nutritious. In addition, some
experiments should assess if calcium-enhanced potatoes still
taste like common commercial varieties.
Since there is some evidence that genetic transformation does
not alter either the ethylene levels or the sugar content of
tomatoes, Dr. Hirschi suggests that transgenic potatoes will
also taste like common varieties.
The research group has carried out some experiments to
determine, whether a higher calcium amount in the tuber affects
the vitamin C content of the potatoes. With greenhouse grown
potatoes, they did not see any difference. In addition, the
consistency appears to be similar, but at this point the
researchers have not finished their work in this area. Dr.
Hirschi told Checkbiotech.
Future studies will also be done to find out if increasing
calcium levels will decrease the incidence of pathogen infection
and post harvest decay, two major problems worldwide. “Our hope
is that the modifications will lower the incidence of
postharvest infection and will not alter the insect attacks on
the plants,” Dr. Hirschi explained.
Plants need calcium for normal growth and development, and it is
especially critical for correct cell wall and cell membrane
formation. It is known that potatoes grown in calcium enriched
soils have an increased tuber quality, whereas calcium
deficiencies in potatoes result in internal disorders such as:
hollow-heart, internal brown spot, brown center and reduced
storage life. Thus, transgenic potatoes containing higher
calcium levels could lead to better yields for farmers and a
more nutritious product for consumers.
Dr. Hirsci looks forward to working with the Vegetable and Fruit
Improvement Center at Texas A&M. This center specializes in
working with growers to bring new produce options to consumers.
Dr. Hirschi and his team have recently carried out similar
studies with tomatoes and carrots, and he noted, “We should try
as many plants as possible.” The results of his study are
encouraging, Dr. Hirschi said. However, a lot more work needs to
be done to show these plants are durable and safe. “Working with
plants is fun, because you can try to help people – but it is a
slow process.”
Katharina Schoebi is a biologist and Chief Science Writer for
Checkbiotech. Contact her at katharina.schoebi@bluewin.ch.
Kendal D. Hirschi et al. Genetic Manipulation for Enhancing
Calcium Content in Potato Tuber. Journal of Agricultural and
Food Chemistry. (2005) 53, pp. 5598-5603
Link to the abstract:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15998121&dopt=Citation
Kendal D. Hirschi et al. Expression of Arabidopsis CAX1
in Tobacco: Altered Calcium Homeostasis and Increased Stress
Sensitivity. Plant Cell. (1999) 11, pp. 2113-2122
Contact:
Kendal Hirschi
Baylor College of Medicine
1100 Bates Street
Houston
Texas 77030
Phone: 713-798-7011
Fax: 713-798-7078
Mail: kendalh@bcm.tmc.edu |